Category Archives: Flying

Electronic Gyro Drift Correction

Flying, Tech
by Mike Clements

Introduction

A magnetic compass is still a required navigation instrument in airplanes. The most common type is called a “whisky compass”, mounted at the top center of the panel. The compass is tilted toward the pilot to make it easier to read. Yet this also makes it accurate only in straight and level flight. When turning, the compass’ balance masses and tilt make its reading lead or lag the airplane’s actual heading.

Most airplanes also have a directional gyro. The gyro’s rotational inertia keeps it in a stable position as the airplane rotates around it. This means it reads accurately when turning. But gyros slowly drift over time. This means during straight and level flight, the pilot must occasionally check the gyro and manually set it to the compass heading. How often is occasionally? Depends on the gyro. For gyros that are entirely independent having no external correction, it’s about every 15 minutes. And this is true whether it is mechanical or electronic.

Gyro Drift

Gyro drift is caused by two factors: the rotation of the Earth, and friction (for mechanical) or noise/errors (for electronic).

The Earth rotates through 360* every 24 hours, which is 15* per hour. The gyro is immobile in space independent of the Earth’s rotation. Thus as the Earth rotates, the gyro “moves” relative to the Earth, and since the Earth is our frame of reference, this causes the gyro to drift. The relative motion of the Earth can make a theoretically perfect gyro drift up to 15* per hour from the pilot’s frame of reference.

Bearing friction and electronic noise are more intuitively obvious causes of gyro drift. With electronic gyros we have the advantage of being able to apply software corrections. Electronic gyros are based on accelerometer sensors, which means the sensor readings must be mathematically integrated to get position. Integration cumulatively amplifies small sensor errors. For example, even if the sensor’s readings consistently average the correct value over time, each individual reading will be slightly more or less. And these accumulate over time into increasing errors.

Drift Correction

One form of drift correction is when the pilot sets the gyro to match the compass heading. Immediately after this we can assume the gyro’s heading is correct. If we store each of these changes, we have a history of how much the gyro has been drifting and can use that to auto-correct and reduce drift.

Correcting this automatically applies only to electronic gyros, since we need a software algorithm to compute and apply this.

Bias and Variance

Errors and noise fall into two categories: bias and variance. Bias refers to anything systematic or predictable, variance is the unpredictable random portion of the errors. We can detect and correct for bias but not for variance. We must be careful, because misinterpreting variance as bias can increase errors rather than reduce them.

The basic idea is that after each pilot correction, we compute the rate of drift of the correction and continue to apply that to the gyro going forward. For example, if the most recent correction was +10* and it was made 15 minutes after the prior correction, the correction is +0.667* per minute, so we automatically apply that to the gyro going forward.

However, it’s not quite that simple so the idea needs refinement.

For example, suppose the drift that the pilot is correcting reverses direction each time. In this case, if we correct as above, we would actually amplify rather than reduce the drift, making things worse rather than better.

The problem is that errors are a mix of bias and variance, yet our idea only works to reduce bias, not variance. One simple way to differentiate bias from variance is to look at whether recent user corrections all go in the same direction. When this happens, there is a simple linear component to the gyro errors: it’s consistently drifting in the same direction, whether clockwise or counter (this is not the only form of bias, but it’s the simplest and easiest to detect). Yet even a blind squirrel sometimes finds a nut, and random errors will sometimes also go in the same direction. When you flip a coin, you will sometimes get heads several times in a row.

Consider that with variance (completely random errors), each pilot correction is 50% likely to be in either direction, like flipping a coin. If you flip a coin twice, you get 2 heads or 2 tails half the time. Similarly, with pure variance and no bias, about half the time each pilot correction will be in the same direction as the prior correction. Three consecutive corrections in the same direction would happen about 25% of the time. Conversely, we can say that in this case the error is 75% likely to have some bias.

So we should not apply our automatic correction unless the most recent N pilot corrections were all in the same direction, and N should be at least 3. Also, we should shrink the auto-corrected rate accordingly. For example when N=3 the error is 75% likely to have some bias, but it will always have some variance too. So let’s assume that 75% of the error is bias and shrink the correction applied going forward to 75% of the pilot-entered value. In the above example, the +0.667* per minute becomes 0.5* per minute.

Oscillation and Damping

At this point we have a simple algorithm that should improve the gyro accuracy. Yet we can improve it further without adding complexity. The idea is that our method to discern bias from variance is always imperfect, and if we get it wrong it makes things worse, not better. It is better not to correct errors, than to make them worse. Put differently, if we are too aggressive with error correction we can make things worse, while if we are too passive or conservative, it still improves things just not as much.

So, we will apply a damping factor to our corrections, shrinking them just a bit. Pick a constant scaling factor between 0.0 and 1.0 and apply that to the correction. For example, suppose we pick 0.85 or 85% and N=3. With the above example, suppose the last 3 pilot corrections were all in the same direction, and the most recent one was +10*, and it was made 15 minutes after the prior one.:

  • The raw error being corrected is 10* / 15 minutes = 0.667* / minute.
  • Because N=3 we are 75% confident there is bias in this error, so shrink it to 75% of its value.
  • Apply our damping factor of 85%.
  • This makes the auto-correction factor 0.667 * .75 * .85 = 0.425* / minute
  • Apply this rate correction automatically going forward

Overall, we end up with a single pilot correction rate that is maintained in cumulative fashion. For example, in the last step above we don’t just set it to 0.425* / minute, but we add 0.425* / minute to whatever the existing value is. And repeat every time the pilot sets the gyro, so the value changes over time, adapting to varying conditions during flight.

Conclusion

This practical example is over-simplified but it illustrates the basic concepts involved regarding bias vs. variance in errors, how to differentiate them and make corrections, and how to increase our confidence that our attempt to reduce errors doesn’t unintentionally make them worse.

