Downdraft induced accidents. Busting the myth...

[Learjet]

Downdraft induced accidents. It's got a catchy ring to it. Maybe too catchy...

Looking through the CAA accidents stats one could be forgiven for thinking that "downdraft" related accidents only happen to gyros. In fact you'll struggle to find too many fix-wings or helicopter accidents directly attributed to downdrafts - but yet quite a few gyro accident reports mention encountering "a sudden and unexpected downdraft shortly after take-off" etc etc which resulted in the gyro executing a forced landing or prang. And yet we all brag about gyros having good wind tolerence attributes - so whats the deal with downdrafts? And why particularly so during / after take-off / climb?

Well, the truth is that there isn't really much of a deal about downdrafts. The problem is that when a gyro experiences a high rate of descent - to the pilot this may FEEL like being in a downdraft. Especially if the reason for the high rate of descent isn't immediately clear. And the CAA accident reports dutifully regurgitates this anecdotal speculation as actual fact. Which is a pity... beacuse I'll venture to say that a sudden and unexpected downdraft is seldom the real cause of a gyro mushing into the ground. (hence the lack of similar aerie, trikes and helo accidents due to downdrafts)

Neither is density altitude. Or excessive MAUW. Oh yes- they are definately dangerous contributing factors... but like these mysterious downdrafts, they are unlikely to be the sole cause but merely a link in the accident chain..

So what is the primary cause?

The answer(s) can be clearly found in the FAA Rotorcraft Flying Handbook... and some of these factors are quite unique to gyros:

Failure to lift-off at proper speed

Failure to establish and maintain proper climb altitude and airspeed

and when the airspeed drops sufficiently...

A gyroplane will DESCEND AT A HIGH RATE when flown at very low forward airspeeds. An unintentional high rate of descent can
also occur as a result of failing to monitor and maintain proper airspeed. In powered flight, if the gyroplane is flown below minimum level flight
speed, a descent results even though full engine power is applied. Further reducing the airspeed with aft cyclic increases the rate of descent.

At the point where maximum power available is being used, no further reduction in airspeed is possible without initiating a descent.

Welcome to flying behind the power curve.


Students are taught to fly by the numbers (which is fine in your average plane or heli) but because of the many variables (some typically unique to gyros)....ie. insufficient airspeed at lift-off in ground-effect, high angle of attack climb, using aft cyclic to climb rather than throttle, density altitude factors, take-off weight loadings etc etc... suddenly the numbers become very blurred as to exactly where this "getting behind the power curve" line is to be found in the flight envelope. We don't have convenient stall warnings. And anyway we were all told by the salesman that gyros don't stall. Perhaps so... but put them behind the power curve and they will fly only marginally better and descend just a bit slower than a falling brick.

Clearly this is an area where there is currently insufficient training by many gyro instructors. There. I've said it. Behind the power curve flying... how to recognise where in the flight envelope this devil lives - and recognising when you're close it. And what do to when it happens. Gyro students needs to be made far more aware of this unique gyro phenomenon or they'll continue to mistake it for being in a downdraft... and the ignorant CAA accident report writer will continue to regurgitate the 'sudden downdraft" rubbish... and we'll continue to see gyros mushing into the ground not long after take-off - with potentially fatal results.

So there it is. In my opinion the downdraft myth is busted.

The real culprit is behind-the-power-curve-ignorance and lack of training. No doubt I'm going to come in for a bit of this whith these statements but it will be worth it if one gyro pilot out there encounters that "downdraft"and instead of pulling back on the stick in vain attempt to get out of the clutches of the "downdraft" - and instead lowers the nose, increases airspeed and flies safely away from behind-the-power-curve (and the ground) it will have made his / her day. And mine!

[Low Level]

Good post Learjet

I started my training in the Sycamore with a Woodcomp VP prop that didn't work too well. This was a very fortunate thing, cause I really had to manage to get in the air. This taught me very valuable lessons. I have changed the prop to an Arplast, and the difference was mindblowing.

What I haven't changed is the way I take off. I will use the entire runway to build speed, sometimes even up to 90 mph before I pull the stick back. A real great sentation that climb rate then.

Now if I do not build speed, there is not enough lift, resultant, get back on the ground cause you WILL not fly. Why push it. Fortunately it hasn't happenned yet.

I have been drilled from the beginning that Sycamores do not like to fly, so with that still in my mind, I make sure speed, speed, speed when I take off.

I think with the newer generation gyros, that part is not drilled in the students' mind. Everybody think they can take off and fly, whatever the conditions, and the pilots want to show off. My gyro can take off in 60 meters, but at what risk.

I say part of the training must be to take off and fly without the use of the turbo. If you cannot get off the ground with 100 hp, stay there.

P.S. ...and I have heard someone say afterwards, look how the Sycamore battle to take off, cause I was using the whole runway.

