PHI Crash in Louisiana Jan 2009 - 8 Dead, 1 Injured
So...night IMC...both of you tucked into your nosh on the way home...George driving....talking about the football pools...and you get a sudden call to arms.
By the time you put down the carry out boxes...pick up your serviette...wipe off the crumbs from your 'statche....deposit the serviette back into the rack....would you possibly be in a situation that would call for a bit of first class handling skills?
By the time you put down the carry out boxes...pick up your serviette...wipe off the crumbs from your 'statche....deposit the serviette back into the rack....would you possibly be in a situation that would call for a bit of first class handling skills?
MITCHAA,
I think that in the event of a major main gearbox failure leading to sudden loss of torque on both engines the result would be both engines trying overspeed.
The overspeed control system would shut down the first engine to hit the target speed (120% N2???) then it would self disable and the second engine would runaway.
This would probably result in an engine explosion as the free turbine gave out.
An indication of what happens when the overspeed system is inoperative is given by the Helikopter service Super Puma that crashed in the mid nineties.
In this case the drive to the main gearbox failed and the thrashing driveshaft cut the cables to the overspeed sensors. The engine FCU control system could not react fast enough and the engine exploded.
None of the above is intended to suggest that this is what may have befallen the S76 in question.
I think that in the event of a major main gearbox failure leading to sudden loss of torque on both engines the result would be both engines trying overspeed.
The overspeed control system would shut down the first engine to hit the target speed (120% N2???) then it would self disable and the second engine would runaway.
This would probably result in an engine explosion as the free turbine gave out.
An indication of what happens when the overspeed system is inoperative is given by the Helikopter service Super Puma that crashed in the mid nineties.
In this case the drive to the main gearbox failed and the thrashing driveshaft cut the cables to the overspeed sensors. The engine FCU control system could not react fast enough and the engine exploded.
None of the above is intended to suggest that this is what may have befallen the S76 in question.
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True the overspeed circuit will shutdown the first engine to 120%. But remember, the engines are N2 governed. The DECUs would be pulling back fuel as fast as possible. So the fantastical Hollywood 190% explosion is not really likely.
HOSS 1
Just to clarify the first engine to reach 120% N2 would shut down completely not stabilise at 120%. The second engine would loose it's overspeed protection.
The explosion situation is exactly what happened to the HS Super Puma.
Working from memory I believe that when the driveshaft failed and the overspeed system was destroyed the electronic fuel control was unable to react in time. The engine oversped and exploded with some parts cutting one of the control linkages to the main rotor resulting in loss of the aircraft. Parts of the engine also penetrated the aircraft hitting some of the passengers.
I believe that the logic behind the overspeed system is that if you loose an engine then you don't want an overspeed system failure to shut down the only good engine.
This clearly is not designed to cater for the simultaneous loss of drive from both engines to the main rotor.
It is possible that the S76 with a more modern system would react faster, but that is outside my knowledge.
Again no intent to suggest this has a direct relevance to the loss of this S76.
At the risk of prolonging this thread creep I just spoke to a friend of mine at work and his take on this is that the overspeed system will cope with a driven overspeed i.e the engine fuel control has failed and is demanding the excess speed. He reckons that an instantaneous loss of the drive would result in an overspeed that even the overspeed system would not contain.
Engineers are clearly like Greek politicians, 3 individuals having 5 different opinions!!!!!!!!
Just to clarify the first engine to reach 120% N2 would shut down completely not stabilise at 120%. The second engine would loose it's overspeed protection.
The explosion situation is exactly what happened to the HS Super Puma.
Working from memory I believe that when the driveshaft failed and the overspeed system was destroyed the electronic fuel control was unable to react in time. The engine oversped and exploded with some parts cutting one of the control linkages to the main rotor resulting in loss of the aircraft. Parts of the engine also penetrated the aircraft hitting some of the passengers.
I believe that the logic behind the overspeed system is that if you loose an engine then you don't want an overspeed system failure to shut down the only good engine.
This clearly is not designed to cater for the simultaneous loss of drive from both engines to the main rotor.
It is possible that the S76 with a more modern system would react faster, but that is outside my knowledge.
Again no intent to suggest this has a direct relevance to the loss of this S76.
At the risk of prolonging this thread creep I just spoke to a friend of mine at work and his take on this is that the overspeed system will cope with a driven overspeed i.e the engine fuel control has failed and is demanding the excess speed. He reckons that an instantaneous loss of the drive would result in an overspeed that even the overspeed system would not contain.
Engineers are clearly like Greek politicians, 3 individuals having 5 different opinions!!!!!!!!
Last edited by ericferret; 16th Jan 2009 at 11:29.
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The engines are required to survive a sudden loss of drive at contingency power during testing. This is done by running the engine on a test stand at the maximum contingency power rating and explosively severing the driveshaft. Tests (or prediction) somehow showed that the N2 wheel would disintegrate at about 150% N2 for the model of engine in the Bell 407. So the initial speed at which the overspeed protection would shut off fuel was set at 110%N2 in order to prevent the N2 from exploding. Later found to be too low (you could actually get the NR to 114-115% if you really worked hard at it in auotorotation. Thanks to the 'soft rejoin' feature of the fuel control, the N2 would run up to try to meet that value when you raised the collective to recover. Ensuing accident where the engine shut down at 110% N2 during a practice turning autorotation required the N2 overspeed value to be re-set at 116% if memory serves me well. Tests showed you couldn't get the NR there.
