Turnbacks

[Genghis the Engineer]

Just to clarify what I mean by "turnback" it's where following an EFATO (pretty much by definition in a single engined aeroplanes) the captain elects to try and return immediately to the take-off runway, landing in the downwind direction.

I've only ever attempted them under (relatively) safe conditions, for my own interest - generally it's left me with a clear view that it's not a manoeuvre I'd attempt under most circumstances. I've however quite a few times had occasion to observe the consequences of a turnback - which invariably seems to have involved destruction of an aircraft (although rarely loss of life).
Working on analysis of yet another case (and yes, yet another destroyed single engined aeroplane), I've been pondering. Has anybody ever done some serious formal research on the turnback? - how best to fly it, go/no-go criteria, relationship to aircraft characteristics? If so, can anybody point me at any publications describing formal turnback investigations?

This is, I should emphasise, pretty much just personal interest in the topic - not any particular case (yes I am working on something at the moment, but no the turnback performance wasn't really relevant to the outcome and anything I learn now will almost certainly have no bearing at-all).

For that matter, is anybody (well, any full time aviation organisatio) anywhere in the world still advocating turnbacks as a standard post-EFATO action?

G

[Genghis the Engineer]

Ahh, I'd forgotten starting a thread on this five years ago (grief, I need to get out more):

http://www.pprune.org/forums/showthread.php?t=66757

However, I don't think it changes my current interest.

G

[D120A]

Transport Canada has been active on this subject, and there is a civil aviation document TP13748E 'An Evaluation of Stall/Spin Accidents in Canada 1999'. It's referred to on the internet in the context of turnbacks, but it's not explicitly available on-line - so it's a suitable challenge for a university library...

I have in my papers somewhere an early 1990s Canadian article on turning back, which contained some numerical analysis and proved to my satisfaction that turnbacks were all but impossible. Now I regard even tree-tops ahead as a better bet.

I shall not rest now until I have found it, and when I have I'll send it to you.

Graham P.

[bushy]

I believe the PC 12 operators look more favourably on the turnback situation due to the better rate and angle of climb of these aircraft, giving a better margin. Obviously there must be a minimum turnback altitude, but in some situations it would be less suicidal than it has been in the past.

[212man]

This a topic that crops up frequently in the GA press, and I well remember writing to Pilot magazine in 1993 in response to a long article that proved it was an impossible manoeuvre!

My only experience of them is on my UAS days flying Bulldogs. It was a standard training exercise for the more advanced students (I was doing them from about 40 hours), and the QFIs were tested on them during their trappers visitis, I seem to recall.

It was only advocated for runways where an EFATO would put you into the spare bedroom of 13 Acacia Avenue! One of the key elements was that it was part of the T/O brief, so was not a spur of the moment decision, and the direction of turn would be briefed as into any crosswind, to minimise the turn radius. The absolute lowest height to attempt it would be 450 ft agl and the aim was to return to the airport environs first, land back on the reciprocal runway, second.

The actual handling of the manouevre required precise and prompt input, and as I recall was something like this (20 years old memory so bear with me!):

Smartly lower the nose to 'nail' Vy
Transfer to instruments and roll to 45 degrees of bank
Maintain an accurate Vy and 45 degrees on instruments for the first 90 degrees of turn
After 90 degrees of turn, ease bank off and visually assess likely landing site
When lined up/wings level consider some engine shutdown drills, but concentrate on the landing


I seem to recall there may have been mention of flaps to 'inter', and even 'nibbling the light buffet', but can't swear to it.

One thing is for certain: if an average PPL decided on the spur of the moment, following an EFATO, to attempt the manouevre, without any prior practice or training, it would most probably end in tears!

[JimEli]

http://jeremy.zawodny.com/flying/turnback.pdf

[cwatters]

I imagine the problem it forcing yourself to fly fast enough with the ground rushing up. Should brief what parts of the airfield or surroundings are landable, just because you took off from a paved runway doesn't mean you have to struggle to get back there. I'm only an ex glider pilot from a big field so I usually had options. Important to pull the cable release not the airbrake in the heat of the moment though :ooh:

[Genghis the Engineer]

Damn you guys are good!

That paper by David Rogers is printing as I type - something to read over lunch today I think, and I shall try from my end to get hold of that Transport Canada document.