Twin Engine Airplanes

Flying
by Mike Clements

It’s common perception that twin engine airplanes are safer. Any for obvious reasons! Who wouldn’t want an extra engine? Yet the details give a more nuanced perspective.

With passenger jets, twin engines are definitely safer – no doubt about it. But with piston engine small aircraft (e.g. light twins), the safety record is more mixed. It boils down to 3 basic reasons:

  1. Having two engines doubles the likelihood of an engine failure.
  2. When one engine fails, the other produces differential thrust requiring immediate corrective action from the pilot to avoid loss of control.
  3. The single-engine performance of some piston twins is so marginal we sometimes say, “the remaining engine always has enough power to get you to the scene of the crash”.

Many people don’t really consider the first point, but when pointed out, it’s so obvious it doesn’t require further discussion.

Regarding point 2: engine failure in any airplane is an emergency, no matter how many engines it has. Yet with a single engine, the immediate pilot actions are pretty simple: keep the nose down so you don’t stall, and pitch for ideal glide speed. The engine is centered, so when it dies the airplane remains inherently stable and will keep gliding even hands-off. With twin engines, when one engine dies the other produces differential thrust that skews the airplane sideways and will flip it over if not corrected. This differential thrust is a double-whammy: the dead engine’s prop creates drag, pulling that side back, while the operating engine produces thrust, pulling the opposite side forward. Not only must the pilot keep the nose down to avoid stalling, but he must also apply heavy opposite rudder (not aileron) and feather the dead engine’s propeller to keep the airplane flying straight. If the pilot fails to do these actions quickly – within seconds – or does them incorrectly, the differential thrust can cause an uncontrollable spiral or spin.

Regarding point 3: one of the reasons people fly twins is for the superior payload and performance. You can carry heavier loads, and fly faster and higher. Yet if you are actually using that performance, you may operating in a way that cannot be supported by a single engine. So when one engine fails, even if you apply the correct inputs to keep flying, you may not be able to maintain level flight even with the good engine at full power.

Overall, the incident/accident statistics for light twins in general aviation is no better than single engine airplanes. Given this, why are big commercial jets always multi-engine for safety? Commercial aviation mitigates these factors:

  1. They use turbine engines, which are more reliable than pistons.
  2. The pilots are better trained, more frequently, and follow more strict operational limits set by both the FAA and their airline.
  3. The engines have much greater power than pistons, capable of maintaining level flight at high altitudes for extended periods of time, even when the airplane is at max weight.

In summary, light twin aircraft can be safer, or more dangerous, depending on the airplane, the pilot, and the mission or how the flight is operated. Pilots considering light twins should consider these limitations, how the airlines mitigate them, and incorporate that into their flying. For example:

  1. Maintain the aircraft above & beyond required minimums.
  2. Train yourself well beyond the required minimums, stay current.
  3. Don’t load to max weight, and fly missions that give you a healthy safety margin below the aircraft’s max performance.

Even then, in my opinion light twins are not safer and the higher performance is not worth the expense and hassles of the higher cost of fuel, maintenance, insurance and training. Speed is proportional to power cubed and drag is proportional to speed squared, so all else equal a twin burns 59% more fuel to go 26% faster.

Here’s where those figures come from:

  • The cube root of 2 is 1.26, so twice the power is 1.26 times as fast.
  • Drag is proportional to the square of speed, and 1.26 squared is 1.59.
  • Fuel consumption is proportional to drag.

Flying VFR International

Flying
by Mike Clements

I fly to Canada occasionally and I haven’t gotten fined or arrested, nor even admonished, so I must not have done anything too terribly wrong. This isn’t covered in private pilot training, so I figured it might be helpful to share my checklists. Note: this is for VFR.

Planning (weeks ahead)

  • Passports for every person on board
  • Buy US Customs sticker and apply on pilot side airplane door
  • Create an EAPIS account
  • Have a 3rd or higher class medical (BasicMed not allowed in Canada)
  • Proof of airplane insurance (required in Canada)
  • Radio station & operator license (legally required but nobody ever asks for it)
  • Get Canadian CFS (their AFD book) and charts
  • Proof of COVID vaccination for every person on board
    • COVID tests not required as of March 2022

Pre-Flight (1-2 days ahead)

  • File EAPIS including all people on board, print and bring the email confirmation
  • Pick an Airport of entry for your first landing after crossing the border
  • Figure out where Customs is at your airport of entry (airport diagram, etc.)
  • Call customs at your airport of entry 2-48 hours before landing
  • File international flight plan in the country you’re departing
  • If in Canada returning to the US, call Flight Service an hour before your flight to get your border crossing squawk code

In-Flight

  • Before crossing border, ensure your international flight plan is activated and you are squawking a discrete border crossing code
    • In USA, when in-flight radio flight service 122.2 or nearby RCO to activate
    • In Canada, call flight service 1 hour before departing to file plan & get squawk
    • Don’t cross a border squawking VFR
  • Fly the plan to your destination airport of entry

Flying into Canada

  • Before entering Canada, contact Canadian approach or terminal
    • for example Victoria Terminal 127.8
  • In all Canadian radio communications, emphasize the “N” at the start of your tail number
  • After landing, taxi to Customs, stay inside your airplane and call Canada customs
  • They will usually clear you over the phone without an in-person visit

After flying in Canada, they will mail you a bill for ATC services. The bill has a flat calendar quarterly rate for every quarter in which you fly in Canadian airspace. For example in 2023 I flew to Canada twice, in July and August, and both trips happened to fall in the same quarter. I got a bill for $24.09.

