"in what circumstances would a pilot need to chop both engines?"

lack of a tail rotor.

phil

Siloe sid, think back to the vector diagram of a blade in auto rotation. If the Nr gets below the minimum 80% in this case you are not going to get it back. High rate of descent gives a very large angle of attack on the blade, the resultant drag will slow the blades very quickly. Disengaged blades on the ground have no rate of descent flow, and a very small angle of attack, so take a while to slow down. I am talking simply about p of f and not speculating on what happened.

Roseland, chopping both engines would be the action in the case if a tail rotor drive failure in the HOGE.

But they weren't in the hover I hear everyone shout!

Well, my 'opinion', based on the report, is that they were slowing to complete the pre-landers, or maybe slowing for a TRGB CHIP light. Anyway, during this slow down, the TRGB gave up resulting in a spin. 'Normally' gaining airflow over the fin would allow a flyaway, but what if the airflow over the fin wasn't sufficient to regain some form of yaw control?

What can you do? Try to get some airspeed on, while spinning, but it just isn't happening ... chop the throttles and where your right boot is, that's where you're going.

As I said, my opinion, and it's the best I can think of given the facts so far.

Sid, I wasn't going at you. You are clear in your posts

Sasless, bet that made you put your coffee down, so to speak.
Although the report says the#2 engine was capable of providing power to the MRGB, there's no mention of the freewheel, so another thing to wait out for.

NR Droop, good point, if that's the case, then it would be a matter of the rotors stopping just at that particular point in time and space where the airframe started to fall vertically with no time/height/forces left to tumble.

Jayteeto, I know. (Soz if sounded otherwise)

A term we use in ATC incidents quite a lot is " The Swiss Cheese" effect. Where a number of small incidents not catastrophic on their own but when all lined up with the holes in the cheese create an incident.......trying to take on board everything that has been spoken about on this forum so far I feel the outcome of this incident will be the same.....one small problem alone not catastrophic but 2 or 3 all lining up with the holes and the end result sadly what we saw here!

Sorry fishbang, as familiar as I am with Swiss cheeses, I am of the opinion that this one is down to one catastrophic thing.

The painfull thing (and I'm a bit reluctant to type this) is that there is basically only one way to have the rotors practically stopped on arrival and that is if something goes wrong during the entry to the AR.
When messing up the flare it is highly unlikely the time spent at high drag will be sufficient to completely stop the rotor.
As I said before drag in a flare will only be less than half of the drag in a stall. And even in the stall it will take a couple of seconds until the rotor stops even though once past a certain point this process can't be stopped or reverted any more.

In my opinion, that something going wrong before entry into AR is something down the back giving up.

Quote from henra:
"As I said before drag in a flare will only be less than half of the drag in a stall. And even in the stall it will take a couple of seconds until the rotor stops even though once past a certain point this process can't be stopped or reverted any more."

So, as a fixed-winger (without the h), can I correctly infer that it's impossible to stall the main-rotor by over-flaring at the end of an auto?

No, not exactly, you can still stall it but the time between when that happens and the bent Heli sitting on the ground will not be sufficient for the energy in the rotor to comletely bleed off.

Sid, would that not depend on the height at which the blades stopped rotating?

What I mean is, if not at the "end of the autotorotate" as in the question posed by Chris, but in general terms. If the collective was fully raised at say, 300 feet, and held up, with both engines producing little or no torque, would the blades not stop rotating, and stay stopped?

No idea ap, that's the puzzle isn't it!

What I do know is that if the engines aren't chopped by the time the Nr gets down to 60%, they will re-engage. So in my opinion, they must have been chopped before then.

Whoa.....that magic number of 60%......now why ever would EC choose that particular number as a trigger to kick the engines back on line?

Were we not told that more than 5% below the Bottom of the Green would be Endex for all helicopters?

Sid.....you might want to carry this discussion over to the Autorotation Thread and compare some notes with Pete or whoever it is that used the 5% concept.

Seems Brother Dixon, ex-SA Test Pilot, mentioned the "60% Range" meaning from 60-70%....I assume as being the critical range.

This bears discussing don't you think? It might be useful over there.

idle rpm
 

Is that not the engine idle speed? No mystery at all.

Post 795, draws a long bow, but perhaps plausible in Scotland IF Fadec failure was the cause of an aborted takeoff that was rumored to have occurred during the Greek olympics at a location near what has been described by TV folk as military strength surveillance equipment.

But since media and police craft operate daily with fadecs not effected by RF, it would have to be a freakish and extraordinary RF event.

Mickjoebill

60%
 

Is there not some confusion between N1 and N2 here? Anyway, if the idle N2 is indeed 60% (seems rather low to me) then with Nr below that, yes there will be a little bit of torque from an idling engine, but the engine won't accelerate and the overall effect won't be much in the context of stalled rotors.

Sasless,
My 60% has nothing to do with the other thread, I'm on about post impact.
To put it in its place;

Nr & N2
60% is idle with twist grips on the red buttons.
75% is normal ground idle (twist grips in normal detent'd position)
97% is flight idle
100% is flight idle with high Nr selected
103% is flight idle on application of collective
100% is flight above 55kts
103% is flight when reducing below 50kts

With these figures, instead of jumping down necks, please remember that when the Nr is above N2 it is not being driven by the engines. Normally, with the tr issue as previously described, the twist grip is rotated to the stop. The engines then wind down to 60% and the MRGB is then disengaged. As there was no rotation of the main or tail rotors on impact, the pilot must have fully wound off the twist grip, beyond the button, otherwise when the Nr reduced to 60% they would be driven once more by the engines ... as I said in my post 1326 that you happily criticised.

...and likely the engines (or 1 of them) would still be running after impact, which they weren't.

Fadec
 

Excuse me, whilst I have nothing authoritative to offer, as I am here for education rather than to educate, but would this (aged) post by 'maxtork' help those of us unaware of the operation of the FADEC systems?

http://www.pprune.org/rotorheads/277...ml#post1787624

Many thanks to all learned contributors here.

Phil