A note to 212man; the RAF I know used to teach turnbacks as a matter of course. I think that these were finally killed off circa 1994 after a Hawk with a QFI+QWI crew with about 6000 hours between them landed on a wingtip at Valley at the end of a practice turnback. This led to the realisation that HMQ was losing more jets practicing turnbacks than she saved by having crews current in the practice and I believe they were banned. I'm not sure how this related to Bulldog practices (possibly your local OP was based upon minimising risk to third parties?). But, one point you mentioned rings true with my own experiments - I've found that nibbling the stall warning always gave the least poor chance of getting away with it. That seems to be borne out by the conclusions in Rogers' paper too, who advocates turning at 45° of bank and 5% above the stall speed in the turn.

G

[212man]

GTE, the paper bears out the practice nicely. I last flew one in 1987, so not sure what happened later, but it wasn't a local procedure, as far as I recall.

Bearing in mind that by that stage we were 'spin/aeros' checked and were familiar with sensing stall bounderies (without a warning buzzing away in your ear: disabled) I'm fairly sure now that we were taught to 'nibble the buffet'. Not all students were taught and it wasn't in our syllabus, but certainly the QFIs were expected to remain current.

I imagine the problem it forcing yourself to fly fast enough with the ground rushing up. Should brief what parts of the airfield or surroundings are landable, just because you took off from a paved runway doesn't mean you have to struggle to get back there

Quite, one of the reasons for going to instruments for the first half of the turn a) minimise height loss by flying accurately b) minimise risk of stalling!

The intent was not purely to get to the runway, and it would not be contemplated if there were decent areas beyond on which to force land (as there were from some runways.)

[Genghis the Engineer]

An observation there. I'm not sure that I remember the Bulldog's stall warning ever being operable - but it did have fairly clear natural stall warning. That may however not be true for all aircraft.

G

[Shaft109]

Having started off on the Vigilant (Grob 109) we were taught turnbacks but they were:

A. planned out for each runway and shown during briefing- 2 runways so four directions that were:

B. graded specifically on height and wind speed direction at the point of EFATO e.g. minimum 300' still air, min 500' wind above 10 knots.

C. as an FSC I was once made to carry through a practice turnback and land in a light tailwind to show it is tricky.

D. captain's discretion - you were not expected to slavishly follow them if it was gusty for example.

E. I'll have a look but the FRC's refer to using the MINIMUM angle of bank to minimise height loss something like 10-15*.

F. I found it very hard not to perform an immediate turnback when I started on the PA28 - those drills were still a sharp instinct 8 years on!

G. Feather the Prop in the subsequent actions.

[212man]

I'm not sure that I remember the Bulldog's stall warning ever being operable - but it did have fairly clear natural stall warning.

It was disabled, hence my comments about being able to judge stall bounderies without the aid of something whining in your ear.

['India-Mike]

What an excellent thread, enjoyable read and the pdf document link is excellent too.

I'm part-way through an FI course. The turnback was a demo and practise on my pre-course entry 'test'; and it's been done once more at the end of one of my air exercises both at height, and in the circuit.

My FI instructor wants me to experience the turnback for two reasons: to sharpen my handling skills; and so that I can talk from a position of strength to my students to give weight to what will be my advice of 'don't try it'. Fair enough.

It'll never, ever be used by a PPL with any success. The vast majority I'm sure won't have the handling skills, or presence of mind to do one successfully. More to the point there's no performance data in the flight manuals regarding turnbacks; there's nothing like the CAA Safety Sense leaflets for turnback guidance (these UK CAA leaflets are great for giving you factors to apply in a variety of take off and landing scenarios) which would give info such as increase airspeed by 'x' or never exceed bank angle 'y' or perhaps most importantly, what to do in wind. The above pdf is great for aero engineers like myself or Genghis, or tp's or the very rare flying techno-anorak. But a ppl won't go anywhere near it - nor will most FI's.

The turnback is all-or-nothing. If you don't get back to the field with it you're arguably in a much worse position than a crash straight ahead. Ground speed in any sort of wind will be greater. Eg in a 10 knot wind I'd rather crash straight ahead at a groundspeed of 30 knots (I fly a DHC-1) than turnback, fail to get to the field, but crash at 50 knots -

[BOAC]

Most definitely a potential 'killer', one of the major problems being the loss of airspeed going from a headwind to a tail and the tendency to stall in the turn therefore.

