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Entering autos: discussion split from Glasgow crash thread

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Entering autos: discussion split from Glasgow crash thread

Old 11th Dec 2013, 23:04
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Entering autos: discussion split from Glasgow crash thread

Gentlemen Ė
This is my first post to PPRuNe. It concerns the tragic accident at Glasgow on November 29. I have read all of the over-800 posts regarding this accident and feel that I may be able to shed some light on what David Traill faced as the helicopter headed down.
The Best Kept Secret in the helicopter industry is how critical it is to immediately apply aft cyclic the moment a loss of power to the rotor system is detected. I have been doing my best to spread this word since analyzing the cause of two law-enforcement helicopter accidents that occurred four months apart in 2002. I call it Cyclic Back.
Very few helicopter pilots realize that if, with a total engine failure, the rotor rpm is allowed to fall more than about 5% below low green, the flight is over. OVER. There is no recovery possible regardless of what actions the pilot may take or how high the helicopter is above ground. This fact is not mentioned in the sales literature for helicopters nor in the approved rotorcraft flight manuals. It is not mentioned in any of our FAA publications having to do with how helicopters fly and how to fly them. It is treated as a deep dark secret, unfortunately.
When power is lost to the rotor system, THE MOST IMPORTANT FLIGHT CONTROL IN THE COCKPIT IS THE CYCLIC! It must immediately be brought aft so that the flow of air is upwards through the rotor system. Bottoming the collective does only one thing: It reduces the rate that the rotor rpm is falling. Thatís all! It NEVER stops the fall of rotor rpm.
Once the rotor rpm has dropped below the critical point, recovery is not possible. The helicopter continues to descend as the rotor rpm falls towards zero and may, in the case of free-turbine engines as used in the EC135, be seen to turn backwards. The rotor blades will show little if any damage when the wreckage is examined.
And as the rotor rpm slows towards zero during the descent, retreating blade stall enters the picture. The normal Vne chart does not mention rotor rpm; it is assumed it is normal for the standard mode of flight. But when rotor rpm falls, Vne falls with it, so Vne is very possible at airspeeds much below those computed via the chart.
This in turn means that as the rotor rpm is falling during the autorotation, the helicopter will roll in the direction of the retreating blades, or to the left in the case of the EC135. Any attempt by the pilot to correct this with opposite cyclic simply adds additional pitch to the blades that are already stalling, thereby increasing the amount of roll.
When a helicopter pilot is faced with a sudden unannounced engine or drive-line failure, here is what must be done:
1. Cyclic back and pitch down, simultaneously or in that order.
2. Pick a place to land.
3. MAKE THAT SPOT!
What about indicated airspeed. At the beginning of the autorotation, the ONLY speed that matters is that over the wings, meaning, of course, the rotor blades, and this is a function of rotor rpm. Pitot tube airspeed (indicated airspeed) is not important at that time, but yes, once the rotor rpm is solidly in the green, indicated airspeed can be helpful in extending the glide or reducing the rate of descent.
Finally, this: During a normal power-off autorotation, the helicopter will respond to all flight-control movements the same way it would if the helicopter is in a flat-pitch descent with the engine running. The only thing it wonít do is a sustained climb. But it will stop, back up, turn in any direction, etc. So when I say MAKE THAT SPOT, Iím saying use the maneuverability of the helicopter just as you would if the engine was running. There is more to life than straight-ins, 90s and 180s!
Based on what I have read so far on PPRuNe, my thoughts are this: For whatever reason, both engines went off line one following the other, but Dave did not apply aft cyclic quickly enough to prevent the rotor rpm from falling below that critical point from which recovery is not possible.
Pilot error, you say? Not at all. Dave did exactly what he was trained to do in this sort of emergency. The blame goes to the simple fact that the critical importance of applying aft cyclic quickly when power is lost to the rotor system has been kept from all of us since rotor blades were invented. Cyclic Back must become an integral part of all helicopter training and publications. This must not remain a secret any longer!
I extend my most sincere condolences to everyone who was directly affected or touched in any way by this most unfortunate accident.
PeteGillies
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Old 12th Dec 2013, 07:06
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Police helicopter crashes onto Glasgow pub

Peter Gillies, thank you for a helpful and, to me, very plausible description of possible events. I am not a rotor head but would like to think I understand enough to appreciate your theory.
The reason the power was lost in the first place remains a mystery but AAIB are the only people that will resolve that riddle.
Thanks for your insight and your patience in reading all postings!
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Old 12th Dec 2013, 07:31
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Thank you Peter Gillies for your very clear and concise comments.
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Old 12th Dec 2013, 07:46
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fish

Peter Gillies's post is most interesting.