Canada aviation regulations and procedures are similar to the US, though here are a few key differences that will help keep you out of trouble:

  • VFR flight plan required for all flights > 25 nm
    • Call to file before flight
    • Plan automatically activates at filed start time – no need to activate after takeoff
    • Must call to close plan upon landing
  • At busy airports, call clearance delivery before calling ground (even for VFR), to get your taxi/takeoff clearance and squawk code, if applicable.
  • Altitude: 10,000 – 13,000 limited to 30 mins without oxygen
  • VFR over the top is restricted
  • VFR night is restricted
  • MF: mandatory frequency; like CTAF
  • Class “E” airports (untowered) have mandatory reporting before entering their airspace
  • Monitor 126.7 continuously, en route, and make occasional position reports in the blind. Also monitored by FSS.
  • Contacts

An ADS-B Troubleshooting Saga

Flying, Tech
by Mike Clements

Introduction

ADS-B is “Automatic Dependent Surveillance Broadcast”. It is an electronic system installed on airplanes that reports their 3-D position in real time. The FAA required all aircraft flying in controlled airspace to have ADS-B by Jan 1, 2020.

My ADS-B system is uAvionix Tailbeacon TSO. I installed it in Oct 2019 and it worked well for about 3 years.

Back in March 2023 I was flying back to KBFI when the tower controller said she didn’t have my Mode C altitude. This sometimes happens even when the transponder is working well, so I reset it. I also reset the Tailbeacon ADS-B just to be safe. The controller then asked if I was ADS-B equipped. This is never good, since it means they aren’t getting my ADS-B data.

The Saga Begins

The next day, a technical representative from the FAA emailed me to tell me my airplane’s ADS-B system wasn’t working, and asked how I plan to fix it. He also provided performance reports from recent flights to show that it was not an isolated case, but a trend. I opened a support case with uAvionix and notified my local airplane shop. My airplane was about to go in for its annual inspection, so I said I’d have them fix during that time. Until then, I self-grounded for a couple of weeks.

When I flew from KBFI to KPLU to drop my airplane off for its annual, the ADS-B performance report (PAPR) was clean. So the Tailbeacon did work properly under some conditions.

Death from Corrosion and Ground Wiring

During the annual, based on uAvionix advice, we improved the fin grounding by running a wire across the hinge to the rudder. We found corrosion on the Tailbeacon circuit board so uAvionix said it should be replaced. Since it was beyond its 2 year warranty, they asked for $400 for the replacement, which is an 80% discount. I asked for a courtesy replacement due to all time, expense, and down-time the failure was causing me. uAvionix granted that and sent it for free.

After annual, the new Tailbeacon worked well enough that ATC did not complain, but it still failed the PAPR. All the data was correct, but the GPS quality flag (NIC) sometimes dropped below minimum required accuracy.

GPS problems are common enough with Tailbeacon that uAvionix has a detailed 16 page manual to troubleshoot it. They sent me a copy. It is marked “company confidential – do not distribute”, so I won’t post it here.

The FAA PAPR is just a summary telling you whether you passed, and if you didn’t why you failed. So if you fail, you know why but you don’t know exactly where. You can email the FAA and they will provide a detailed GPS log in KMZ format, showing every message your ADS-B system sent, color coded GREEN for good and RED for bad. This is essential for troubleshooting ADS-B systems. You can load this into Google Earth and easily see exactly where it failed.

Radio Interference

In the detailed track log, it was mostly green, but red in a few spots. I noticed that one of the spots it turned red was over the rock quarry SE of Boeing Field, exactly where Boeing Tower asked me to report my position. Could my radio transmission have jammed the Tailbeacon GPS? It seemed unlikely because I was transmitting on 118.3 MHz, while GPS is at 1.5 GHz, more than 10x higher frequency.

The uAvionix troubleshooting doc says that radios can jam the GPS from harmonic distortion. Specifically, around the 12th or 13th harmonic. When this happens, you can install lowpass filters on the comm antennas to block that distortion. But those lowpass filters are expensive, and the GPS track also turned red in places I wasn’t transmitting, so I wasn’t sure if that was the problem.

I have 2 comm radios, an MX-385 and an RT-385. I removed one from the panel and made a test flight. Then I reinstalled it, removed the other, and made another test flight. The PAPR for these flights still failed, but it improved. With the MX-385 removed, there were fewer GPS drops.

Next, I tested it on the ground. I turned on the Tailbeacon while monitoring its data with the uAvionix app on my phone. I watched it get a good GPS fix. Then I transmitted on different frequencies on each of my radios. The MX-385 would cause the Tailbeacon to lose GPS completely and instantly. The RT-385 did not. But it would jam the GPS while flying. So ground testing is informative yet not authoritative.  I also made test flights with the Emergency Locator Transmitter (ELT) turned off and antenna disconnected.

So I needed to install filters. But what kind? From what I read, Garmin makes them and so does TED. The TED filters are more than twice the price, but user comments suggested they are more effective. The TED 4-70 is -52 dB at 1.5 GHz. I ordered 2 of them.

The filters should be easy to install: each goes inline and has a BNC connector on each side (one male, one female). So I crawled underneath my airplane panel with a flashlight. I discovered that the comm radio antennas do not have any BNC connectors. They are hard-wired to the back of the radio rack, and the cable runs straight to the antennas on the roof of the airplane. I spent hours removing interior panels to follow those cables looking for a connector, but alas there were none. So the only way I could install the filters was to cut the antenna cables and install new BNC connectors.

I studied to find out what kind of coax cable the antennas use, ordered a set of male and female BNC connectors, a cable stripper, and crimp tool. When they arrived I spent several more hours contorted upside-down under the panel with a flashlight, cutting the cables and installing the connectors. When I finished I ground-tested the radios. One worked, the other didn’t. Apparently, a strand of wire went astray when I installed the BNC connectors. So I did it over again. Finally, both radios worked.

I made a test flight and the PAPR was much improved. The GPS NIC never dropped to zero, but only dropped to 6. It should be in the range of 7-9. So it still failed, but it nearly passed.

I bought another pair of TED 4-70 filters, this time used from eBay to save money. I installed one on the ELT antenna and kept the last as a spare. My next flight still failed the PAPR, but it was still improved.

