Entering autos: discussion split from Glasgow crash thread
Gentlemen –
This is my first post to PPRuNe. It concerns the tragic accident at Glasgow on November 29. I have read all of the over-800 posts regarding this accident and feel that I may be able to shed some light on what David Traill faced as the helicopter headed down.
The Best Kept Secret in the helicopter industry is how critical it is to immediately apply aft cyclic the moment a loss of power to the rotor system is detected. I have been doing my best to spread this word since analyzing the cause of two law-enforcement helicopter accidents that occurred four months apart in 2002. I call it Cyclic Back.
Very few helicopter pilots realize that if, with a total engine failure, the rotor rpm is allowed to fall more than about 5% below low green, the flight is over. OVER. There is no recovery possible regardless of what actions the pilot may take or how high the helicopter is above ground. This fact is not mentioned in the sales literature for helicopters nor in the approved rotorcraft flight manuals. It is not mentioned in any of our FAA publications having to do with how helicopters fly and how to fly them. It is treated as a deep dark secret, unfortunately.
When power is lost to the rotor system, THE MOST IMPORTANT FLIGHT CONTROL IN THE COCKPIT IS THE CYCLIC! It must immediately be brought aft so that the flow of air is upwards through the rotor system. Bottoming the collective does only one thing: It reduces the rate that the rotor rpm is falling. That’s all! It NEVER stops the fall of rotor rpm.
Once the rotor rpm has dropped below the critical point, recovery is not possible. The helicopter continues to descend as the rotor rpm falls towards zero and may, in the case of free-turbine engines as used in the EC135, be seen to turn backwards. The rotor blades will show little if any damage when the wreckage is examined.
And as the rotor rpm slows towards zero during the descent, retreating blade stall enters the picture. The normal Vne chart does not mention rotor rpm; it is assumed it is normal for the standard mode of flight. But when rotor rpm falls, Vne falls with it, so Vne is very possible at airspeeds much below those computed via the chart.
This in turn means that as the rotor rpm is falling during the autorotation, the helicopter will roll in the direction of the retreating blades, or to the left in the case of the EC135. Any attempt by the pilot to correct this with opposite cyclic simply adds additional pitch to the blades that are already stalling, thereby increasing the amount of roll.
When a helicopter pilot is faced with a sudden unannounced engine or drive-line failure, here is what must be done:
1. Cyclic back and pitch down, simultaneously or in that order.
2. Pick a place to land.
3. MAKE THAT SPOT!
What about indicated airspeed. At the beginning of the autorotation, the ONLY speed that matters is that over the wings, meaning, of course, the rotor blades, and this is a function of rotor rpm. Pitot tube airspeed (indicated airspeed) is not important at that time, but yes, once the rotor rpm is solidly in the green, indicated airspeed can be helpful in extending the glide or reducing the rate of descent.
Finally, this: During a normal power-off autorotation, the helicopter will respond to all flight-control movements the same way it would if the helicopter is in a flat-pitch descent with the engine running. The only thing it won’t do is a sustained climb. But it will stop, back up, turn in any direction, etc. So when I say MAKE THAT SPOT, I’m saying use the maneuverability of the helicopter just as you would if the engine was running. There is more to life than straight-ins, 90s and 180s!
Based on what I have read so far on PPRuNe, my thoughts are this: For whatever reason, both engines went off line one following the other, but Dave did not apply aft cyclic quickly enough to prevent the rotor rpm from falling below that critical point from which recovery is not possible.
Pilot error, you say? Not at all. Dave did exactly what he was trained to do in this sort of emergency. The blame goes to the simple fact that the critical importance of applying aft cyclic quickly when power is lost to the rotor system has been kept from all of us since rotor blades were invented. Cyclic Back must become an integral part of all helicopter training and publications. This must not remain a secret any longer!
I extend my most sincere condolences to everyone who was directly affected or touched in any way by this most unfortunate accident.
PeteGillies