Having personally flown nothing more modern than an AS355N I have no idea how these rigid rotors react in auto, but I'm willing to believe his theory. Don't know how much use it would be from a hover though.

The most important nugget to draw from his post is that once NR falls below critical, the flight is OVER.

From then on the pilot has control which very rapidly diminishes to zero, and there is no input to any control that will alter the trajectory and final impact site.
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Old 12th Dec 2013, 08:04
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Peter Gillies post gives a very plausible reason for loss of control after a double engine failure. Since the AAIB report indicates that there wasn't an immediately obvious mechanical failure and that there appeared to be sufficient fuel in the tanks to return safely to base, what could cause the engines to fail.

My question is, are their any 'forbidden' attitudes when fuel levels are low, although no low fuel indicator lights are illuminated? For example if the helicopter adopted a steep nose up or nose down attitude, or banked sharply, would the fuel feed pipes be exposed to air above the fluid levels in the tank(s)?
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Old 12th Dec 2013, 08:36
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The Best Kept Secret in the helicopter industry is how critical it is to immediately apply aft cyclic the moment a loss of power to the rotor system is detected.
The transition from normal stable flight to stable autorotation flight has always been a messy affair requiring the application of lots of inputs quickly. Nr drops, nose drops, nose yaws, body rolls all of which must be fixed quickly and it can be fixed easily enough. We tend to generalise this maneuver when we talk about it by only mentioning lowering of the collective but what we really mean is going from one mode of flight into another. I'd like to think this wasn't a secret to this experienced pilot.
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Old 12th Dec 2013, 08:48
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Dave did exactly what he was trained to do in this sort of emergency. The blame goes to the simple fact that the critical importance of applying aft cyclic quickly when power is lost to the rotor system has been kept from all of us since rotor blades were invented. Cyclic Back must become an integral part of all helicopter training and publications. This must not remain a secret any longer!
????? Since when has this been a secret? I learned it when doing my PPL around 16 years ago. "Down, right, back" as my instructor put it over and over again, referring to collective, pedal, and cyclic, but meaning that I should do all three together. Everyone else learned this too. As an instructor, that's what I learned in my FI course, and subsequently taught. It's in all the books. It's common knowledge. No secret, and I find it ABSOLUTELY impossible to believe an experienced police pilot didn't know about it and do it.
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Old 12th Dec 2013, 08:48
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Peter Gillies
Really don't agree with your theory. When we did practice auto entries in the BK 117 they were done from the most probable scenario, that is the pilot pulling power off the incorrect engine. We first simulated a single engine failure, then reached up (overhead throttles) and reduced the second engine to idle, then reached back down and reduced collective to address the RRPM light and horn.
No flare, and no rotor stall. I have done this exercise many times, and the instructor who did the checks would have done it innumerable times. Had he felt there was such a problem he would undoubtedly have mentioned it.
High hover zero speed autorotations, admittedly I have only done in high inertia machines, close throttle, collective down and forward cyclic, further unloading the rotor. Dive for a bit then aft cyclic and flare, pitch and cushion.
Never considered rotor stall in entry
Further to the discussion, if the accident aircraft was 1.5 NM from the pad he was probably no longer in cruise flight but beginning a gentle deceleration.
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Old 12th Dec 2013, 09:57
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Angry

Page 61 of this thread should be pulled in its entirety after what Peter Gillies posted. It was followed by a load of bollo*s from newbie pruners some who actually admitted they new little or nothing about helos but liked his theories.
Munneyspinner / GordonField and that extra special contribution from...wait for it: Gouli. Mmmmm

Whirlybird is right - what secret??

Let me clarify - for the sake of every helicopter pilot in the WORLD, that what Peter Gillies has stated requires a health warning. Let me set the record straight so helicopter pilots around the world are NOT confused after reading the rubbish he has just pushed out.