The original value had been chosen too conservatively without due regard to the big picture.
But the overspeed protection is required to demonstrate that it can survive an explosive immediate loss of drive at the worst condition.
The original value had been chosen too conservatively without due regard to the big picture.
But the overspeed protection is required to demonstrate that it can survive an explosive immediate loss of drive at the worst condition.
Interesting that Bell fit an overspeed system to a single. I believe that most singles don't have an overspeed system.
I was told that Bell 205's had a sytem and normal procedure was to pull the C.B so it wouldn't operate (ye of little faith) inadvertantly.
I suppose it's a balance of risk, which is worse an uncommanded shutdown where there is nowhere to land or an exploding engine if the drive fails.
I was told that Bell 205's had a sytem and normal procedure was to pull the C.B so it wouldn't operate (ye of little faith) inadvertantly.
I suppose it's a balance of risk, which is worse an uncommanded shutdown where there is nowhere to land or an exploding engine if the drive fails.
Bell 205's had an over speed system?
Where did you hear that?
Where did you hear that?
Man, now the two brain cells are fired up! Wasn't it the 206L-1 that had an overspeed system at first? Which we ended up disabling because it activated prematurely?
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The above poster is correct. Once an engine is shutdown by the overspeed system, protection is inhibited on the remaining engine. But, I wonder just how high the N2 would get. The DECU is pretty responsive. But, purely conjecture on my part.
HOSS
HOSS
Last edited by HOSS 1; 19th Jan 2009 at 18:10.
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Yes, the Bell 205 had an overspeed system but there were too many faults with the frequency counter resulting in spooldowns at the most inappropriate time. You will find the press switch contained in a round guard at the top middle to right hand side of the pilot instruments.
I will see if I can find an instrument panel view.
Regards
carholme
I will see if I can find an instrument panel view.
Regards
carholme
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205 Overspeed Trip System
Here is the schematic showing the overspeed trip system installed between the gearbox and the N2 tach on the -13 engine.
http://www.huey.co.uk/images/service...tem%206-22.jpg
and here is the instrument panel showing the guarded swith just to the top left of the ADI.
http://www.huey.co.uk/images/service...tems%203-1.jpg
carholme
http://www.huey.co.uk/images/service...tem%206-22.jpg
and here is the instrument panel showing the guarded swith just to the top left of the ADI.
http://www.huey.co.uk/images/service...tems%203-1.jpg
carholme
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Can We Get Back to the PHI Crash Please
Can we get back to the PHI crash now and have another discussion about engine overspeed systems on various types on another thread please? I keep thinking I may see something relevant to the accident or S76 and find it isn't so and as I fly the S76 I'd be happier if I didn't keep thinking I may find something which may be relevant to what I'm doing. Thanks, Soggy
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The engine overspeed systems are part of the design of all turbine engines so that any runaway is stopped before the engine speeds up so much that it comes apart. In older engine designs, the protection system was mechanical, in newer aircraft (virtually all by now) it is electronic, and predicts the failure state by using both trhe actual N2 speed and the rate of increase of the N2 speed. Thus it triggers at lower N2 if the turbine is accelerating.
SASLESS
No idea where I got that bit of info from.
Chickenhawk is my normal source for Huey info!!!!!!!!!!!!
However in this case I think it came from someone who had worked with the Sultan of Omans armed forces. I shall now agonise over who it might have been.
Specifically I understand that S76's are now subject to a repetitive inspection of all indicating chip detector systems every 25 hours. 4X Gearbox and 4x Engine chip detectors.
Full system test.
Not sure if this is Sikorsky driven or just local.
No idea where I got that bit of info from.
Chickenhawk is my normal source for Huey info!!!!!!!!!!!!
However in this case I think it came from someone who had worked with the Sultan of Omans armed forces. I shall now agonise over who it might have been.
Specifically I understand that S76's are now subject to a repetitive inspection of all indicating chip detector systems every 25 hours. 4X Gearbox and 4x Engine chip detectors.
Full system test.
Not sure if this is Sikorsky driven or just local.
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Bell 205's had an over speed system?
Where did you hear that?
Where did you hear that?
Brian,
My alcohol ravaged brain must be getting really bad....flew the civvie 205A-1 a bit but don't recall that bit of kit....but then I have trouble remembering my address after dark too.
I know the UH-1 series did not have it.
Of course....most of the places I flew the civvie 205 would not have had it working after the first time it got snagged either.
But as they say....we live and learn....and forget.
My alcohol ravaged brain must be getting really bad....flew the civvie 205A-1 a bit but don't recall that bit of kit....but then I have trouble remembering my address after dark too.
I know the UH-1 series did not have it.
Of course....most of the places I flew the civvie 205 would not have had it working after the first time it got snagged either.
But as they say....we live and learn....and forget.
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As a former PHI C++ pilot, I can pretty much assure you the pilots were not flying in 3 cue. We were trained to climb/decend in 3 cue, but not cruise in 3 cue. We were aware of the torque demands due to wind gusts so cruising in 3 cue was pretty much prohibited. 