[Johannes Cronje]

Learjet

Thanks for that, I see it as a clear and well worded answer to my question in this viewtopic.php?f=20&t=12131&start=30 topic.

[Low Level]

I have seen three attempted take-offs behind the power curve, with gyros at Rhino park. Fortunately in all three situations the pilots realised it, and checked out before it was too late. Three different pilots, three different makes of machines, and no, not one was Sycamore.

That is the nice thing about a gyro, and it MUST be taught. You can bail out at a very late stage and be safe. I remembered Pierre asking the question when he started training, cause somebody told him you get blade flap when you bail out. Just shows the ignorance out there.

[t-bird]

Hi Learjet

Nice post

I have ended up once behind the power curve.
Pitch of prop was too coarse and engine could not get to 5000 RPM ,heavy on a hot day at altitude.
We didn’t take long to get into the air but we just could not gain speed and get out of the ground effect.
Running down the runway trying to convince myself that speed will pick.
Then try to climb out at 50 mph but could not maintain it once out of ground effect climb like a grand piano.
Had to put it back on the ground in a huge cloud of dust.
Nothing got hurt except my ego.

Cource pitch ,weight and density altitudes were factors, but continuing and ending up in the grass were pilots error.

[Gyronaut]

Good one Dave!

I agree with you entirely! Speed speed speed.

Learjet

Thanks for that, I see it as a clear and well worded answer to my question in this viewtopic.php?f=20&t=12131&start=30 topic.

In that topic, notice Frans says things like... "By now my airspeed also diminished to a point i did not like" and "Just before i put her down i am sure i was behind the power curve.I might have been able to give more nose down..."

Bingo!

As long as we all learn from the experiences and each other its all good!

Fly safe, fly lekka!

Len

[Learjet]

Thanks for the positive feedback guys.

And before I sound too lecturing or sanctimonious. Yes it nearly happened to me too... !!!

Looking back now with the benefit of a few more hours... it's even clearer. Inadequate airspeed during lift-off in ground-effect and very nearly getting my arse caught behind the power curve...

viewtopic.php?f=1&t=8516

I had fairly recently qualified on my gyro and decided to fly to the Piketberg Fees. Weather was perfect and softness and I soon found ourselves touching down on the local golf-gourse fairway which had been roped-off for use as a make-shift micolight landing strip for the duration of the festival.
The day turned out to be a scorcher - and by 3pm when we decided to depart (so as to still have plenty of daylight at our home field for landing), the temperature was still in the 30's!
Lining up on the long fairway I was quite happy that there was more than enough usable runway to compensate for the DA / heat. Started my take-off roll with a light cross-wind breeze, rotor rpm came up nicely to 220 and the nose-wheel started to lift...
Eased the stick forward to build up more ground-speed before gently pulling back expecting to lift off...but the main wheels just kept glued to the grass! Now I could see the putting green or whatever you call it with the little flag-pole at the end of the fairway approaching and with my hand a mili-second away from closing the throttle and aborting, the wheels lifted and we were airborne. But not out of the woods yet... even with the throttle aginst the stops and turbo engaged the gyro seemed reluctant to climb out of ground effect and we passed over the flag pole and the golf course boundary wall with a lot less clearance than I'd liked to have had. Recognising that I was perilously close to getting behind the power curve I wanted to push the nose down to get airspeed, but doing so meant sacrificing altitude - and I had precious little to give. Turning into the wind would provide more lift but the golf-course was situated on a mountain slope and that would mean turning into the rising slope. Straight ahead, the flood-light towers and trees at a nearby sports field were looming and the only option left was to turn downwind, push the nose down and thank my lucky stars that the downward sloping terrain fell away more quckly than my dip in altitude as I gathered airspeed and got everything back on track! Softness, oblivious to my sweaty palms could only comment as to how nice it was that the people waved as we zoomed over their heads...

BIG Lessons learned:
1. DA bites hard. And even though I was happy with my DA calculation I hadn't taken into account...
2. The rolling resistance of the fairly thick fairway grass!
3. Substantially less lift created by a cross-wind as opposed to a headwind.
4. My lack of having not properly determined an abort point prior to commencing my take-off roll.
5. My lack of having identified "escape routes" prior to take-off.

[franss]

Hi Learjet,
Mabe it is time to revise the calibrated and generic way of training.Gyros differ and therefore the method in training must be different.What Jetranger is doing is a step in the right direction,and he is probably going to get some stick for it,but for the sake of safety we cannot stick to the system because that was the way our forfarthers did it.The dangerous time in flying is probably the time between qualifying and experience.Exponential function.I have been trained to not exceed 60mph on the runway,and my instructor will kill me for saying this,but i just do not feel safe doing it that way.I also agree that the DA situation should be broadend in both the thoeretical and practical syllabus.Flying behind the power curve in flight is another example.Iam talking about in flight and not during take of and even landing.Flying circiuts with touch and go almost always brings you behind the power curve every time but at least now you expect it and know what to do.The better the training the better the pilots.
franss
Sycamore Eagle Mk 1
ZU-CCN
Rhino

[Eggbeater]

Hi All

When I was training we occasionally used to take off from a wheel balance by adding power until the mains unstuck and then lowering the nose to gain speed before climbing out. The problem is that you forget and I did until it was reinforced by a few lessons again - taking off downwind (slightly) close to noon at Kitty.

