Autorotations: The Bible is Wrong

The most important thing that I learned about at Heli-Expo wasn’t on the show floor and I decided that it merited its own posting. I attended a two-hour seminar on autorotations. It seems that the stress on lowering the collective in the event of an engine failure is misplaced and that this emphasis starts in the FAA’s Rotorcraft Flying Handbook, i.e., the Bible as far as Private helicopter students are concerned.

Flying a helicopter may well be the most dangerous job in the U.S. (TIME magazine puts “pilots” in at third most dangerous but they are lumping in scheduled airline pilots, whose jobs are not hazardous at all, with helicopter pilots and Alaskan bush pilots) A real-world emergency in which an autorotation becomes necessary is not common but being prepared may mean the difference between life and death.

I decided to write an article on how to teach autorotations, incorporating the best ideas from the seminar.

8 Comments

  1. kokonutz

    March 5, 2014 @ 3:40 am

    1

    Precision autorotation: http://www.youtube.com/watch?v=vPZso34nJdY

  2. Joshua Levinson

    March 5, 2014 @ 11:08 am

    2

    Will maintaining that attitude preserve main rotor RPM for longer? I know just about nothing about helicopter flying, but I seem to recall from one of your articles that the R22 has under a second and half before the main rotor RPM degrades to below-recoverable levels in an engine failure. That would seem to make it a pretty urgent item, no?

  3. philg

    March 5, 2014 @ 1:24 pm

    3

    Joshua: Pulling back on the cyclic will quickly boost the rotor RPM. Letting the nose drop will allow the rotor RPM to fall. In the long(ish) run one does have to lower the collective to preserve rotor RPM but it is a more immediate fix to pull back on the cyclic (assuming that you had at least some airspeed).

    I had heard that the R22 has about 1.7 seconds of rotor inertia before things get unrecoverable and that a Jet Ranger or R44 are closer to 4 seconds. However, the panelists could not confirm these folkloric numbers and in fact it seems that without at least enough cyclic input to preserve a level attitude even a heavy helicopter such as a Huey will become unrecoverable in less than 2 seconds. We do know from FAR 27.143 that the collective cannot be lowered in less than one second from a “cruise condition”. So all helicopters have at least one second of rotor inertia. And the FAR says “No corrective action time delay for any condition following power failure may be less than…” so I think the question becomes whether or not factory and FAA test pilots are putting in some cyclic following simulated power failures. I have a feeling that they are and simply aren’t considering it part of the “corrective action” (i.e., the “corrective action” is lowering collective).

  4. Joshua Levinson

    March 5, 2014 @ 3:23 pm

    4

    Philip,

    Thanks! Very interesting. I really will need to treat myself to some helicopter lessons one of these days.

    So, I suppose the only real condition in which you wouldn’t pull back on the cyclic in the event of an engine failure is if you were in a high-altitude hover? Maybe that’s why the FAA has kept with the collective advice, so that there can truly be a one-size-fits-all response, rather than a “If you have forward airspeed, then pitch up, otherwise pitch slightly down?”

    That would support your assertion of it being an attitude maneuver though. “In the event of an engine failure, immediately apply cyclic to bring the ship to an autorotation attitude (this may require fore or aft cyclic)”.

  5. philg

    March 5, 2014 @ 3:28 pm

    5

    Joshua: Definitely from a high-altitude hover you would have to commence a dive at some point to build up at least 45 knots of airspeed that would subsequently be used in a flare to arrest the vertical descent. So it is true that lowering collective is always going to be a good idea after an engine failure at altitude whereas a cyclic pull is only sometimes going to be the best idea. But on the third hand, unless you’re right on the hairy edge of the height-velocity curve, a cyclic pull to maintain pitch attitude, and therefore a slight delay before the dive starts, won’t be fatal.

  6. Joe McGuckin

    March 5, 2014 @ 10:34 pm

    6

    >starting from the rather high cruise speed of the Mosby crash, aft cyclic needed to be applied within 2 seconds or the aircraft would be unrecoverable

    What happens if you’re at cruise and don’t pull back on the cyclic immediately?

    If you’re moving forward, isn’t the rotor disc tilted forward? If you lost power, wouldn’t the airflow through the disc keep the blades spinning?

  7. philg

    March 5, 2014 @ 11:04 pm

    7

    Joe: The Rotorcraft Flying Handbook, referenced above, is available online as a PDF. It explains how autorotations work aerodynamically. Basically you need airflow from below the disk to keep driving the rotor system. So you wouldn’t get that from going forward, even if the disk were tilted to a substantial extent (which it typically isn’t).

  8. Mark

    March 9, 2014 @ 5:50 pm

    8

    Hi Phil,

    I recall a very demanding instructor becoming so frustrated with me glancing at the gauges during practice autos that once, as we entered an auto, he shoved a piece of cardboard over the instrument panel and shouted: “Get your eyes outside and get the ship into proper attitude!”
    It worked. I trusted my eyes and flew the heli in its proper attitude instead of constantly scanning the instrument panel. Consequently, my autos got much better.
    PS
    What did the panel surmise the autorotation error to be in the EMS crash that was caused by fuel exhaustion? Did the pilot panic and yank the collective up?

Log in