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The difference between an expensive falling brick and a helicopter is rotor RPM. Pete Gillies talks about the importance of aft cyclic in autorotation entry to conserve rotor RPM in this episode.
Depending on where you did your training this might come across as ‘yeah, of course, doesn’t everyone do this’. If that’s you then fantastic and this interview will give you an in depth refresher on the ‘why’ behind it. If you’ve been out of regular autorotation practice for a while or didn’t have this emphasied then you’ll walk away with a new understanding to take on your next flight. There are a number of links further down the page that go on to show that this knowledge may not be as widely spread as it should be.
As with any flying advice you should discuss it with an instructor that you trust if unsure and apply some sense in how to use it – Pete is obviously not trying to say that aft cyclic must be used religiously for example in a hover engine failure.
[Tweet “A bad ending of an autorotation is usually survivable, but a bad beginning is usually not”]
“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. That’s 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 won’t do is a sustained climb. But it will stop, back up, turn in any direction, etc. So when I say MAKE THAT SPOT, I’m 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!” – extract from a post that Pete made on PPRuNe in 2013 (with a typically PPRuNe spirited debate that runs to 28 pages)
As for in Pete’s earlier interview back in Episode 39 we have Douglas Williams to thank for capturing this audio.
[Tweet “The pilots had not applied aft cyclic at the time the engine quit, they had gone for airspeed – Pete Gillies”]
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In the episode:
02:04 Competition for a custom aviation drawing
03:00 Listener comment Andy, 28 SQN OCU
04:20 B206 engine start clip last episode
05:50 Old Bell Helicopters magazine article, 1975 – ‘Keep the rotor in the green’ – redundant?
09:20 Studying accident reports – LAPD Astar overrunning clutch failure
11:15 MD500D ran out of fuel then crash wreckage
15:40 1992 MD500E engine failure on takeoff with crash onto busy road
20:55 Sherif MD600 steep 180 turn following engine failure
25:10 Law enforcement recurrency training using crash photos – lightbulb moment!
28:40 “Immediately put the collective down and immediately nosed over to get airspeed”
30:20 The aircraft were turning left due to below green rotor RPM – retreating blade stall
33:00 Impact on Vne of low rotor RPM
35:00 Emphasis at the time on airspeed in RFM
36:30 Fixed wing comparison
38:45 Reaction time and immediate actions
41:00 Overrunning clutch – engine overspeed but rotor blades slowing
42:00 ‘Aft Cyclic’
44:10 Autorotation training changes
48:15 Completely consumed with the mission and not thinking about engine failure
50:10 The helicopter is happiest in autorotation
51:30 Multiengine helicopters and applicability of ‘Cyclic Back’
53:20 Engine off vs engine idling training autorotations
1:00:20 ‘Aft cyclic’ to be in print, training programs and RFM
1:02:30 Advice on a real world engine failure from Pete
1:08:50 Thanks to Doug Williams
Links from this week’s episode:
Western Helicopters
Marc Philipp Veenendaal’s Instagram account with aviation drawings
Episode Sponsors trainmorepilots.com
Video – Pete Gillies talking to impromptu group about autorotations
[leadplayer_vid id=”56C9A0F1E2306″]
[Tweet “One wing falling off in flight – equivalent to a too low RRPM #helicopter”]
[Tweet “In a real autorotation there is no upper red line on RRPM. Just worry about the bottom one.”]
You can also find more reading on ‘cyclic back’ online: AStar Accident Shines Light On Autorotation Training, AIN Online 2013 , ‘Failure to Enter’,VerticalMag 2013 , Rotorcraft Safety: No Margin for Error, Aviation Week 2014 , Enough skill to kill yourself, 2015
Chapter 43 ‘Autorotation Concerns’ of Helicopter Aerodynamics Volume II by Ray Prouty also cites Pete Gillies when talking about the need to instigate airflow into the disc during autorotation entry in forward flight.
The following is an analogy penned by Randy Rowles, 2013 HAI Instructor of the Year, that provides another approach to discussing the application of aft cyclic in an autorotation entry.
“First, I would like to add to Mr. Pete Gillie’s (Chief Pilot, Western Helicopters) comments regarding the importance of applying aft cyclic when entering an autorotation. As a longtime flight instructor, I couldn’t agree more with Pete as I’m sure most experienced instructors within the helicopter industry would as well. The key to getting the point across on such subjects is providing an example that is relevant to the topic.
I would suggest we provide an example taken from a baseball analogy. Applying aft cyclic when entering an autorotation is a key consideration because you want to present the rotor system to the changing relative wind. This is very similar to explaining HOW to catch a ball in a glove. In simple terms, you MUST present the open glove to the ball for the glove to function.