JetRanger, is this the sort of thing that you had in mind?

[Gyronaut]

Yes Eggbeater, its the same kind of thing. The way I like to do it is to climb to 1500' agl, (do the HASELL checks of course) and ask the student to maintain a constant altitude at different airspeeds without exceeding 29' Manifold Air Pressure. (The speeds I quote are examples only and of course differ from machine to machine, weight on board and DA bla bla...) He quickly discovers that at 70 all is well. at 55 he is climbing and needs to reduce power... at 90 he begins to descend a bit... at 65 all is well again at 30 he simply cannot hold it at that altitude... then I ask for 20 and allow him to roll on more power ... and more while holding it at 20... eventually he is sitting at full power and yet still descending positively unable to maintain altitude. Back to 29' MAP, ease the nose forward to 55-65 and well I never, suddenly he is maintaining altitude or even climbing again. The penny seems to drop then and the significance of airspeed management is understood. Its a fallacy that Gyro's can't stall, they just do so gradually and elegantly.

Another good exercise, once the one above is done, is to do a relatively flat (low) final approach, limit the MAP on final approach FORCING him to find the best glide speed or he simply won't make the runway at the given power setting. Once over the runway, in ground effect at the same power setting you are flying along merrily at 3 or 4 feet above the runway, now slow him down gradually telling him NOT to land and VOILA... he ends up doing his best landing yet since the slower he flies the more he descends and pulls the nose up instinctively... eventually the gyro simply cannot and will not fly anymore and gently sinks onto the runway; sits there with the nose wheel off the ground, engine power on and stick way back. (of course a bit of a headwind down the runway helps).

I know I have waffled on a bit and bored most of you (forgive me) but I hope I have managed to make myself clear?

Fly safe and have fun!

Len

[M I Claase]

RAF 2000 GTX SE F1
The following is my opinion based on experience.(Hope it make sense)

I can not agree more with Jetranger.

Downdraft, beyond the power-curve is just another name for "stall" which is defined as; A malfunction in the flight of an aircraft in which there is a loss of lift
that results in a downward plunge. To me this means a gyro can stall. When a wing shaped section (aerofoil) moves through air in a straight line (not forgetting the angle of attack), lift is the result. Once the airflow (speed) is increased more lift and vice verse.The moment you start to lose airspeed you increase the straight line angle to maintain lift up to a point where drag and weight has the upper hand resulting in a plummet to earth and this phenomenon is called "stall". I can hear gyronaughs say my rotor speed was 335 rpm (as for the RAF at 6000 ft) how can speed be a factor? Remember once you are falling out the sky your forward speed and downward speed affects the line of airflow over your rotor aerofoils effectively stalling it! Thus when you are doing a hover maneuver it is only the "stall" drag that limits your fall speed which increases the longer you sit there and has to be managed very carefully.( If you are doing it in a fairly strong wind and with enough altitude you will be flying backwards and extra care should be taken to get out of it). Although a gyro rotor blades are set at an fixed angle the airflow angle over the rotors can be managed with stick and power within a envelope suited for each situation and gyro type.To get out of a "stall" situation you need forward airspeed at the correct angle (any angle before the "stall" angle,the further the better if you have altitude!) by pushing for nose down with controlled power application or plenty power if you do not have altitude! All movements must be smooth or it can cause other less manageable situations such as blade flapping,to steep descending angle resulting in over speed in forward motion and rotor over speed when you pull out to hard!

Regarding take off; I don't believe ground effect plays such a big role as energy management does.

In my opinion the moment you rotate is the most dangerous as this is the point where every thing comes together! By this I mean everything! From planning your trip, preflight,fuel, all up weight,weather(including DA),wind direction,oil pressure, airspeed,engine rpm,etc. All the forces off lift,drag,weight and thrust comes into harmony and if planned correct you will take off and do what only birds are supposed to do but if planned incorrect this is the point where you wil find out what you did not do!
Getting to the point. While you are doing your take off roll, after pre rotation, the gyro weight are supported by the main wheels and the result is insufficient rotor speed to create enough lift to support weight for take off. Your airspeed hits 60mph you start pulling back on the stick and wallah you are airborne by 2-3 meters. But all of a sudden you start falling (stalling) back to earth. Airspeed has fallen from 60 to 50mph and rotor speed jumped from 260 to 330 (your forward energy gets taken up by your rotor to support the weight and if your blades are longer and/or heavier this change off energy is more speed consuming!) which puts you in a downward plunge (as defined in stall), called stall, and now only power or a downhill will prevent you from going back to the runway!
Bearing in mind the above I totally agree with Low Level rather make your take off roll longer and safer.Avoid pull back, rather take the forward pressure off the stick as you get to your rotating speed and energy transfer will be a lot smoother. Try impress only yourself because every take off to me is impressive whether I'm in the airplane or just witnessing it.Every pilot has his or her envelope within the airplanes envelope and the closer these envelopes comes together the closer you are getting to disaster!