Nothing wrong, with a QFI onboard to show you what it is like, but keep the carb heat on and a hand on the throttle:eek:

As D120A and others say, there is normally a better option ahead, +/- 45 deg, unless you are in a high-power machine when you may well have enough altitude. They were always exciting in glider training after a lowish cable break.....................

[ShyTorque]

212man, your recollection of the Bulldog turnback drill isn't quite correct. (I still have my QFI CFS lesson plans and copy this directly from them):

Under the "Stalling in Manoeuvre", Advanced Turns (2).

(6) TURNBACK (n.b. This being the final part of the lesson)

LOWER NOSE TO 80 KT ATTITUDE
SMOOTH AND POSITIVE 45* ANGLE OF BANK
LEVEL WINGS ON RECIPROCAL
FLAP A/R
MIN HT 350' AGL.

I think at Vy you might well come a very nasty cropper, hence this being included in the "Stalling in manoeuvre" sortie.

Yes, we used to practice them and be standardised on them; I used to brief EFATO drills for every takeoff and although the turnback option (or not) was included in the brief, I would be reluctant to do one for real if there was any other option.

[ProfChrisReed]

BOAC wrote:

... one of the major problems being the loss of airspeed going from a headwind to a tail ...There's no loss of airspeed turning out of a headwind to a tailwind. If there were, you couldn't fly a 360 at height without constantly changing attitude to maintain a constant speed, 'cause there's head and tail wind there as well.

However, if you turn back in these circumstances your groundspeed increases markedly (by 2 x windspeed once you've completed 180 degrees). As all this is happening close to the ground, and is thus very noticeable, the natural tendency is to pull off the apparently excessive speed. Result, tears all round.

At height you don't notice the apparent speed change in relation to the ground, and so this doesn't happen.

[John Farley]

There's no loss of airspeed turning out of a headwind to a tailwind

A bit sweeping and does not apply if you change heading very rapidly. The numbers matter.

JF

[john_tullamarine]

I have a couple of links to papers by Rogers on the tech log sticky which may be of interest ..

[pithblot]

During 2FTS the RAAF used to brief the manouvre for every T/O in the Macchi. I don´t remember the details - maybe someone Dunnunda in Godzone knows. I wonder if the RAAF still do turnbacks in the PC9 or Hawke?

PITHBLOT

[27mm]

Some time ago on the RAF Hawk, we briefed turnbacks at Valley, but only after achieving a minimum of 250kt - and also Valley was fortunate in having an alternate runway not too far round from the main.

[212man]

Shytorque, without trying to make excuses (apart from the memory loss!) I thought Vy was 80 kts. Now I'm curious: what was/is it?

[actus reus]

I think there is a great deal of discussion that could be had re a loss of airspeed turning out of a headwind into a tailwind. Apart from the obvious discussion of CAS versus EAS and the ramifications of very slow speed aircraft such as a First World War type, there is the issue of slip and skid, whether the aircraft is in or out of balance, whether the aerodynamic forces involved are in equilibrium and, not a insignificant factor, the rate of turn used. Instantaneous effects, regardless of the overriding criteria of 'airmass movement' versus 'ground observation' will show that the airspeed will change during such a turn. If you take the manoeuvre to criticality, i.e approaching the 1G stall speed, in a twin engined aircraft with the critical engine (the engine inop that requires the highest speed for controllablility) shut down; I think you will very well find a decrease in airspeed turning out of a headwind. All thoughts welcomed.

[Flash2001]

Neville Shute Norway in his autobiography gives an interesting account of F/L George Stainforth, who had been hired to test fly the first Airspeed aircraft, sitting for hours in the cockpit prior to the first flight asking the same questions over and over again and running his hands over the controls repeatedly. The Airspeed group actually became rather uneasy in the situation and thought perhaps they had selected an unsuitable test pilot. On the first flight the engine stopped at less than 500ft after takeoff, F/L Stainforth did a 180 and returned to the field for a perfect downwind landing. Then they figured it out.
After an excellent landing you can use the airplane again!
(and Airspeed did)

[Megaton]

In a previous life I used to occaisonally back-seat while the pilots practised turn-backs. I seem to recall that certain combinations of height/speed/wind in the Hawk would provide insufficient energy for a safe return to the field. It was always interesting and sometimes exciting to watch :-)

[tonker]

The RAAF did indeed look into this matter and there is a document on the net somewhere detailing the results of their experiment.