To properly catch a ball, the glove must be open and presented so that the ball will hit the glove in the pocket. If the glove is face down, the ball will hit the glove and be driven to the ground. If the glove is held upside down, the ball may roll up the glove and hit the person in the mouth, both non-desirable outcomes.
So you see…an analogy of catching a ball in a glove presents the reader with a basic fundamental principle of the interaction of the ball in flight, and how to properly engage the design of a glove to catch the ball. The rotor system works exactly the same way. No matter what the speed of the aircraft, the rotor system MUST immediately be presented to the airflow through the use of aft cyclic during all autorotative entries.”
Custom drawing competition – Final hours
Your chance to win a custom aviation drawing my Marc Veenendaal is almost gone. Leave a comment on Episode 39 about your first helicopter experience to go into the draw.
Do you have an opinion on ‘Cyclic Back’ or additional information to share? Perhaps you’ve had an engine failure and can share how it turned out. Join in the conversation below in the comments
Sorry if this is a little longer than you hoped…!
My first helicopter flight (and second flight ever I think) came about one morning in the October of 1997. I was just a normal 13 year old boy walking the family dog in the half term holidays when I was interrupted by my father who came to pick me up. This was strange as he was meant to be at work that morning but all would become clear very soon.
At the time we lived at RAF Northolt, located in the London Borough of Hillingdon where my father was stationed as a helicopter navigator. This posting was no ordinary posting for him however; this posting was with number 32 (The Royal) Squadron.
My father’s career had seen him start on the Avro Shackleton AEW Mk2 before a ground tour and then a transition to Rotary; predominantly flying the Westland Wessex in the Search and Rescue role.
As such my life up until this point in time had been fixated around helicopters with many a happy hour spent in crew rooms and hangars, better than any playground I had ever been in. I was in awe of these machines and longed to be able take flight and put into reality the sensations that my imagination had conjured up from the times I had spent ‘pretending’ to fly whilst playing in cockpits.
Back to that October morning…my father was clearly in a rush to be somewhere. That somewhere was work, only he needed my mother, sister and I. It transpires that a ‘family’s flight’ had been authorised and keen for us not to miss out he had gone on a foraging mission to round us up.
As we waited for the safety brief it was very apparent that this would be no ordinary flight, or ordinary helicopter. The helicopter that I would take my first ever flight on was the helicopter that remains today to be my favourite, the Westland Wessex. However, this was no ordinary Wessex but one of only two HCC4 variants.
The HCC4 were ordered to equip The Queen’s Flight (which became No.32 (The Royal) Squadron in 1995) and were built to HC2 standard but with the main cabin having a VIP interior finish, furnishings and sound proofing plus an external folding step below the cabin door and additional Decca navigation equipment installed on the flight deck. The Wessex HCC4 was sadly retired in 1998 with No.32 (The Royal) Squadron giving up the task of providing helicopters for the Royal Family. The Royal Household awarded a ten year contract to a civilian entity flying S76’s.
I remember vividly that the hanger floor was buffed to a perfect shine and my father had told me that the steps up to the elevated cockpit of the Wessex were plated in gold, although I wonder if he was pulling my leg. Either way this was not some
grotty flying club but something quite different, quite special. I remember thinking that the lower passenger cabin, where we now sat, was not quite as plush and grand as I thought it ought to be especially considering the people that normally occupy the seats that now supported a 13 year old with a grin that was literally from ‘ear to ear’!
Anticipation and excitement grew as things began to happen, new sounds and smells began to fill the air as my father and his pilot set about their business of preparing this graceful machine for flight. Soon the rhythmic thumping of rotor blades filled all of my senses and as the collective was raised and we became ‘wheels light’ I was in awe. All of these years wondering what it must be like to fly in a helicopter was now about to be answered.
Before long we were airborne and heading east from Northolt for a flight along the London heli-lanes. The engineer in the cabin with us (the HCC4 was normally crewed by a pilot, navigator and ground engineer with the engineer being responsible for the helicopter always being serviceable and to act as a VIP Steward!) gave me a headset briefly to hear what was going on in the cockpit
some feet in front and up from my position in the aft port seat of the cabin. It was somewhat surreal for me as I looked around in wonderment at how my morning dog walk had evolved into sitting a few thousand feet over London in the Queen’s helicopter whilst listening to my father and the pilot discuss the building of a new clothing store at our local retail park in between ATC transmissions and performing checklists. For them it was another day at the office but for me something quite different, an experience I will never forget.
As we entered the heli-lanes soon famous and noteworthy landmarks I had visited many times whilst touring London came into sight; Wembley Stadium, the River Thames, Big Ben, the Houses of Parliament, St Paul’s Cathedral, Buckingham Palace, Canary Wharf and the Millennium Dome which, in 1997 was nothing more than a large circle carved into the ground awaiting construction to start in earnest before its grand opening in less than 3 years time.