Switches and Connectors

I mentioned that my flight from KBFI to KPLU with the old Tailbeacon pass the PAPR. Just before that flight I exercised the panel switch for the Tailbeacon about 10 times, to scrape off any internal corrosion and improve the connection. These panel switches are OEM, so they are over 40 years old. I exercised all of them again to see if that would help.

Well, three of them broke while I was switching them back and forth! At home, I wired a shunt from 16 gauge wire with dual male spades, soldered together. Then at the airplane I plugged the nav light direct through the shunt instead of through the switch. The next test flight still failed, but almost passed, a further improvement and closest I had yet come to passing.

Re-Evaluation

At this point I had done everything in the uAvionix guide, and it still wasn’t passing the PAPR. It was working well enough that ATC was not complaining. But it needed to pass the 91.227 requirements, which are more strict.

uAvionix escalated my case to Lou and we spoke for about an hour covering the history, all the things I had tried, and what to do next. We agreed that I would replace the panel switches in my airplane, test it again. If it didn’t pass, uAvionix would send me another warranty replacement unit. But Lou said they were out of stock and it would take 4 weeks.

So, I dropped my plane at Spencer Avionics to get the switches replaced. Spanaflight had new switches in stock and Spencer installed them. My next flight worked as well as the prior one with the shunt, so the new switches definitely helped. And I needed them anyway, since some broke. But it still didn’t pass.

At this point Lou called me and said that even though uAvionix was out of stock, he had one at his avionics shop and he would send me one, via 2 day FedEx.

Another Warranty Replacement

When it arrived I flew back down to Spanaflight and, working alongside Karl, we replaced the old Tailbeacon with the new one. At my request we soldered it instead of using crimp connectors. I turned it on and did the initial set-up. Then on my flight back to KBFI I flew the long way around in order to make the flight long enough (at least 30 mins) to get PAPR. After I landed, I pulled the report and it passed! I forwarded it to the FAA rep, who agreed it passed. Problem solved, case closed.

Happy Ending

So that is the end of the saga. Here’s a summary:

  • Original Tailbeacon developed corrosion on its circuit board, after 3 years of service.
  • It failed intermittently especially in freezing temperatures.
  • The new warranty replacement Tailbeacon also failed, due to weak GPS (low NIC).
  • All other fields (tail #, squawk code, etc.) were correct. The only failure was NIC.
  • We improved the ground by wiring across the hinge from the rudder to fin. This improved things but didn’t fix it.
  • We installed notch/lowpass filters on both comm radios and the ELT. This improved things but didn’t fix it.
  • We replaced the panel switches to the nav light. This improved things but didn’t fix it.
  • We replaced that Tailbeacon unit again, with another new warranty replacement.
  • During installation we soldered it instead of using the crimp connector. And we covered the connection with insulating shrink wrap.
  • The new Tailbeacon passed the PAPR on the very first flight and the FAA representative signed it off.

If this new one had failed, my only other option would have been to stop using uAvionix Tailbeacon and install a Garmin GDL-82 system instead.

Airplane Engines

Cars, Flying
by Mike Clements

Introduction

Most small airplanes are powered by piston engines. Car engines are sometimes used for kit or experimental airplanes. It seems like a logical thing to do since most car engines are reliable and less expensive than aviation engines. Yet while some car engines have performed well in aviation, they are the exception that proves the general rule to the contrary.

Here I’ll discuss some of the important ways in which airplane engines are different from car engines.

Rotational Speed

A typical prop for a small airplane has about a 76″ diameter (more or less). That’s a circumference of about 20′, which is how far the tips move in each revolution. The speed of sound is about 1100′ per second (sea level standard conditions), so that’s 55 revolutions per second, which means at 3,300 RPM the tips of the prop are moving at the speed of sound.

When the tips move faster than about 85% of the speed of sound, they start to lose efficiency. The airflow changes and they start making more noise & turbulence, and less thrust. And it creates unnecessary stress on the prop. So we need to limit the prop to about 2800 RPM. But we need to limit a bit more than that, because the airplane flies at high altitude where air is colder and sound travels slower. So typical small airplanes like this have a prop redline of 2700 RPM, plus or minus (lower for bigger props).

Power moves the car, or the airplane, or anything else that moves. In an engine, power is torque * rotational speed. Cars have a transmission enabling the engine and wheels to spin at different speeds, so they can rev up the engine to make good power, then gear it down to the wheels to maximize performance. To avoid the complexity, weight and reliability issues of a geared engine, in most airplanes the prop is bolted directly to the engine crankshaft. Thus, limiting the engine to 2700 RPM limits the power it can produce.

Consequently, most aviation engines don’t make much power for their displacement (for example the popular Lycoming O-360 which makes 180 HP from 360 ci), but they are designed to produce their rated power continuously while being lightweight and reliable.

Duty Cycle

Cars spend a lot of time in traffic constantly changing speeds. And cars rarely use their full rated power, but spend most of their time producing only a small part of it. For example, it takes about 30 HP to move a typical car down the freeway at 60-70 mph. For a car with a 150 HP engine, that’s only 20% of its rated power. A car engine is optimized for this duty cycle: to be efficient and reliable while producing a low % of its rated power.

Airplanes spend most of their time in cruise flight moving at a constant speed. The engine is running at a constant speed at or near wide open throttle, producing a high percentage of its rated power. For example, cruising at 70% power is typical. Airplane engines are designed to operate efficiently and reliably while generating their full rated power.

Lightweight

The value of light weight in an airplane engine is obvious. Consider the Lycoming O-360 mentioned above. It is a large displacement 360 ci engine that weighs only 260 lbs. A typical car engine of similar displacement weighs more than twice as much.

Of course, that 360 ci car engine would produce more than 180 HP. So for a fair comparison consider a modern car engine making 180 HP, like the Mazda Skyactiv 2.5. It produces 180 HP and weighs 260 lbs. In power and weight it’s similar to the Lycoming. But that Mazda is not designed to produce its rated power continuously. If you ran it constantly at wide open throttle at 6000 RPM it would not last very long.