In short 3 of the 5 test pilots on the programme died during the trials.

[H Peacock]

Turnbacks were (are) routinely practiced by QFI/QPNIs at Linton in the Tucano. You needed to have achieved a speed/height combination (130kts/500ft from memory). Without this you simply did not have the energy to complete the manouvre and still have the energy to safely eject if needed!

Options available included a 'dumb-bell' back on to the reciprocal, or a less agressive turn onto finals for another runway. The former involved a hard turn (45aob), invariably away from any crosswind (to help blow you off the centre-line) prior to a swift reversal onto the the reciprocal. (A bit like a procedural turn!). In the Tucano a top-tip was to take mid-flap ASAP to arm the stick shaker and give a slight increase in performance. Needed plenty of awareness re: speed, height , bank, ROD, seat parameters etc. The aim was simply to see if a safe landing could be made rather than jettisoning the aircraft! Landing ahead in a field was not recommended.

Another option was to simply fly a 180 as discussed by others, but this was rarley practiced due to relative position of the ac v runway. It was only an option when the ac was well upwind or had more energy when you could glide ahead for a few seconds prior to initiating the turn.

Take-off brief included details about the turnback to be flown if required. You needed to think about the planned departure; you were often planning to turn at 500ft anyway, so this needed to be included in the turnback plan. Have heard some people call it a turnback only if you go for the reciprocal rather that an alternate runway.

Finally...

There's no loss of airspeed turning out of a headwind to a tailwind. If there were, you couldn't fly a 360 at height without constantly changing attitude to maintain a constant speed, 'cause there's head and tail wind there as well.

However, if you turn back in these circumstances your groundspeed increases markedly (by 2 x windspeed once you've completed 180 degrees). As all this is happening close to the ground, and is thus very noticeable, the natural tendency is to pull off the apparently excessive speed. Result, tears all round.

I'm with JF - I don't agree! Wish I could prove it with the numbers, but I can personally testify that, in a light/slow aircraft, a slick turn onto a downwind trk will temporarily reduce your IAS and not instantly increase your gnd speed.

:)

[rotorfossil]

Having practised turnbacks as a routine exercise as a QFI in the RAF, I have always had the thought as a possibility in later life, but practices (at a safe height) in draggy light aeroplanes convinced me that it wasn't a goer. However, for the sake of interest, I investigated the technique again as a part owner of a Europa. I found that in this slippery aeroplane, it was perfectly viable given certain criteria as mentioned by previous guys above.
You must pre brief before takeoff as to the lower and upper height limits and the direction of turn. These vary considerably depending on the wind but roughly in the region of 600 - 1200 ft for this type.
Looking out in the initial stages of the turn is not good news due to the apparent slip and acceleration over the ground. Once round the problem is nearly always a tendency to overshoot rather than drop short.

[ShyTorque]

Shytorque, without trying to make excuses (apart from the memory loss!) I thought Vy was 80 kts. Now I'm curious: what was/is it?

Do you know- I can't remember it and can't find that figure in my notes - which are now some 18 years old! Having said that, turning at 45 degree AOB increases the stalling speed by a factor of approximately 1.2 hence the need for an increase over normal glidespeed.

I don't think "Vy" is the correct term to use for a glide because it refers to best climb speed. :)

[Flying Binghi]

This may be simular to an EFATO turn back and what may be encountered -

When I do fixed wing or Heli cattle mustering, I encounter what I think of as a rolling turbulence (Roller) that travels with the prevailing wind.
In the Super cub, when I do a come back turn to keep the pressure on an animal,[ once I have the aircraft pionted down hill ] I sometimes get the down side of this roller and see the effect of a rapid increase in ground speed and desent, yet a fixed airspeed (Due to the position of the main herd and other facters, a musterer can not always operate into wind)
What I do in windy conditions to compensate for rollers and wind shear, is to manuver at a faster airspeed, then if all is right when pointed down hill, I do a full slip to slow down if needed.

[172_driver]

I used to believe myself turning from headwind to tailwind would result in a marked decrease in IAS, but this is a common mistake. The only loss in IAS results from increased induced drag during the turn.

