more on helicopter crash.

[The Natural Philosopher]

This is getting weirder and weirder. No signs of at least one engine or any gearbox failure.

[The Natural Philosopher]

is the actual preliminary report.

[Bob Henson]

The PM on the pilot may be more conclusive. The fact that the crash was so sudden that no mayday call was made, and that no attempt to miss the building was made, would, in the absence of obvious catastrophic mechanical failure, indicate something like a heart attack.

[GB]

The AAIB report indicates that the rotors were stationery. That's a bit odd. Why would he have done that?

[Dave Liquorice]

engine or

Ta, saves me wandering over to get the story from the horses mouth without the mejia filter in place.

Most odd, apart from fuel starvation. Crud in the fuel and/or tank blocked filter. Would fit with the witness report of loud misfiring, silence and nothing going round on impact.

[Dave Liquorice]

The pilots funeral was a couple of days ago. One would assume that if anything like that had been found in the post mortum it would have been published. Doesn't really fit with misfiring, silence, stationary rotors, and near vertical decent.

[Jb]

Would a massive electrical failure do it? No engine, no avionics/radio but why did the rotor actually stop? Do they not 'freewheel' with a loss of power?

[Terry Fields]

The rotors would take a bit of stopping; the helicopter took about 7 or 8 seconds to fall and even if the engines had failed they'd still be turning after that interval, especially if the attitude was 'flat' the airflow would also assist them turning even if there was no power.

The report says that the power train from No 2 engine was OK, but does note that this couldn't be tested on No 1 due to impact damage.

ATM I'm thinking that some sort of possibly-temporary seizure of the drive train stopped the rotors, and the disrupted gas flow through the engines caused the backfiring sound reported by at least one witness.

[Bob Henson]

I didn't know they'd had the PM. If must be a major mechanical failure then.

[The Natural Philosopher]

yebbut blades don't stop going round cos the pilots snuffed it.

[The Natural Philosopher]

Perhaps he ran out of balsa wood an a sheet of A4 was all he had left?

[The Natural Philosopher]

Ok...wouldn't that actually simply cause autorotation, and surely gas turbines don't 'backfire'

[Nightjar]

The AAIB report does not actually say no engine failure, it only says no major mechanical damage to the engines. There are other failure modes, such as a flame out, which cannot yet be ruled out. The report of sounds like a car backfire would be consistent with compressor stall, although the type of engine management system fitted is supposed to stop them from happening.

Colin Bignell

[Dave Liquorice]

I don't for sure, no mention has been made but I'd be *very* surprised if there hasn't been one.

[Nightjar]

If the compressor stalls, they can* and the noise will be similar. However, the helicopter had an advanced engine management system that should have prevented compressor stall.

• Airflow through the compressor is disrupted, compression stops or is seriously reduced and air that has already been compressed reverses direction and comes out of the intake. The engine will probably flame out through lack of air and stop.

Colin Bignell

[newshound]

"powered by two Turbomeca Arrius 2B2 turboshaft engines. These full-authority digital electronic control (FADEC) equipped engines"

Don't know anything about FADEC but *presumably* the two engines would have had completely independent "engine management systems". I wonder if someone missed a common mode failure fault (unlikely though that seems).

[Dave Liquorice]

misfiring,

Doesn't autorotation require hefty levels of main rotor pitch? Cruising flight wouldn't have much pitch. There may also be too much drag/inertia with the main rotor still coupled to the engines for them to spin freely.

They can flame out. Intermittent fuel supply at odd pressures would screw up the fuel/air mix possibly allowing unburnt fuel into the hot exhaust part of the machine where it ignites but not confined by a combustion chamber, BANG!

[Graham.]

The day after, one of the red-tops was heralding the pilot as a hero, for averting a more serious incident in the final seconds.

Just how much worse did they expect it to be?

[Dave Liquorice]

Would it? Isn't the airflow in the opposite direction? In flight the rotor is shoving lots of air downwards. Lose power and start falling the airflow through the rotor is now upwards, without changing the rotor pitch that will tend to slow the rotor...

As you say "the rotors would take a bit of stopping", with a seizure all that kinetic energy gets disipated at the place of seizure, that is going to leave a mark like stripped gear teeth etc. The report says "Clear impact distortion of the structure had caused a splined shaft on the drive train from the No.1 engine to disengage ..." nothing about any other damage that could be expected if the gearbox or bearings has seized.

[mick]

One item that I read mentioned that there are 2 fuel tanks of different capacities. I'm guessing that each of these feeds its own engine, so you would need both fuel systems to fail.