Twin engine aircraft are still the backbone of the corporate transportation world. The Beechcraft King Air is emblematic of what “makes business work” in the US. To “fly corporate” you still need to obtain a multi-engine rating and build that “twin time.” After the piston comes the jet and most of these seem to have at least two burners. Unfortunately it is increasingly difficult to find a light twin in the GA training fleet or on a local airport. Slippery carbon fiber, single engine, fixed gear planes are largely the small business airplane and as they move up the single engine turbo-props like the TBM, PC-12. Meridian have taken over in the flight levels.
Along with the disappearance of the Apache, Aztec, TravelAir fleet, the knowledge and craft of multi-engine flying is also getting harder to obtain and master. This is a brief overview of the subject with a special emphasis on the problems of flying on one engine in the event of a problem. Further articles on this subject are available on our multi-engine training page…this is just a warm-up to the subject.
The wake-up call for small twins came in 1979 with a special NTSB report that studied 2,229 accidents over a five-year period. Of these, 610 accidents involved fatalities and 123 resulted from engine failures. In nearly 3/4 of these cases pilots lost control of their plane due to the asymmetric thrust from the lost engine. The percentage of fatal accidents involving engine failure was more that four times greater in light twins than in single-engine planes. Lack of pilot proficiency was cited as the cause of most of these accidents and four specific recommendations were put forth by the NTSB. Lets first examine the problem and then we will look at the NTSB ideas.
The basic problem is obvious if you study a wing mounted twin-engine aircraft. If one of those fans goes bad that plane sure is not going to fly straight or pretty. But the major problem is more what my former chief pilot used to say when he saw a light twin take off. “You know why that plane has two engines? Because it needs them both” Once you understand how pathetic the performance is on one engine, you are a long way toward safety. When an engine fails in a light twin you absolutely must sacrifice performance and achieve control instead. A good rule is only try to achieve performance once you have controlled the adverse forces. Though you lost half the power when an engine goes bad, 80% or more of your climb potential just died too.
It did not help of course that manufacturers like Piper used to put pilots in the field who would demonstrate single-engine take-offs in the Aztec. This was fueling a myth that only could be achieved with superb piloting skill (and a very light aircraft) Load it up with fuel and family and most light twins can barely maintain altitude on an ISA day with an engine out. Pilot handbooks of the day were often written more to sell airplanes rather than insure safety.
Here is a fun experiment for a single-engine pilot to understand the dilemma of a twin engine failure. At a safe altitude, after clearing carefully, fly your plane at the best glide speed (Max L/D…minimum drag) at a level altitude. Make sure you get very stable and control the left-turning tendencies with rudder and carefully note the power setting. This number in most small singles will be 60-70% of your rated power (this assumes max gross weight) In a C-172 this is around 1900rpm.This is the minimum power it takes to maintain level flight. The difference up to maximum rpm is your climb potential. Usually, with coordinated controls, you can assume about 100fpm for each 100 rpm of surplus power available…500-700fpm. (Here is where power to weight ratios really count and streamlined airframes rule!) The point however is to gain respect for how little climb potential ANY plane has. A twin with an engine out lost 50% of it’s power but most of that is what allows it to climb! The remaining 50% just might keep you in the air (if you do everything correctly)
“Doing everything correctly” in the heat of the moment is a tall order for a twin driver. Imagine you are climbing out at max power and max gross, (gear up) when an engine suddenly fails. That operating engine will twist the plane dramatically and the loss of lift on the dead engine side will dramatically amplify this force. One major flight control opposing this yaw and roll is the rudder. Since this air control surface loses effectiveness rapidly as you fly slower, it’s critical to lower the nose and maintain airspeed (while holding the nose straight) as a first remedy. Sacrificing climb to get control is essential. Once basic control is regained, there are many tweaks to optimize the performance that are beyond the scope of this article. Suffice to say we must avoid flying slowly of we lose rudder authority and the yaw force will win. The critical loss of control occurs at what is called Vmc or “velocity for minimum control.” There are detailed specifications for Vmc in CFR 23.149 for manufacturers to determine and mark this airpeed (the “red line”) The important take-away must not get near this minimum speed or we will lose control. It would be better to reduce both throttles to idle and crash level lie a single than to roll over from the asymmetric forces. The high AOA coupled with the huge yaw lead to loss of control pretty quickly.
The joke in multi-engine training is that since you mostly fly on one engine anyway (since the goal is mastering this exotic skill) shouldn’t the cost be half? You get exceptionally fast and skilled after five to ten hours of training (and hopefully even pass an FAA check ride and add this too your certificate). Unfortunately like all carefully honed skills this quickly becomes rusty without practice. Hours of successful multi-engine flying lead to complacency. This is why the NTSB statistics are so grim on twins.
And the recommendations? More recent currency for pilots, better POH information and increased outreach to rated pilots on light-twin problems. An interesting idea still not implemented by the FAA is for multi-engine pilots to accomplish their flight review in a twin to retain their privileges. Hopefully the overview is clear; the price of flying a multi-engine aircraft is maintaining superb currency. If you don’t do this “luck and hope” are two of your primary flight planning tools…never a great idea. Here is a great article to dig deeper…good luck! ©2015, David St. George