It’s not easy to produce a lightweight engine that can operate reliably while continuously producing its full rated power. From a power / weight / reliability perspective, the Lycoming O-360 is comparable to modern car engines in 2022. This is especially notable when one considers that the Lycoming is a design from the 1950s.

Efficiency

Modern car engines are fully computer controlled. The driver applies a certain amount of throttle, and the engine computer determines the valve timing, spark timing, air/fuel ratio, etc. and constantly changes or adapts these settings to the conditions.

Airplane engines are manual. The pilot sets the throttle, RPM, and mixture manually. How can a human compete with a computer? Pretty well, it turns out, because the airplane spends most of its time in cruise flight, running at a constant power level, RPM, and altitude. This gives the pilot time to carefully optimize these settings and leave them there for hours.

One way to measure efficiency is miles per gallon. That Mazda gets about 40 miles per gallon on the freeway. A Cessna 172 in cruise gets about 18 miles per gallon. The Mazda wins, right? Well, it’s not really a fair comparison because the Cessna is going twice as fast. If you drive that Mazda twice as fast (say 130 miles per hour), it’s going to get about 1/4 the fuel economy, which is 10 miles per gallon (or less). So at the same speed, the airplane is almost twice as efficient. Indeed, other airplanes like Mooneys are far more efficient than the Cessna.

Yet this method of measuring efficiency is more about air resistance or drag, than the engine. Airplanes are just inherently more efficient than cars. What if we ignore that and focus on the engine itself?

Another way to measure efficiency is BSFC: brake-specific fuel consumption. That is, how much fuel does the engine consume to do a certain amount of work? One way to measure this is horsepower per gallons per hour.

Let’s estimate this for the Mazda. Suppose it’s getting 40 miles per gallon at 65 miles per hour. Each hour it burns 65/40 = 1.625 gallons of gas. Traveling that fast takes about 30 horsepower, so it produces 30 / 1.625 = 18.46 HP per gallon per hour.

Now consider the Cessna 172. It’s getting 18 miles per gallon at 130 miles per hour. Each hour it burns 130/18 = 7.2 gallons of gas. But how much horsepower is it generating? That is about 65% power, which is .65 * 180 = 117 horsepower. It produces 117 / 7.2 = 16.25 HP per gallon per hour.

So here the Mazda engine is about 13% more efficient (18.46 versus 16.25). However, keep in mind that this is when producing only 30 / 180 = 17% of its rated power. The Lycoming was producing 65% of its rated power. When you open the throttle to make the Mazda produce 65% of its rated power, its efficiency drops significantly, well below the Lycoming.

Note that each engine, car or airplane, is more efficient than the other when operating within its typical duty cycle.

Reliability and Durability

If an aircraft engine fails in flight, the airplane stays in the air but not for long; it becomes a glider that is going to land somewhere very nearby, very soon (within minutes), and most likely off-airport. It is an emergency situation that can lead to injury or death. If a car engine fails, you coast down and simply pull over to the side of the road. It’s an inconvenience, not an emergency.

Airplane engines are designed for reliability. Their spark plugs are powered by magnetos, so (unlike a car) the engine keeps running even if the electrical system fails. Each piston has 2 spark plugs, so if one fails, the piston still produces power. They have 2 separate magnetos and half the plugs are fired by one magneto, half by the other, so if one magneto fails, the engine keeps running. They are air cooled, so there is no water pump that can fail, no radiator that can leak. Also, they spend most of their time in cruise operating around 2500 RPM, so they have static spark timing optimized for that speed – no need for timing advance means simplicity and reliability.

Plenty of historical examples demonstrate the problems using car engines in airplanes. In the 1980s, Mooney made a plane that was optionally powered by a Porsche engine. It had so many problems, the changes needed to make that engine reliable in aviation were so extensive, Porsche gave up and discontinued it. Thielert had a similar situation building Mercedes diesel engines for aviation use. You can google the details on these and other examples.

Yet how do we reconcile this history with the fact that aviation engines use technology that is more than half a century old? A pilot’s pet nickname for Lycoming is “Lycosaurus”!

Consider how any engine becomes reliable: start with a good design, then tweak a little it every year to address any problems discovered in the field. Cars follow this pattern. They come out with a new engine, the first year has some issues, each year it gets a little better, then 5-10 years down the road, just when the engine is reaching its peak, they scrap it and start all over with a new design incorporating new technology. Imagine how reliable car engines would be if they never scrapped it, stuck with the design and continued that incremental improvement for 50 years. The engine would be “low tech” for sure. And may not be as efficient. But reliable? You betcha – they’ve seen just about every failure there is and incorporated changes to address it.

This is what a typical Lycoming or Continental certified aircraft engine is: the result of more than 50 years of incremental improvement on a design that was pretty good to begin with. It’s ancient technology, yet it’s highly optimized and adapted in an incremental, evolutionary way.

Production

Last year, Mazda built more than half a million engines. Lycoming produced about 4,000 engines. Yet this difference of more than 100:1 understates the difference, because there are many car manufacturers while there are only two manufacturers of certified aircraft engines: Lycoming and Continental. For each aircraft engine built, more than 1,000 car engines are built.

To produce reliable engines at such low volumes, aircraft engine manufacturers use completely different production methods. Each engine and all the parts in it are individually hand-built, inspected, and tested before leaving the factory. Visit a modern car engine factory and it looks like a scene from a sci-fi movie where robots have taken over the world. Visit an aviation engine factory and it looks like you’ve gone back in time to a boutique hot rod custom engine building company.

Conclusion

Cars and airplanes are completely different applications with different requirements. It should be no surprise that engines optimized for one are not well suited to the other. High technology is not and end, it is a means to an end. The end or goal is meeting the requirements for the application. Pilots building their own kit / experimental airplanes can use any engine they want. Yet most of them still prefer certified aviation engines from Lycoming or Continental, despite the high cost and low technology compared to car engines. This is not irrational, but backed by some of the reasons discussed above.

