Friends have been asking me about TransAsia 235, an ATR 72-600 turboprop that crashed in Taiwan.
First, why a turboprop, where jet (turbine) engines drive propellers? A pure turbojet, such as a Boeing 737, burns more fuel at lower altitudes and needs more runway. So turboprops are ideal for short flights and/or flights to small airports.
Managing an engine failure in a twin-engine turbojet is pretty easy. You push the two thrust levers full forward, which gives you max thrust from whichever engine is still running. Then you use the ailerons and rudder to keep the airplane flying more or less wings level. This compensates for the torque of just one engine spinning out on one wing and prevents the plane from rolling. If the engine occurs right around takeoff, which it always does in the sim(!), you make sure that the gear and flaps are retracted so that drag is reduced. After you’ve climbed to a safe altitude you start running checklists. At high altitude airports or in very high temperatures your climb performance will be reduced but it will always be sufficient if pre-flight planning is done correctly (mostly not loading up the plane with people, bags, and fuel).
Managing a real-world engine failure in a twin-engine piston-powered airplane is beyond the skills of most pilots, leading to the adage “the second engine takes you to the scene of the accident.” There are six power levers in a piston twin. If an engine fails near the ground there is an emergency situation created by the drag of the windmilling propeller. The pilot must figure out which engine has failed and pull the propeller control lever back on that side of the plane, turning the blades of the prop into knife edges relative to the slipstream. This is called “feathering” the propeller. If this is not done, the airplane will sink because the second engine is not powerful enough to fight both gravity and the windmilling prop’s drag. If the still-running engine is feathered by mistake then a single-engine scenario is turned into a zero-engine scenario. I wrote about this in 2006 during my own multi training (“Unsafe at any speed… Philip and a piston twin”):
When an engine quits, the pilot is supposed to push up the two mixture controls, the two prop speed controls, the two throttles and then make sure that the gear and flaps are up. After that it is identify and verify the dead engine by pulling back the throttle and seeing that there isn’t any yaw. Finally one is supposed to pick the correct prop speed control from among the six power levers and pull it back to feather. I thought I’d done just this and was a bit surprised by the fact that the airplane was yawing as I pulled the lever back. I kept pulling. My instructor, Jim Henry, is normally the soul of cool and calm. He jumped out of his seat and pushed my hand out of the way. “Maybe you shouldn’t pull back the mixture on the good engine.”
Are there piston-powered twins in airline service today? Yes, Cape Air is a notable example and they have an excellent safety record due to careful hiring and an excellent training program. However for bigger planes that need more horsepower it is impractical to make piston engines that meet modern reliability standards. There were plenty of heavy powerful piston-driven airplanes during World War II, e.g., the B-17 and B-29 bombers (example engine) but a turbine engine spinning a prop is a much more practical way of generating anything more than 500 horsepower.
Turboprops are bigger, more complicated, and more expensive than piston twins so generally they include an auto-feather capability. If an engine isn’t producing power the propeller associated with that engine will automatically be twisted to a low-drag knife-edge position. The historically popular Beech King Air, for example, has this feature, which has contributed to its excellent safety record relative to similar-size twins that don’t auto-feather. The “engine failure after lift-off” procedure (online checklist) for the King Air is pretty similar to what one would do on a turbojet (max power both engines, flaps/gear up, clean-up after reaching a safe altitude).
The Guardian says that a dead engine on the ATR 72-600 will auto-feather (but then the article gives an incorrect explanation of feathering as “reduced thrust to the propeller”) and this is confirmed by some information for flight simulator enthusiasts that I found online. So it is unclear at this point why the pilots were doing anything with the engine controls other than pushing them full forward so that could concentrate on flying the airplane.
A friend asked about the pilots’ thousands of hours of experience and recurrent training. Depending on the carrier and their agreement with the FAA or local equivalent, airline pilots get recurrent training every 9-12 months. For an airliner this means simulator training. The New York Times said that the pilots had 5000 hours and 7000 hours of experience. An airline pilot typically flies about 1000 hours per year. Initial sim training on single engine procedures might last for 15 hours. Recurrent training might be just 5 hours of single engine work. So a pilot with 7000 hours of total time potentially would have only about 50 hours of experience in flying the ATR 72 on one engine.
Too soon to say definitively what caused this accident, but as background for understanding the news, keep in mind the best thing that a pilot can do in the event of a single-engine flame-out in an otherwise properly functioning ATR 72 is basically nothing. Contrary to Hollywood portrayals doing nothing is the best course of action for a lot of in-flight issues (for example, everyone on Air France 447 would be alive today if the pilots had sat in their seats with arms folded for a minute or two).
Related: the crash of Atlantic Southeast Airlines Flight 529, in 2001. The propeller itself failed and could not be feathered. The achievements of the crew, pilots Ed Gannaway and Matt Warmerdam and flight attendant Robin Fech, were chronicled in a superb book: Nine Minutes, Twenty Seconds
(also a great audiobook).
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