[Learjet]

Welcome M I Claase! - and thanks for the great introductory post. I look forward to reading more.

[FO Gyro]

Guys I am enjoying the new posts here, and welcome to MI Claase, but I think we should try and keep our posts accurate, otherwise those that read these forums will come away saying gyro's stall. Cierva would be horrified, and would turn in his grave, if he knew that gyro pilots said his gyros stalled!

A stall is defined as a reduction in the lift coefficient generated by an airfoil as angle of attack increases. By definition, this occurs when the critical angle of attack of the airfoil is exceeded. The driving portion of the rotor blade definitely never reaches a point where the critical angle of attack is reached, even in a vertical descent with 0 airspeed (the angle of attack only increases by around 2 degrees in a vertical descent!).

A stall in a well behaved fixed wing, that doesn't drop a wing, and simply mushes, might seem to act like a gyro when flying behind the drag curve, but the two aircraft are in two totally different states. The fixed wing is hanging on the prop, with the boundary layer breaking away early on the top surface of the wing, and is stalled, with a huge loss of lift, whereas the gyro is still flying. A stall also brings about with it a sudden change in the flight control response, and how the stick feels. A stall is an uncontrollable state, whereas a gyro flying at say 20 mph, is fully controllable.

Gyro's can stall, but this type of stall is not through slow airspeed, but is caused by unloading the rotor, allowing the rotor RPM to decay, and then suddenly pulling back on the stick to load it again. If a gyro had to perform a loop, this is what would happen, and the results would be unrecoverable. In normal flight, from zero airspeed, to max. speed, a gyro doesn't stall.

I agree very much that a lot of gyro pilots don't pay enough attention to their speed, and get behind the curve when heavy, and close to the ground. Taking off from too short a field, when the density altitude is high, eats up runway quickly when trying to accelerate to your climb speed.

As gyro pilots, let's not lower our standards and say our gyros stall, but rather admit that speed control is very important as failing to maintain the proper speed will result in the gyro settling back to the earth again, without achieving the climb performance we wanted. Keep the posts coming...

[FLYNOTE]

Fantastic work Learjet ! This is a fascinating topic and you have it on the button. Well done also FO. We need more of these conversations . I am an 'expert' on ending up behind the power curve , I have to admit to my embarrassment ... you know all about my Bushveld 'landing' previously related ! Eina ! Your posts will highlight the importance of being aware that a gyro bites if you are not aware of this critical danger and obviously to understand what this is all about and how easily it can happen. Forewarned is always forearmed.
Everyone also aware of virgo and microbursts which effect can maybe be interpreted as 'downdrafts' in your aircraft ?
Maybe worth a mention / reminder in this discussion.
"Microbursts are downdraughts that accelerate to the surface from the clouds. As precipitation evaporates, it cools causing it accelerate downwards. Once it reaches the surface, it has to spread out. The result is a toe-shaped outward flow along the surface which is visible in wet microbursts (precipitation mixed with dust). During dry microbursts, only dust is observed blown outwards. Some microbursts can be very severe causing aircraft to crash when landing or taking off."
On especially the Highveld when there is cloud around , look for areas under the clouds where you can see precipitation. You sometimes see the rain falling from the cloud but it evaporates before hitting the ground.This is called virgo and is due to the warm air evaporating the precipitation before it can hit the ground.. So underneath that cloud we have a big helping of cooled down air due to the evaporation and cold air goes down ... and if you are in that vicinity you go down with the air ... right down to the ground.Stay well clear of virgo on a hot day !! Do not attempt to fly near that area. The effects of microburst has pushed huge blik aeries into approaching terrain or the ground on take off. ( FO , remember the Boeing at Windhoek many years ago ? )
Happy flying !

[Gyronaut]

And here I thought Virgo () is the sixth astrological sign in the Zodiac? Wys jou nou net mens is nooit te oud om te leer nie!

By the way, I always thought these microbursts were inclined to dissapate very close to the ground due to 'backpressure'? I know one can't count on it but isn't it possible that once very close to the ground one might find the phenomenon to have less effect? If it does happen and you cant get out of it... Fly the machine as far into the crash and see I suppose...?

Good posts LearJet, Flynote and everyone else.