All that said, much of the reason aviation engines are so low tech and expensive, is certification. The cost to certify an aircraft engine is so high, and production volume is so low, they can never break even on a new engine design. Over the years, this forced them to differentiate and improve their products with incremental tweaks to existing designs. One can view this as an unintended consequence of over-regulation: certification rules that were intended to promote safety, led to technological stagnation. Or, one can view it as a beneficial outcome that optimizes for reliability in their intended application, which is crucially important with aircraft engines.

WRIAD: White Rim in a Day

Bikes, Exercise, Flying, Travel
by Mike Clements

Summary

The White Rim Trail is SW of Moab Utah. It follows the Colorado River SW to its junction with the Green River, then NW up the Green River, making a rough “V” shape, then a mix of dirt & paved roads connect the top of the V. It makes a loop measuring 100 miles, about 8000′ of cumulative climb. The trail ranges from simple dirt/rock, to sand, to rugged steep technical with big rocks. Along the route there is no food, water or services. And mostly no cell/mobile coverage.

Most bicycle tours take 3-4 days to do this trail, supported with 4WD vehicles providing food, water and shelter. It is possible to ride it in a single day, but it’s a big physical effort that also takes some planning. It helps to have a gung-ho friend named Stefan to convince you to ride it with him.

Stefan rode WRIAD solo in Oct 2020, and he and I rode it together in Oct 2022. This describes what it was like and how we prepared for it.

Pics here: http://mclements.net/Moab-202210/

Here’s the GPX track overlaid with Google Earth, which underestimates the mileage and elevation because it over-smooths and simplifies the track. The red flag is our start/end point. The spike in speed around mile 75 is a GPS glitch.

Preparation

I’ve done some big tough MTB rides over the years. La Ruta, Kokopelli’s Trail, OTGG, and others of Stefan’s and my own devising. From a fitness perspective I knew what to expect. It takes several months to a year of serious training to get into the best physical fitness you can. You’re going to be pedaling for 10-12 hours over rugged terrain, miles of tire-sucking sand, and incredibly steep grades (> 25%) that make its 100 miles feel more like a 200 mile road ride.

The best time to ride WRIAD is in spring or fall. This means near the equinox, so you’ll have about 12 hours of daylight.

You need a day-use permit that you can get a day or two in advance, cost about $6. And you need to pay another $15 to enter the national park.

Plan on 11-12 hours total if you stop only once or twice during the ride. That means enough food and water to carry you through. Everyone is different; here’s what worked for me. I had 224 ounces of water: two 100 oz camelback bladders, plus a 24 oz. water bottle. I used all but 12 ounces of it. For food, bring some real food for lunch (sandwich, burrito, etc.) and about 240 cals per hour to eat while you’re riding. Have this food ready to eat while riding because if you stop to eat every hour, you might not finish the ride in daylight.

Have a bike that you trust, proven to stand abuse. A bike mechanical failure that strands you along the trail can keep you there overnight and become a life threatening situation. Make sure the entire drivetrain, axles, etc. are new and fresh. Several sections of the trail are too rugged for a gravel bike. You will need a true mountain bike, hard-tail or full suspension, with knobby or semi-knobby tires at least 2″ or 50mm wide. I used Maxxis Ardent Race, 2.2″ / 57mm and they were great. Anything narrower wouldn’t work, anything wider would make a hard ride even harder.

Clockwise or Counterclockwise?

This is a common question. Both ways are doable. Either way you go, you’ll descend into the canyon then climb back out again. These two points are Shafer on the NE side and Mineral Bottom on the NW side.

Here’s the Shafer grade. Red marker is poised at the top.

Here’s the Mineral Bottom grade. Red marker is half-way up, blue marker is our start/end point.

Also, along the trail in the canyon are 2 big notable climbs, each close to 1000′ with some sections too steep to ride. So no matter which way you go, you’ll have 3 very big climbs, in addition to the constant up and down of the trail.

Climb 1, Murphy Hogback, the up side:

Climb 1, Murphy Hogback, the down side:

Climb 2, Hardscrabble Bottom, the up side:

Climb 2, Hardscrabble Bottom, the down side:

We went clockwise starting from the NW corner of the route: the parking lot and toilet just at the top of the Mineral Bottom Climb. This means starting with a 12 mile dirt/gravel road ride that gradually climbs about 1200′, then turning right onto the paved road that runs into the park. Total distance to the Shafer descent where you enter the canyon trail is about 20 miles. Then you ride another 79 miles along the White Rim Trail, and then climb up Mineral Bottom back to where you started. It’s about 1000′ in 1 mile.

The east half of the ride is easier than the west half. It’s just a bit flatter, less sand, less rugged & technical. So the drawback of the clockwise route is that the toughest riding is in the second half of the ride. However, the Mineral Bottom climb, as tough as it is, isn’t quite as rugged or long as the Shafer climb.

Getting There

I flew from Seattle into Moab in my C-172, Stefan drove from Boulder, and we met at the Moab airport KCNY. We stayed at the Moab Apache Inn. It’s not fancy, but it’s a good place with truly excellent service/management.

Sunrise was at 6:45, so that’s when we started. Temps in early Oct were in the mid-high 50s at the start and got into the 70s during the day. This was fortunate!

The Ride

Our start point was at 4800′ MSL. The way we rode, we started along the dirt/gravel road on a long gradual climb. This was nice because it was cool out and the climbing kept us warm so we didn’t need to bring jackets that we would only doff later and carry all day. At mile 12 we reached the paved road (Hwy 313) which is near the peak elevation of about 6000′. We turned S towards the park. After entering the park, a short distance more put us at the top of the Shafer grade with 20 miles on the odometer.