Before I knew it we are on approach back into Northolt and soon in the car again to travel back home. I don’t recall much else about that October half term but to this day I remember that flight, made even more special that it was shared with my mother, father and sister.
I now have roughly 260 hours of flying experience of which the rotary time is the time I have the greatest fondness for. I have always dreamt to fly in helicopters particularly as a Search and Rescue Winchman partly due to wanting to follow in my father’s footsteps and partly due to the captivating nature rotary flight has had over me since my first flight.
My dream has so far not come to fruition due to my eyesight however, I plan not to give up and as a qualified paramedic I am hopeful that one day I will become an aircrew paramedic with a HEMS operator and then hopefully a Winchman with the new civilian SAR contract here in the UK.
Thank you for reading.
Great episode. Even tho I am not a pilot, I can certainly see with what knowledge I do have about helicopters and how they fly how applying aft cyclic would make for a safer autorotation. Fascinating stuff.
Another master class from a man with 45 + years of “real world, in the field” experience.
To discuss within any brieffing room in the rotary world.
Hi.. I too agree with the concept but qu also highlight.. Two more things to this…. One….flare effect as we call…as a result of cyclic back will result in increase in air flow leading to increase in authoritative force as a result of conservation of angular momentum and also because of reduction in the downwash on tail stabiliser there will be need to pull cyclic back else nose dips fwd…
I think Pete has got this backwards a little bit. Yes pushing forwards on the stick reduces rotor rpm and pulling back raises it but that’s not how I teach someone to fly past lesson one. Flying is about selecting and holding an attitude/height/bank/etc to achieve your aim not selecting the stick left right or back to make the helicopter move.
When you enter an auto with just collective, the nose will drop which reduces the beneficial effect of lowering the lever when recovering rotor speed. You need to hold some aft cyclic to hold the original attitude. I teach that when your engine fails you control rotor speed. The pilot has several tools to do this and he needs to learn how to use all of them. You cannot do an autorotation with just collective or just cyclic. You need to manipulate them and trade them off against each other to get your desired effect. I have done some avoid curve testing and if you mechanically apply aft cyclic without any regard to what your attitude is doing, you are going to crash, particularly if you don’t do something with the lever simultaneously. You should be teaching pilots to achieve the required attitude/height/heading using the appropriate control but not using the control in a certain direction. For example, during take off in a French helicopter, an instructor should direct the pilot to hold the heading with pedal not hold the heading with right pedal. It develops the right mentwl process. Telling him to apply right pedal on takeoff would lead to issues when changing to an american type. In simple terms Pete has it right in that collective is not the only action but only cyclic is not the way either. As Pete says, the exception to his rule is auto from the high hovers that law enforcement helicopters have to do. Mechanical aft cyclic is not going to help; you have to coordinate the controls on entry to maintain rotor speed; you are going to selecting an accelerative attitude at some point – note I did not say forward cyclic. I showed a pilot a auto from a high hovers just above the avoid curve and he remarked that noone had ever shown him how much height you lose before you are at a landable speed. With aft cyclic only at first it would take even longer
That said, I love the podcast and love the discussion it generates. Keep up the good work Mick (I’m working backwards from episode 44 to catch up)
G’day George! Thanks so much for the input and hope you enjoy the other episodes too.
There is an awesome video about autos here too: https://www.youtube.com/watch?v=YOl2YEH6wFU What I take away from ‘Cyclic Back’ is that maintaining the attitude is not enough. Most of us will have some amount of disbelief and hesitation when the engine stops during which time the RRPM is decaying. Pete is emphasising the immediate reaction of cyclic back to give us a chance to extract energy from whatever forward motion we have at the time to restore RRPM and get the RRPM up above and away from the point it can’t be recovered. So its more than maintaining the initial attitude. ‘Cyclic Back’ provides it as a short memorable instruction.
Agreed though that normally we would refer to the thing we want the machine to do rather than the control input.
Mick, thanks for replying. I flew the full suite of engine off profiles this week (Variable flare, constant attitude etc) and tried to analyze what I did with the cyclic. I couldn’t – I definitely fly by trying to achieve the output through whatever options I have rather than making specific inputs. It really worries me that a baby helicopter pilot might latch onto the “cyclic is the first thing to use in an engine failure” mentality. Its a shame there’s not a cheap high fidelity simulator for this manoeuvre.
Anyway, keep up the good work. Really enjoying the V22 episode. It would be really great if you could get Shawn back to do a chat about how Automatic Flight Control Systems work (from stabilisation through attitude hold to the higher order modes). I wish I’d known more back when I started out.
Great topic idea George. I’ll see what I can do!