Police helicopter crashes onto Glasgow pub
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Grenville, I think you misinterpreted my comments. I did not suggest that a stalled rotor should be recoverable. I said that I believe a stalled rotor with the collective fully down may still have some small rotation. The AAIB have stated that there was zero rotation, so even if there had been a double engine failure and a completely mishandled entry into autorotation (which I would not expect from this pilot) this doesn't quite add up. No rotation suggests to me a mechanical failure in the transmission. As the AAIB are indicating against that, I am quite puzzled by this accident.
rotor accelration
henra said
In support of that I noticed the following.
Regarding a different EC135 incident[1] the NTSB report said:
125% is 3 times the Knietic Energy of 73% (square law)
It seems that aerodynamic forces can change the rotor speed very rapidly (at least upwards).
With the same input force (or is it power, or are they the same thing?) it would go from 0% to 73% (or the reverse) in about a second and a half.
[1] NTSB Report Untitled Page
and stop the rotor in a frighteningly short time.
Regarding a different EC135 incident[1] the NTSB report said:
1741:48, the main rotor RPM had decreased to 73 percent.
...
a rapid increase in main rotor rpm, to about 125 percent, which occurred at 1741:53
...
At 1741:56, the pilot transmitted, "mayday mayday, our engines are out, we're going down."
...
a rapid increase in main rotor rpm, to about 125 percent, which occurred at 1741:53
...
At 1741:56, the pilot transmitted, "mayday mayday, our engines are out, we're going down."
125% is 3 times the Knietic Energy of 73% (square law)
It seems that aerodynamic forces can change the rotor speed very rapidly (at least upwards).
With the same input force (or is it power, or are they the same thing?) it would go from 0% to 73% (or the reverse) in about a second and a half.
[1] NTSB Report Untitled Page
None but a blockhead
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Yes, I'm most surprised that the engine control positions haven't been mentioned yet. I'm of the understanding that this information is of primary importance in incidents where there's no obvious, gross mechanical failure, even more so in the case where there's no flight data recorder, and one - if not the - first thing to be noted when the investigators get access.
Also, I'd appreciate some informed info on the basic physics. If the rotors had stopped by the time the helicopter impacted, then what had happened to their rotational energy such that the helicopter itself wasn't rotating? Is there anything other than a managed reduction in speed that could do this?
Also, I'd appreciate some informed info on the basic physics. If the rotors had stopped by the time the helicopter impacted, then what had happened to their rotational energy such that the helicopter itself wasn't rotating? Is there anything other than a managed reduction in speed that could do this?
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Try this
The pilot used ALT ACQUIRE on the auto pilot at a fairly low level, he was close to the helipad I think.
He was distracted and did not apply power (only 3 axis autopilot).
Helo slowed and then decended and entered vortex ring. Pilot (brain scrambled)
applied full collective. Over pitch etc etc.
Can you stall the blades with both engines working?
He might have shut down the engines before impact. Feds did not say if 'speed select levers' shut off.
The pilot used ALT ACQUIRE on the auto pilot at a fairly low level, he was close to the helipad I think.
He was distracted and did not apply power (only 3 axis autopilot).
Helo slowed and then decended and entered vortex ring. Pilot (brain scrambled)
applied full collective. Over pitch etc etc.
Can you stall the blades with both engines working?
He might have shut down the engines before impact. Feds did not say if 'speed select levers' shut off.
It's a pity there is no mention of the position of the engine control switches and/or throttles
I suggest that there was at least another 60 litres in the system.
Re the rotor brake, a unit once did start a 135 up with the rotor brake applied but I don't think it stopped the donkeys and the blades still turned.
the AP just tried to continue the flight?
To the poster who asked about AVGAS in one of the tanks, yes, it was a stupid question.
No.
Once fully stalled, the position of the collective makes little difference.
It may reduce the AoA temporarily by 5°.
Once much above 10°, AoA will increase quickly, once RPM decays and Rate of Descent builds.
If you arrive at AoA >12-13° with lever full down, it's game over. Watch the Rotor stop in about 5s from there. (In a low inertia rotor system, a high inertia one may expand that towards 10s).
Edit:
After having read @Grenville's comment I think me too misunderstood the point you were trying to make. See my comment to @Grenville.
Last edited by henra; 9th Dec 2013 at 21:42.
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Chopjock
Because that (given the facts of a call to enter the zone at 2218 and catastrophe at 2222) WOULD have been grounds for a PAN call at least when the first engine failed and the pilot being incapacitated or some similar catastrophe when the second engine failed.
