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:
- Having two engines doubles the likelihood of an engine failure.
- When one engine fails, the other produces differential thrust requiring immediate corrective action from the pilot to avoid loss of control.
- 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:
- They use turbine engines, which are more reliable than pistons.
- The pilots are better trained, more frequently, and follow more strict operational limits set by both the FAA and their airline.
- 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:
- Maintain the aircraft above & beyond required minimums.
- Train yourself well beyond the required minimums, stay current.
- 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.