Or *couldn't* get it right.
Once rotor RPM drops below a critical
It has to be be of those disaster heaped on crisis moments Losing one, or two engines is not unheard of and is covered in training.
Being unable to them move the pitch to autorotate is not covered, because it's curtains if it happens.
Hmm... yes, I don't doubt what you say for a moment (and yes, what I said might well be complete and utter bollocks :-)), but what if the pilot wasn't able to use the collective to change the angle of attack of the blades, for an as yet unknown reason? Is there a 'fail safe' position that the blades automatically assume even if all the power fails? Your last sentence ("Stalled blade"=falling out of the sky) does seem to describe what happened, all too realistically.
It might be a 'never event' for the blades to start to auto-rotate in the opposite direction in normal circumstances, but something very unusual must have happened there, otherwise we would probably already have an explanation.
I'm sure it is but I wonder if there may have been a short temporal separation between the two engines failing leading to the pilot being halfway though a sequence for dealing with a single engine failure and then suddenly having to "change tack" as it were. At low altitude with little forward speed the window for getting it right is probably very small.
Tim
I don't think that there is a "fail safe" position that the blades adopt. From all the reading that done on the subject recently it would seem that in the event of engine failure, the actions of the pilot must be instinctive and almost instant. It's a comforting idea that things might be designed so that a helicopter but make the transition automatically with no active pilot input but that isn't how it works.
Tim
Then if it is anything other than around one rotor diameter (10m) from the ground in a hover then it's contacting the ground, very hard. If it is in that position then you could roll off throttles, or kill the engines and it's time for a hard but probably survivable landing.
No, and you wouldn't be able to select one that is universally good under all circumstances.
The 'wing' , in this case the rotor, needs to be able to fly at all times the aircraft is in the air.
It is. In flight the blades must always stay within the rev limits (in this case 85% -> 112%.) They never 'reverse' Nor do they stop the instant a successful autorotation is completed. You need lots of revs and lots of pitch on the rotor during the end of the transition from flight to landing. On the ground a freewheeling rotor keeps going for a long time.
The only time blades go the opposite way (i.e. clockwise from above) is on 'French'* and Russian helicopters, and then they always rotate the other way and still never reverse.
Could be a simple problem or combination of problems that won't leave any evidence. That it hasn't happened before in 2.5 million flying hours on 1000+ aircraft points to it being a rare event.
Has anyone considered Vortex Ring?
As I have said elsewhere, the reported sounds like a car backfiring would be consistent with compressor stall. To affect both engines that would need some external event, such as bird strike or turbulent air at the inlet, and that would stop them both virtually simultaneously.
Colin Bignell
In message , Brian Gaff writes
no I don't think so. (for starters the report says it had 95L of fuel).
I think it's just that the crash has caused them to do extra checks and be a bit more cautious about things
I would imagine that bird strike would leave some evidence and there's been no suggestion that this happened (so far). What would cause sufficient turbulence at the inlet to stall a turbine? Just interested...
Tim
Wouldn't fit with the rotor being stationary at the time of crash.
Tim
Could a bug in the software stop both engines?
Chris Hogg formulated the question :
No they always rotate in the same direction. All that changes, is the angle of the blades, from the normal of pushing down under power, to the opposite angle needed for auto rotation, so rotor speed is built up and the rotors speed and energy can be used to soften the landing. At the last second, the blade angle is set as it would be when driven.
So the rotor only stops once on the ground.
Tim+ submitted this idea :
The idea of auto-rotation is that height is used to build up speed on the un-powered rotors, then at the last second that speed which has been gained, is changed into lift. They only get one chance to get it right.
I suppose it is possible that the pilot very badly misjudged it in the dark and pulled back on the stick too early, used up all of the acquired rotor speed, the rotors came to a complete stop whilst still at some height. The aircraft would then drop like a stone, with no rotor movement.
I only mentioned bird strike because it is the most common cause of compressor stall. I would expect some damage if it had been the cause and the report says there was none. As for causes of turbulence, that can be difficult to determine. I don't see any unusually tall buildings in the area, which are often associated with unusual air flows, nor industrial chimneys nor cooling towers. There are no reports of other aircraft in the area, which could produce wake turbulence. However, those are only the more common causes. Severe contamination is another. We can probably rule out volcanoes though:
The amount of fuel removed should have been enough for about 20 minutes flying time, so fuel exhaustion is probably ruled out and fuel starvation seems unlikely with such an experienced pilot in a twin engined aircraft.
Colin Bignell
I fully understand and accept your point. Others have also made it. But my point is, what happens if for some reason that change in angle of attack doesn't happen and the blade pitch isn't reversed, then what? From what's been said here, it requires manual intervention by the pilot to make that change, by moving the 'collective' lever. If the pilot was distracted, or was incapacitated, or the lever had become jammed, or.... or.... and he didn't change the blade pitch, then if the power failed, as the helicopter descended under gravity the blades would stop rotating and the helicopter would fall out of the sky, which is what happened.
Yes, it's speculation, but that's all anyone can do at the moment without concrete evidence, and this thread isn't short on speculation!
Critical aviation systems are normally multiple redundant systems. I would think it highly improbable that all the systems on both engines would fail at the same time in the same way on an aircraft in normal flight.
Colin Bignell
I was thinking of a logic fault that, in effect but not by design, ends up saying:
If the fuel in the main tank runs dry AND It's between 10 and 11 PM on 29 November (say) THEN Stop the engines.
Not much point in having a low fuel warning light if the only action suitable is a controlled emergency landing? Surely it should tell you to go to the nearest re-fuelling place, like a car?
Anything is possible until we get the official report, but software related accidents are more usually associated with the pilot doing something the software designers had not anticipated. This aircraft was in normal flight. In terms of probability, I would put it as rather more likely than a micro black hole suddenly appearing and sucking the aircraft out of the sky.
Colin Bignell
could the pilot have suffered a heart attack?
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