Also, no sane helicopter pilot given 1 minute or so warning of complete power loss (ie first engine stopping) fails to do everything necessary to cope with the imminent event. Which, with one spectacular exception in my 37 years knowledge, has never previously involved a twin simply running out of fuel.
Now, if you are postulating two entirely independent failures, I am willing to discuss the odds. Would you like to play poker?
Because that (given the facts of a call to enter the zone at 2218 and catastrophe at 2222) WOULD have been grounds for a PAN call at least when the first engine failed and the pilot being incapacitated or some similar catastrophe when the second engine failed.
Also, no sane helicopter pilot given 1 minute or so warning of complete power loss (ie first engine stopping) fails to do everything necessary to cope with the imminent event. Which, with one spectacular exception in my 37 years knowledge, has never previously involved a twin simply running out of fuel.
Now, if you are postulating two entirely independent failures, I am willing to discuss the odds. Would you like to play poker?
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How do we know for sure that one engine was not already out and the pilot was expediting the trip home, perhaps on AP and then when the second engine quit the AP just tried to continue the flight?
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There was a comment from the AAIB that the lifting of the helicopter from the site and other factors regarding the aircraft being disturbed in situ meant that the extent of the examination of the aircraft was limited.
How do we know for sure that one engine was not already out and the pilot was expediting the trip home, perhaps on AP and then when the second engine quit the AP just tried to continue the flight?
In your scenario....What does the Pilot do?
How long could you sit there like a Knot on a Log before you punched off the Autopilot?
You 135 Pilots out there.....if your last and final engine quits...thus taking the "only" generator with it....do you simultaneously lose the Autopilot system?
Would not every Helicopter Pilot's default position be to lower the Collective upon the second engine failing....regardless of the Status of the AutoPilot?
Apologies to Blade Crack....we posted at the same time.
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Grenville, I think you misinterpreted my comments. I did not suggest that a stalled rotor should be recoverable. I said that I believe a stalled rotor with the collective fully down may still have some small rotation. The AAIB have stated that there was zero rotation, so even if there had been a double engine failure and a completely mishandled entry into autorotation (which I would not expect from this pilot) this doesn't quite add up. No rotation suggests to me a mechanical failure in the transmission. As the AAIB are indicating against that, I am quite puzzled by this accident.
Yes I agree. Even a "stopped" disc would have some small rotational movement created by the upward airflow. That this disaster is puzzling is without question.
The AAIB have stated that the aircraft impacted the building at "a high rate of descent" and when "neither the main rotor nor the fenestron tail rotor were rotating."
This means that between the "popping" sounds reported by eyewitnesses (and recorded in the bulletin) and sometime prior to impacting the building (a handful of seconds presumably) the drivetrain came to a complete and sudden stop, almost as though it struck something, which we know it didn't, and which is why I was expecting to read about a catastrophic mechanical failure.
I am in no way implying this is what happened, and I am content to wait the year or however long it takes for the AAIB to do their usual thorough job, but heres a scenario for the seasoned 135 chaps to comment on. How do you get yourself out of this one?
We are transitting back to base, I say we, you are the handling pilot and I'm sat next to you. I develop a very sudden and massive panic attack, (never had one before, no idea why it's happening, could be stress, but there you go, it's happening). All I know is, I want this flight to end now and I want to be back on the ground right now.
In my blind panic to get back on the ground, I reach over and flick the engine switches to off. Immediately followed by pulling the rotor brake on. My actions are irrational, but thats a panic attack for you.
As soon as you try to correct either the eng switches or the R/B, I push down on the collective to speed up my return to the percieved safety of the ground.
How long would it take for us to reach the ground from 700(ish)'?
Supposing you manage select one engine switch back to flight, (before I turned it off again), would the relight on a hot engine go with a 'pop', similar to a 'misfiring' engine?
Once again, not saying this is what happened.
We are transitting back to base, I say we, you are the handling pilot and I'm sat next to you. I develop a very sudden and massive panic attack, (never had one before, no idea why it's happening, could be stress, but there you go, it's happening). All I know is, I want this flight to end now and I want to be back on the ground right now.
In my blind panic to get back on the ground, I reach over and flick the engine switches to off. Immediately followed by pulling the rotor brake on. My actions are irrational, but thats a panic attack for you.
As soon as you try to correct either the eng switches or the R/B, I push down on the collective to speed up my return to the percieved safety of the ground.
How long would it take for us to reach the ground from 700(ish)'?
Supposing you manage select one engine switch back to flight, (before I turned it off again), would the relight on a hot engine go with a 'pop', similar to a 'misfiring' engine?