The Shafer descent is just rugged and steep enough to keep you on your toes. If you slide out and miss a turn it could lead to a fatal fall. It was no problem on my full suspension bike but you would not want a gravel bike or skinny tires. It’s incredibly scenic. A short distance and about 1000′ of descent later, you’re in the canyon on the trail. To call it scenic is a grave understatement. It’s stunning.

Here (red marker) is where we had lunch, around mile 55:

For the next 43 miles or so you ride along the rims of canyons, weaving in and around following the contours. Then you reach one of the big steep climbs at Murphy Hogback Canyon. Some parts of this are too steep to ride. It just goes up and up. The top levels off for less than a mile then you go down an equally steep opposite side.

The next 20 miles or so is a gradual downhill, but don’t let the word “downhill” fool you. It’s got long sections of soft sand which sucks down tires, forcing you to pedal hard at slow speed despite the downhill grade.

At this point I encountered nutrition difficulties. I brought Kind bars to eat throughout the day, because they are low sugar and worked great for me in all-day rides over the years. Yet starting around mile 65 I couldn’t keep them down; as I ate them I got a strong urge to barf them back up, so I had to stop eating them. Fortunately, Stefan had some spare Fritos and I had no problem eating those. I never considered chips to be an ultra-endurance food, but sometimes during adversity we learn new things about ourselves. In hindsight it makes sense: Fritos are simple carbs (but no sugar), plenty of salt, and calorie dense. I don’t think the problem was electrolyte loss because I had Nuun mineral tablets in all my water.

Then you reach the second big climb, Hardscrabble Bottom. It’s every bit as tough as the Murphy Hogback climb, ultra steep with some sections too steep to ride. Ride along the top for about 2 miles or so, rolling up and down varying from decent to rough technical conditions. Then back down the other side takes you to around 4000′ MSL about the level of the Green River.

Now ride along a decent quality trail following the Green river for about 15 miles or so, mostly flat. Then around mile 99 you reach the right turn to go up Mineral Bottom. Only 1 mile to go, but it’s very steep, nearly 1000′ climb.

At the end of the ride I didn’t feel right – eating or drinking would have triggered vomiting. I think it was temporary over-exertion because over the 1st post-ride hour I slowly sipped 12 oz of water and kept it down, and over the next hour I felt fine. An hour later we ate a big dinner in town, no problem.

Conclusion

WRIAD was a bucket-list ride for me. The preparation and execution consumed nearly a year of my life. I got into the best physical condition I’ve ever been, similar to doing La Ruta over 20 years ago. Even so, it was one of the toughest rides I’ve ever done, if not the very toughest. I’m pretty sure I’ll never do it again, but big rides like this come with satisfaction and confidence equal to what you put into them. Thanks Stefan for suggesting this one! It was an epic adventure.

Cessna 172 Rear Seat Removal

Flying
by Mike Clements

Why?

Removing the rear seat significantly increases the cargo space, which opens up new mission possibilities. For example, I can normally fit 1 bicycle in the back of the plane, but with the rear seat removed I can fit 2 bicycles. This makes it possible to take a friend and make cross-country trips to explore some of the best bicycling across the pacific northwest.

Regulations

For my 1980 172 (built in 1979), the POH equipment list does not mark the rear seat as required, so the airplane is airworthy with, or without, the rear seat. But can a pilot remove the rear seat himself? FAA regulations part 91 section 43 governs the maintenance pilots can perform. Appendix A, (c) says:

(c) Preventive maintenance. Preventive maintenance is limited to the following work, provided it does not involve complex assembly operations: 
...
(15) Replacing seats or seat parts with replacement parts approved for the aircraft, not involving disassembly of any primary structure or operating system.

The rear seat comes out with 4 simple bolts and nothing has to be disassembled. To get the rear seat out of the airplane one of the front seats has to be removed, then reinstalled. This can be done without any tools at all.

Conclusion: it is legal to fly the airplane without the rear seat, and it is legal for a pilot to remove and install it.

Of course, this changes the weight & balance. So the pilot removing/replacing the seat must note the removal/replacement in the airframe logs, citing the above paragraph as his authority and the maintenance manual as his reference. Also use the weight/arm info from the equipment list to make appropriate modifications to the empty weight/arm of the aircraft in his W&B computations for flights with the seat removed. And, of course, the pilot would have to be sure that at most only two people were in the plane when it is being operated.

How-To

So now that we know it’s legal, how do we actually do it? It turns out to be quick and easy.

Before you start it looks like this:

Step 1: remove one of the front seats

This makes room to remove the rear seat from the airplane, and you can reinstall it afterward. I removed the right / co-pilot seat.

 

Aviation, Lycoming, Mixture, Efficiency, Cruise Flight

Flying
by Mike Clements

Flying my Lycoming O-360-A4M powered Cessna 172-N, I learned through experience something that should be in the POH, but is not. It’s about fuel efficiency during cruise.

These engines are conservatively rated (180 HP from 360 cubic inch displacement), designed to operate continuously at or near full power, and their 2700 RPM redline is a prop limitation, not an engine limitation. Also, at cruise altitude full power is a smaller fraction of their rated power; for example, at 10,000′ the air is so thin, a normally aspirated engine can only produce about 70% power. All this is to say, when cruising above 5,000′ MSL, you can run the engine at WOT. This is considered normal operation; the POH cruise tables support this configuration. It is not hard on the engine, in fact, these engines are designed to run best at or near WOT properly leaned.

However, you shouldn’t run it at WOT, but slightly less. Here’s why, based on 2 reasons:

The carburetor for this engine (and for the O-320-D2J it had before) has an enrichment circuit that mechanically engages at WOT. This adds a margin of safety against detonation, with a minimal loss of power since the power vs. mixture curve is asymmetric, tapering slowly on the rich side, steeply on the lean side.