IN THE CRUISE:

If you experience a total engine(s) failure you lead with collective down followed with aft cyclic IF you need to. The reason for doing it in that order is because the down collective contains the decaying Nr and the OPTION to select aft cyclic provides you with the choice to go for range or not. Selecting aft cyclic without a second thought means that after you have done this and THEN find you won't make your spot you have to reselect fwd cyclic to gain range. The important part of this above statement is LEAD with collective and NOT aft cyclic.

HOVER:

Absolutely under NO circumstances whatsoever do you lead with aft cyclic- PERIOD.
In the hover after sustaining total engine(s) failure you lead with down lever. You ALLOW the attitude of the a/c to nose down. Down lever contains decaying Nr and nose down allows you to gain fwd speed which in 'most' cases is what you want.
If you lead with aft cyclic in the hover and then follow with collective you will enter a whole new world of hurt.


These above processes have been tried and tested - probably hundreds of thousnads of times in training and in real life scenarios.
There is no SECRET Never has been ...never will be.

Does Peter Gillies honestly think 99% of all helo pilots out there are harbouring a myth? Does he honestly think there really is some "secret" being kept from end users about this. Stop reading your Marvel mags PG and join the rest of us in the real world.
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Old 12th Dec 2013, 10:24
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TC - a little harsh, and as or more misleading than PG! Clearly, in the hover or low speed, aft cyclic would be wrong. I don't think PG was trying to say it was needed, he was in his own mind just covering the case of being in the cruise etc.

As for the cruise case, perhaps it depends on the type. In the case of the Super Puma family with which I am familiar, lowering the collective lowers the nose slightly. If you floor the collective without moving the cyclic, everything flies up to the roof as the g falls below zero. Zero or negative g means no significant autorotational effect and the Nr plummets whilst IAS increases. After a while and once the rate of descent has built up the g bites and the Nr rockets up from too low to above max in about 1 second. It is a most horrendous way to enter autorotation for both its effect on the aerodynamics and the disorientation that negative g brings to the pilot.

You say lead with the collective to contain Nr, but dumping the collective does not arrest the Nr decay, you need aft cyclic as well (from cruise speed) to do that. Yes dumping the collective will eventually restore the Nr but it will have decayed much further, especially if the entry is unexpected, possibly beyond the point of no return, if aft cyclic is not applied.

By contrast a simultaneous aft cyclic movement makes it all a benign and gentle manoeuvre. From cruise speed, little or no airspeed is lost on entry, you are still well above best range speed even into a headwind so your point about airspeed and best range is plain wrong. If you enter autorotation from around Vy, less or no aft cyclic is needed of course, but then most people spend most of their time in the cruise, not flying around Vy.
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Old 12th Dec 2013, 10:27
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From PeteGillies:

When power is lost to the rotor system, THE MOST IMPORTANT FLIGHT CONTROL IN THE COCKPIT IS THE CYCLIC! It must immediately be brought aft so that the flow of air is upwards through the rotor system.
Total nonsense when applied to the high-hover scenario. A simultaneous lowering of collective and application of FORWARD cyclic to accelerate to a suitable autorotative speed would be a more appropriate response (assuming you had enough height to achieve forward airspeed).

In all cases the cyclic and collective are equally important. Collective lowered to preserve Nr and cyclic to determine a suitable speed and direction towards your nominated landing point.

Slight thread drift I know but I thought it important to quash spurious advice!
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Old 12th Dec 2013, 10:30
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Re: Peter Giles' post #1205

Well kept secret??

Your statement implies to the many non-pilot types reading it that actually knowing how to fly a helicopter is a well kept secret from the pilots actually flying them!

Lowering the Collective and moving the Cyclic back go hand in hand!
Any pilot worth their salt will know how aerodynamics go, and more so will know what to do and when.

I would not wish to fly with a pilot who considers this a well kept secret!

Presumably anyone writing posts in reply, to the effect of "Thank you for your insightful words and revelation into the most surprising secret of helicopter flying" are not helicopter pilots.

In fact it has always been a well known fact that you have very little time to get RPM under control in the event of power loss to the Rotor System. And of course the flight is over if there is no RPM. After all, it is the turning of the Rotor that make a helicopter fly. It's a bit like saying "Do you know? If your car leaves the road and enters the air, your drive is OVER" - well of course it is; your wheels are no longer on the road!