Once again, not saying this is what happened.
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MightyGem
So I presume then the AP maintains alt by use of cyclic fwd and aft. So in my scenario then,second engine stops then aft cyclic would be applied by AP in order to try and maintain alt?
Then what, if no pilot action?
If by AP you mean the Auto Pilot, the AP doesn't work the collective.
Then what, if no pilot action?
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We are transitting back to base, I say we, you are the handling pilot and I'm sat next to you. I develop a very sudden and massive panic attack, (never had one before, no idea why it's happening, could be stress, but there you go, it's happening). All I know is, I want this flight to end now and I want to be back on the ground right now.
In my blind panic to get back on the ground, I reach over and flick the engine switches to off. Immediately followed by pulling the rotor brake on. My actions are irrational, but thats a panic attack for you.
As soon as you try to correct either the eng switches or the R/B, I push down on the collective to speed up my return to the percieved safety of the ground.
In my blind panic to get back on the ground, I reach over and flick the engine switches to off. Immediately followed by pulling the rotor brake on. My actions are irrational, but thats a panic attack for you.
As soon as you try to correct either the eng switches or the R/B, I push down on the collective to speed up my return to the percieved safety of the ground.
Anyone touching my engine switches, or any other switches for that matter without my say so would rapidly receive a knuckle sandwhich style reminder of their error of their ways.Its all getting a bit tedious again
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jimjim1's note on rotor speed when windmilling
Rotor rpm increasing from 73 to 125 per cent does indeed correspond to a kinetic energy increase of almost 3.
However, the power available to a windmill goes as the airspeed through the disk to the third power, and so the ability to pick up speed is very sensitive to the airflow conditions.
With the right blade angle of attack, a ~2m/s change in the flow speed through the disc could power the rotor as much as the ~1 MW from the engines. (Power ~ disc area * rho * v^3)
The kinetic energy of the rotor is also not large compared with the power flowing through it. A 200-kg rotor, 10-m in diameter has something like 2MJ of energy at 200rpm (~20 radians/s). But, it's driven by about a MW of power, so in the right/wrong conditions, it could stop in a few seconds.
Edit: The energy in the turning rotor's overestimated here.
Moment of inertia for 200kg, 5-m radius, turning at 200rpm is about (1/3)*200*25~1500 kg m-squared. At 20 radians/s, energy is only 750*400J~0.3MJ.
However, the power available to a windmill goes as the airspeed through the disk to the third power, and so the ability to pick up speed is very sensitive to the airflow conditions.
With the right blade angle of attack, a ~2m/s change in the flow speed through the disc could power the rotor as much as the ~1 MW from the engines. (Power ~ disc area * rho * v^3)
The kinetic energy of the rotor is also not large compared with the power flowing through it. A 200-kg rotor, 10-m in diameter has something like 2MJ of energy at 200rpm (~20 radians/s). But, it's driven by about a MW of power, so in the right/wrong conditions, it could stop in a few seconds.
Edit: The energy in the turning rotor's overestimated here.
Moment of inertia for 200kg, 5-m radius, turning at 200rpm is about (1/3)*200*25~1500 kg m-squared. At 20 radians/s, energy is only 750*400J~0.3MJ.
Last edited by awblain; 27th Dec 2013 at 19:33. Reason: Momentum of inertia wrong?
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The AAIB report is clear that the engines and gearbox have not yet been dismantled or examined in detail. All we know is that they could be rotated when externally inspected. The detailed stripdown examination could still reveal surprises. For example a bearing could sieze when overheated but still be free when cold.
Stopped doesn't necessarily mean exactly 0 RRPM, although even that might be possible.
Even AAIB probably didn't mean this litteraly. But it doesn't make any appreciable difference if you arrive with 50 RRPM or 0. (<1% in Lift).
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Essentially this is an AFCS malfunction whereby the software receives no signal from the collective jack as to what position it is in. So the software goes looking for it, and demands inputs of huge magnitude at a very swift rate. It is absolute mayhem in the cockpit ( Been there!) as the collective hydraulic jack goes from maximum to minimum unpteen times per second.
The kinetic energy of the rotor is also not large compared with the power flowing through it. A 200-kg rotor, 10-m in diameter has something like 2MJ of energy at 200rpm (~20 radians/s). But, it's driven by about a MW of power, so in the right/wrong conditions, it could stop in a few seconds.
Based on awblain's explanation, rapid loss of Nr is possible if power is absent meaning that in order to achieve the AAIB's initial findings of a high speed impact with nil apparent Nr we are looking at a total power failure followed by sustained pitch on the main rotor?