No matter how rapid your heading change is when turning head- to tailwind there will be no decrease in IAS. Your airspeed doesn't change in relation to the cube of air you are in, only to the ground beneath you.

In order to understand this fact (and I still doubt myself sometimes) I imagine the cube of air being still and the ground map moving like on a rollerfloor below me. No matter what my heading is the air around me doesn't move and IAS remains the same. On the other hand, my speed in relation to the ground does vary with my heading.

[John Farley]

I72 driver

Sorry – not very fair of me to set you up like that. Well not you particularly but just anybody who came out with that sort of standard comment.

I said it depended on how quickly you turned and I stick by that. If your mount is a 172 you are quite correct. However some other aircraft have very different capabilities which can change the situation.

Take a Harrier on a heading of north doing 60 kt IAS on a flat calm day. Now change you heading to south in three to four seconds (easily done with full rudder I can assure you). I think you will probably be able to see for yourself that you will end up not with less than about 45 kt of backwards velocity (a bit more than a ‘reduction’ in IAS) because the drag that acts for that brief period of sideways flight is fighting the momentum of eight tons of aircraft doing 60 kt and does not represent much of a 'brake'.

Do this same downwind turn manoeuvre into an original headwind of say 30 kt and you might even die should you let the nose drop a tad so that the backwards IAS can get under your tailplane and blow it over your head.

BOAC knows ‘cos he was a Harrier pilot too.

Regards

JF

[Islander2]

We are dissuaded from turning back following EFATO because experience has shown this all too often ends with stall/spin fatalities.

Am I alone in thinking that, in advising this maneuver be flown using a 45deg banked turn at 5% above the stalling speed, Rogers must therefore be completely barking?

Whilst some pilots may remain calm and collected following an actual EFATO, thereby fitting the profile of the simulator test group that supported the feasibility of his recommendation, I suspect the vast majority would exhibit varying levels of panic and be unlikely to fly with a very high degree of precision. Is it a terribly good idea, then, for pilots to be taught to turn back flying a profile where, using numbers appropriate to Rogers' Bonanza example, a stall would follow from them getting the speed low by 4 kts or from overbanking by 5deg?

I do believe that a turn back plan is good to have in mind ... but one where the numbers are based on Eckalbar's 45deg bank at 1.3x Vs45deg (which Rogers rubbishes as "from the popular aviation press" and "demonstrably incorrect") strikes me as infinitely preferably to the truly dangerous approach proposed by Rogers!

[Islander2]

JF wrote:
Take a Harrier on a heading of north doing 60 kt IAS on a flat calm day. Now change you heading to south in three to four seconds (easily done with full rudder I can assure you). I think you will probably be able to see for yourself that you will end up not with less than about 45 kt of backwards velocity (a bit more than a ‘reduction’ in IAS) because the drag that acts for that brief period of sideways flight is fighting the momentum of eight tons of aircraft doing 60 kt and does not represent much of a 'brake'.

BOAC knows ‘cos he was a Harrier pilot too.
Not that I was Harrier pilot (although I was a design engineer at Hawker's during your era, JF!), but do you not think your momentum example applies equally in turning upwind? Just posing the question!

Regards
Islander2

[172_driver]

JF, now I see what you mean but then we aren't talking about the wind anymore, are we?

I'll try to turn around quickly next time I ride my Cessna... maybe if I pull really hard? :ok:

[H Peacock]

So guys (specifically the increase groundspeed - don't lose airspeed - when you turn downwind brigade). Tell me why you think you feel any turbulence in a gusty wind? If your little box of air keeps moving about then surely your aeroplane inside does???


Turning a slow Harrier, or helicopter downwind is still flying. The 'problem' is that they can both achieve high rates of turn and fly at a low IAS at the same time, so the problems of losing IAS when turning downwind are far more pronounced.

:ugh::ugh::ugh:

[212man]

invariably away from any crosswind (to help blow you off the centre-line)

Surely if you are trying to return to the runway, you want to minimise the turn radius and therefore the number degrees of turn required to return. In a utopic world you would turn about the centre axis and point back at the reciprocal heading.

[Islander2]

H Peacock wrote:
so the problems of losing IAS when turning downwind are far more pronounced.You were doing fine, until you got to that bit. Maybe the head-banging, though, suggests you won't want to re-consider!

