The dreaded H-V curve discussion redux

[PPRUNE FAN#1]

One thing I don't like to do in a helicopter is the botched-auto-go-around. That's the one where you see it's not going to work out so you go, "Err, let's get out of here and try this again, shall we?" But now that I think of it, doing auto-rpm checks require the same maneuvre, and I don't like doing them either. While helicopters transition easily enough from powered to autorotative flight, the reverse does not seem to be true. They seem awkward and uncomfortable.

On the JustHelicopters free-for-all/discussion board, an operator of R-22's rather courageously reported that he now had two of the little birdies lying on their side. Seems that in both cases, the instructors (one of which was him) were letting the students practice "autorotative glides" with a termination at (low) altitude. In both cases, the engines failed to respond and the resulting real autos were not successful for a number of reasons.

Let's say that you were in an autorotative descent at...oh, 60 knots. Let's say that you had just decided to resume powered flight. Let's say that you've increased the power (to max or top-of-the-green or whatever value you choose). Further, let's say that the aircraft has not yet arrested it's rate-of-descent. Now let's say that the engine picked exactly that moment to quit. How fast would the main rotor rpm bleed off? Any of you wannabe (or actual) test pilots want to go out and try it? (Perhaps Nick will give it a go when he starts test-flying the "new" Sikorsky/Schweizer/Hughes VHS-300.)

Not me. I've never been brave enough to - especially in a recip, but I believe that the results would probably be startling. It would probably look like somebody pulled the rotor brake handle.

After his first R-22 accident, the confessor on the JustHelicopters melee/discussion board decided that the altitude he'd previously selected as his "floor" for the maneuvre was too low to get the auto re-established and get everything back within the proper parameters. Ergo, he increased it as a matter of school policy to 150 feet agl. (We can only surmise what the floor was prior to that.) This still proved too low, of which the second accident is unfortunate proof.

Regulatory agencies require applicants for a certificate to demonstrate "autorotational descents with a power-recovery and power-recovery to a hover." Two different things. In both cases, there are a lot of human variables possible in the performing of the procedures. In the first, if the student were to let the nose rise a little, thus bleeding off airspeed, it could get quite uncomfortable as both P1 and P2 realize that the engine isn't responding to the lash and the rotor rpm is going down faster than the governor of New Jersey on a good-looking Israeli guy.

We've had many discussions on this board about the Height/Velocity curve and how it is derived. We believe that the worst-case scenario involves an engine failure in a climb at a high power-setting and low airspeed. But I wonder if there isn't another even-worse-worst-case scenario?

[Flingwing207]

Hm.

Glib ripostes aside, I think that going around to terminate an autorotative descent is not a particularly dangerous practice.

No more dangerous than hovering with a student at the controls, or practicing slope landings, or steep approaches or max-performance takeoffs or quick stops.

Bad luck in the form of mechanical failure, and/or inexperience, inattention, or pilot error will get you no matter what you're doing. Flying every autorotation to a flare is not practical, and anyway many more helicopters get pranged in the power recovery at the flare than in a go-around.

Some would say every auto should be to the ground. This precludes simulated engine failures, which are the most important part of the training - if a student doesn't learn the proper reactions in training, they stand a good chance of dying in the real thing. Anyway, a full-down on every auto would make for a lot of bent Robbies.

Is it an American trait or just a human trait to get worked up when the same thing happens twice. This spring in our area we had two R22s roll in as many days, both while terminating normal approaches. We still make normal approaches in Robbies.

I'll still make autorotation go-arounds, but I'll be darned sure to keep the threat of real engine failure in the front of my mind (and I'll fly a 300CBi). This is what I take away from PBH's recent incidents.

[PPRUNE FAN#1]

Flingwing207:Glib ripostes aside, I think that going around to terminate an autorotative descent is not a particularly dangerous practice.Just to be clear, I never said that go-arounds out of autorotations were scarily dangerous and should be avoided. If I didn't make myself clear (not unusual for me) I was merely making the point that such a maneuvre should probably be initiated at a higher altitude than we might otherwise assume was safe...like 500' or above, say. I never really gave it too much thought. Randy Rowles found out the hard way. And now he's got his County Commission pouncing on him like former Swift Boat Vets attacking a presidential hopeful.

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PF#1 - my first question would be why did the engine fail to respond on 2 occasions?
Is it because the R22 engine is unreliable? I don't think so.

Is it because the student didn't open the throttle sufficiently to prevent Nr decay as he raised the lever? We'd have to ask the instructors in question.

Was the carb heat selected on? Low power in a descent followed by rapid application of power to an iced up engine is a classic recipe for engine failure even at high OAT.

Were they relying on the governor and asking too much of it? I remember being taught autos on a governor equipped robbie with the governor turned off.

Before deciding not to carry out a manoeuvre that has been successfully completed on thousands of occasions - one needs to know the real reason for the accidents.