Last edited by Old Age Pilot; 13th Dec 2013 at 10:00.
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Old 12th Dec 2013, 10:32
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OAP - you'd better not fly with TC then!
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Old 12th Dec 2013, 10:33
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Forgive me, but can someone explain to this fixed-wing pilot precisely what PeteGillies is recommending in the following statement?

1. Cyclic back and pitch down, simultaneously or in that order.

Does he mean "pitch-down" in the sense of the a/c attitude, or is he referring to the collective lever?

(BTW, not planning on trying to fly a heli any time soon!)
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Old 12th Dec 2013, 10:42
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Helicomparitor. I agree 100% with your last post. By the time most of us have recognised what has happened, aft cyclic is definitely required almost simultaneously. I think PGs post is a little naive thinking we do it know this but technically sound.

I do not have much experience on smaller helis anymore but I do remember a 500 foot hover throttle chop exercise in the B206 OPC required for filming. Lever fully down, nose fwd for some speed leading to immediate flare" cannot remember the heights/speeds associated with the fare though. 'twas exiting but not too difficult even for me!!

DB
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Old 12th Dec 2013, 10:43
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Helicomparator: PLease please do me the courtesy of reading my post very carefully. DO NOT add words that did not exist in the original script. I have chosen my words carefully. I chose the word: LEAD not DUMP. Do you understand the difference between the two and why the use of the word 'dump' (apart from the obvious!!) would lead to all sorts of confusion in the cockpit.
The descriptions are simply explained, they could not be more succinct I would suggest but adding strange volatile words like "dumping the collective" will of course trash the purpose of my message.

You know and all of us know that the physical act of initiating the immediate actions after an engine(s) failure is done in a co-ordinated and smooth fashion whether we are in the hover or cruise. Whether one selects aft cyclic or retain its present position is a matter of choice (IN THE CRUISE) dependent on reaching the landing spot, but (a) it is NOT compulsory to select aft cyclic and (b) it is dangerous to do so from the hover.

Your comment about Old Pilot are inaccurate, yet again you fail to read what is written correctly. At NO point does he mention AFT cyclic...................in fact what he talks about is the aspect of this procedure being kept a secret. C'mon now HC read carefully please?

Double Bogey....Interesting comments:
Helicomparitor. I agree 100% with your last post. By the time most of us have recognised what has happened, aft cyclic is definitely required almost simultaneously. I think PGs post is a little naive thinking we do it know this but technically sound.

I do not have much experience on smaller helis anymore but I do remember a 500 foot hover throttle chop exercise in the B206 OPC required for filming. Lever fully down, nose fwd for some speed
Fascinating......
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Old 12th Dec 2013, 10:57
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TC

IN THE CRUISE:

If you experience a total engine(s) failure you lead with collective down followed with aft cyclic IF you need to. The reason for doing it in that order is because the down collective contains the decaying Nr and the OPTION to select aft cyclic provides you with the choice to go for range or not. Selecting aft cyclic without a second thought means that after you have done this and THEN find you won't make your spot you have to reselect fwd cyclic to gain range. The important part of this above statement is LEAD with collective and NOT aft cyclic.
I thought you knew what you was talking about until I read this.

If I was in the cruise, 125Kts in my 500 and the donk quit, I would most certainly not lower the collective until slowed to at least 80 Kts. I would be pulling in aft cyclic first.
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Old 12th Dec 2013, 11:07
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PG Tips:

I hope fixed wing pilots also learn the world's second best kept secret of not pulling back on the control column while attempting to recover from a stall.

Seriously, if all these pilots you know haven't already learned the basics of helicopter control, who is training them and do the FAA know what is going on?
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Old 12th Dec 2013, 11:12
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I hope fixed wing pilots also learn the world's second best kept secret of not pulling back on the control column while attempting to recover from a stall.
Sadly, AF447 would prove otherwise.
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Old 12th Dec 2013, 11:14
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Chopjock,

If I was in the cruise, 125Kts in my 500 and the donk quit, I would most certainly not lower the collective until slowed to at least 80 Kts. I would be pulling in aft cyclic first.
And if was flying at 155 kts in my twin, I would lower the lever slightly and carry on straight and level. It will fly at 125 kts on one engine.

Problem here is that some are trying to find "one size fits all" advice. The correct actions for one aircraft may differ from another. The actions in one set of circumstances need to be quite different for the same aircraft in different circumstances.
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