There is no difference between turning down upwind and turning downwind at a given IAS. In both cases, a high rate of turn will result in loss of airspeed due to inertia and/or increased induced drag (the amounts of each depending on how the turn is achieved), and the loss will have exactly the same magnitude in both cases.

For conventional fixed wing aircraft flown according to IAS, there is no difference between upwind and downwind turns in steady wind conditions.

Harriers and helicopters are different! A hover stationary with respect to the ground is the extreme example. Turning infinitely rapidly from a 30 kts headwind to a 30 kts tailwind produces a 60 kts loss of airspeed. Vice versa produces a 60 kts increase in airspeed. This is the effect JF got very accustomed to and very much needed to concern himself about with respect to the wind getting under the tail. BUT, the IAS is completely different in each case. Turning downwind, you're starting with an IAS of +30kts. Turning upwind, you're starting with an IAS of -30kts. Try this now without keeping the Harrier (or helicopter) stationary above a point on the ground, and arrange for it to have a +30 kts IAS in both cases, and the loss of airspeed will now be 60 kts whether turning downwind or upwind.

[Genghis the Engineer]

Am I alone in thinking that, in advising this maneuver be flown using a 45deg banked turn at 5% above the stalling speed, Rogers must therefore be completely barking?


I was reading through it yesterday, and it did strike me as a nice bit of theory mercifully untainted by too much contact with the real world.
Whilst without doubt, it's a useful start to any analysis of the problem, and that you'd not really want to go below 1.05Vs - one's ability to fly this would depend a lot upon the aeroplane. I dont know the Bonanza personally, but if taking (say) a PA28 or Hawk you'd probably get away with it - the combination of high stick forces, clear audible stall warning at about that margin, and benign stalling characteristics should allow you to nibble the stall effectively. On the other hand, there are aeroplanes with poor stall warning, low pitch forces less than glaring stall warning, and less than benign stalling characteristics that I suspect would offer much too high a probability of crossing over into the stall - most likely fatally.

Oh yes, and how accurately can somebody hold a precise bank angle? At low level, under stress, and whilst keeping looking out of the window most of the time.

I certainly hope that if I'd been asked to referee that paper (I wasn't), I'd have had the sense to ask for inclusion of a few test points in support of the theoretical analysis. Without that, the drawing of conclusions is arguably a bit over-confident.

G

[H Peacock]

Surely if you are trying to return to the runway, you want to minimise the turn radius and therefore the number degrees of turn required to return. In a utopic world you would turn about the centre axis and point back at the reciprocal heading.

212man, I take your point, but the aim here is to use the crosswind to help in a different way. A jink away from the crosswind will quickly displace the aircraft from the centreline such that a tight 180 (plus a bit) will line you up back on the reciprocal. If there is no crosswind and you fly a turn you will clearly be displaced by the diameter of the turn from the centreline. If you had a very strong crosswind you could dispense with the jink away and just turn towards it, this would again reduce most of this displacement. Wish I could draw it to make things clearer.


Islander2 wrote
For conventional fixed wing aircraft flown according to IAS, there is no difference between upwind and downwind turns in steady wind conditions.

Harriers and helicopters are different! A hover stationary with respect to the ground is the extreme example. Turning infinitely rapidly from a 30 kts headwind to a 30 kts tailwind produces a 60 kts loss of airspeed. Vice versa produces a 60 kts increase in airspeed. This is the effect JF got very accustomed to and very much needed to concern himself about with respect to the wind getting under the tail. BUT, the IAS is completely different in each case. Turning downwind, you're starting with an IAS of +30kts. Turning upwind, you're starting with an IAS of -30kts. Try this now without keeping the Harrier (or helicopter) stationary above a point on the ground, and arrange for it to have a +30 kts IAS in both cases, and the loss of airspeed will now be 60 kts whether turning downwind or upwind.

Think I follow that and agree; your IAS will change when turning upwind/downwind.

There is no difference between turning down upwind and turning downwind at a given IAS. In both cases, a high rate of turn will result in loss of airspeed due to inertia and/or increased induced drag (the amounts of each depending on how the turn is achieved), and the loss will have exactly the same magnitude in both cases.