The intake manifold for this engine is immediately downstream of (fed from) the carburetor. On any engine having a single carb upstream from an intake manifold, the A/F ratio to each of the cylinders will never be exactly the same. There will always be one cylinder that is slightly richer or another slightly leaner, than the others. This A/F balance across the cylinders is “mostly even” but exactly how even varies depending on the throttle position & mixture.

These 2 factors combine to form an important aspect of engine operation that should be (but isn’t in) in the POH. The carburetor’s WOT enrichment circuit impairs the distribution of mixture to the cylinders. When it engages, it increases the difference between the richest & leanest cylinder. This means, in order to avoid roughness (the leanest cylinder running rough), you must set the mixture richer than it otherwise would be when this circuit is not engaged.

You might wonder, why not apply WOT, then lean the mixture to compensate for the enrichment circuit? You certainly can do this. The problem is, the mixture setting will be richer than it would be, if the enrichment circuit weren’t engaged.

Put differently: when you pull the throttle back from WOT just enough to disengage the enrichment circuit, the mixture distribution across the cylinder is more even. There is less of a difference between the richest and leanest cylinder. Thus, the mixture setting for any equivalent power level (peak, 50 RPM below peak, or whatever) is leaner.

The difference is significant: about 15%. That is, if you fly at WOT and lean the engine properly, you will burn about 15% more fuel than if you pull the throttle back from WOT just enough to disengage the enrichment circuit. The POH tables reflect non-WOT operation, so at WOT you will burn 15% more fuel than the POH indicates. That is, if the POH says 8 gph, you will actually burn about 9 gph.

Procedure: High Speed High Altitude Cruise

So what is the best procedure for high speed high altitude (above 5,000′) cruise?

  1. Apply WOT.
  2. Gradually lean until slightly rough.
  3. Very slowly pull the throttle back.
  4. Since the engine is already lean, when the enrichment circuit disengages the engine will suddenly get much leaner, and you will get a sudden drop in RPM and increase in roughness.
  5. Leave the throttle in this position, then enrich mixture to desired setting, typically peak RPM, or 50 RPM below peak.

Typically, step (4) happens about 1/2″ back from WOT.

Miro Quartet at Orcas Island

Flying, Music
by Mike Clements

Michelle and I flew in for the Orcas Island Chamber Music Festival this year and caught the Miro Quartet playing with Aloysia & Jon on Tue Aug 13. Our last-minute decision afforded stage seating, stage right behind the musicians. We really liked this. The experience and sound is different and quite wonderful, reminding me of my own weekly chamber music rehearsals years ago.

Miro opened with the Mozart quartet K 458 The Hunt. Their sound struck me like a velvet hammer: big, round, smooth, rich and fat yet detailed. A huge grin spread across my face and the back of my neck tingled. I especially noticed their dynamics, micro and macro, and their tight timing playing off each other handing the lead back & forth every few bars like a great jazz ensemble, yet with all the musical refinement that Mozart demands. The menuette bounced and the adagio soared, breaking tradition as they came in that order. The allegro set it on fire and summed it up.

Kevin Puts entered the stage and introduced his piece, Arcana for solo cello and string quartet. He described how watching the sun rise over a volcano on Maui inspired him to write this impressionistic piece. Julian Schwarz (son of Gerard Schwarz, prior conductor for Seattle Symphony, who was also visiting the OICMF this year) and Aloysia Friedmann joined Miro to play the lead cello and supporting violin, respectively.

The guest musicians left the stage and Miro played Schubert’s Death and the Maiden. More specifically, the andante which is an absolute classic of the chamber music repertoire and structured as a theme and variations. It ranges widely from lyricism to flaming virtuosity giving each musician a showcase and the Miro quartet just nailed it. The piece had a few moments in the lyrical sections when Ching (lead violin) sounded just slightly off in timing or intonation, but it could have been my own ears.  That’s part of the character and expressive joy of live music performance: every piece is unique rather than perfect in the robotically sterile way that recordings sometimes can be, and this enhances the experience. A robust standing ovation delayed the intermission.

Upon returning, Jon Kimura-Parker was scheduled to play a Clara Schumann piece, but instead played Schubert Impromptu Op. 90 # 3, one of my favorites of the solo piano repertoire. He played with a depth, delicacy and refined power that perfectly suits this piece. The performance reminded me of Radu Lupu’s style, but Jon made it his own. For me, this piece was the highlight of the concert in terms of emotional intimacy.

Last yet certainly not least, Miro joined Kimura-Parker on stage to perform the famous Brahms piano quintet in F minor Op. 34. A few years ago when Michelle and I last attended an OICMF concert they also played this piece, so I knew we were in for a treat. We were not disappointed. We were sitting just behind Kimura-Parker so close we could have reached out and touched him. I was reading his tattered and heavily annotated (in different colors!) sheet music as he played and his daughter turned pages for him. We could hear and feel the power and wonderful woody resonance of the Steinway Model D in the FFF sections. The strings were no less in the game as they brought the piece to its fiery and satisfying conclusion.

Headwinds & Tailwinds

Flying
by Mike Clements

It seems obvious that head and tail winds are equally likely. That is, assuming the direction of the wind and your flight are both random, head and tail winds should be equally likely. But it’s wrong.

Of course, even if head and tail winds were equally likely, you would spend more time flying in headwinds, simply because they slow you down. But that’s not the reason I’m talking about here.

The reason is simple. When the wind is 90* to your direction of flight, you have to turn toward it slightly to maintain your desired direction of flight, so it slows you down. Visualize the entire 360* circle that the angle of the wind can have relative to your direction of flight. If wind at exactly 90* slows you down, then more than half of the range of the circle slows you down. A wind from the side must be slightly behind you in order for the loss of speed turning toward it, to be countered by the gain in speed it adds pushing you along. In other words, when the wind is from the side, it must be slightly behind you to break even.

Of course, the same applies to boats. But not cars, because you don’t need to steer into a crosswind when driving (well you do, but it requires so much less correction as to be insignificant).