Not true. A slow aeroplane/helo does not need lots of 'g' to achieve high turn-rates, and therefore doesn't suffer from a big increase in induced drag.
In the Puma I could fly downwind with a low (+ve) IAS, but when I flew a balanced turn in to a strong headwind the IAS would rise dramatically before steadily reducing back to the steady state condition. You gain IAS turning in to the wind, lose it turning downwind - is that what you're saying?

:)

[Islander2]

H Peacock wrote:
Not true. A slow aeroplane/helo does not need lots of 'g' to achieve high turn-rates, and therefore doesn't suffer from a big increase in induced drag.Have another look at my post: it doesn't say that, does it? What it says is: "a high rate of turn will result in loss of airspeed due to inertia and/or increased induced drag (the amounts of each depending on how the turn is achieved)"

Also:
You gain IAS turning in to the wind, lose it turning downwind - is that what you're saying?Categorically not, 'cos it isn't necessarily true! What I am saying is: 1) that the impact on IAS is identical whether you're turning upwind or downwind for the same pre-turn IAS, and 2) that whether or not the turn results in an increase or decrease in IAS is dependant solely on whether the IAS is initially greater or less than zero, and is nothing to do with whether the turn is upwind or downwind. If the pre-turn IAS is positive ahead of a very rapid turn, you'll temporarily lose an equal amount of airspeed whether turning downwind or upwind; if it's negative (obviously not a usual fixed wing scenario, but certainly one for Harrier pilots), you'll temporarily gain an equal amount.

Edited to add an intepretation note: all the above assumes a steady wind.

[Croqueteer]

:)As has been said already, go and practice at a safe ht. In most light a/c including the Pitts, from an engine failure in the after take off mode (again, at a safe ht) I can be pointing the other way with a loss of 200ft. Just stuff the nose down, roll on 60 or so bank, pull to the stall warner, roll out on heading. After a bit of practice you can refine it of course! It is a bit of fun just seeing what your a/c can do. Please no hands up in horror replies.

[Flying Binghi]

eeeerr... Croqueteer, You are an experienced (?) aerobatic pilot.The advice you are giving may not be suitable for low time/experience pilots. I think what most in this thread are thinking of is how best to approch turn back training and theory.

[Croqueteer]

:)Yes, and I'm not saying who this is for, or advising it, just saying what can be done. The captain is in charge!

[rotorfossil]

After reading all he posts and adding my experience, i would sum up by saying that turnbacks are a possibility given a whole lot of considerations:
a) it should only be considered in aircraft which have good L/D ratios and don't lose speed quickly at higher angles of attack (like most non aerobatic biplanes). It also helps if the aircraft has a high power to weight ratio and therefore gains height quickly after takeoff.
b) Given the above, the exercise must be practised at a safe height first.
c) Before takeoff, the parameters off minimum and maximum heights and therefore distance from the airfield must be decided.
d) The direction of turn must be considered depending on the crosswind (personally I favour turning towards the wind) and the airfield configuration.
e) The absolute necessity of maintaining safe airspeed in relation to the dynamic stall speed in a steep gliding turn.
f) Lastly, the first part of the turn must be flown without reference to the visual effects of slip and apparent increase in groundspeed.
Given all the above, anyone presented with a donk failure after takeoff and hasn't made all the correct decisions beforehand is better off going straight ahead.

[cwatters]

I'm not sure how applicable this is to GA but low turns in gliders are discussed here..

www.abqsoaring.org/misc_files/NM_Glider_Accidents.pdf

Basically the message appears to be not to float around the turn slowly trying to preserve height but the get the turn done positively. A slow turn has a larger radius, longer distance to fly therefore potentially a greater loss of height. I don't believe this contradict the advice to fly the turn "just above the stall" because the stall speed will vary with rate of turn.

See page 15..

"A common mistake by many glider pilots in situations when height becomes critical is to instinctively keep the glider’s nose up while entering a dangerous low-banked turn with the intention of preserving height. Knowing that at the same rate of lift while circling in a thermal, a lower bank angle will increase the rate of climb the pilot assumes that a lower bank angle in a no lift situation will preserve height due to the lower sink rate. This can be a fatal mistake. As demonstrated above a low-banked turn takes much longer to complete and as a result significantly extends the glider’s exposure to its inherent sink rate. This leads to a higher total loss of height despite the lower inherent sink rate as compared to a high-banked turn with its relatively higher inherent sink rate."

continues..

[Kit d'Rection KG]

The perspective, visual horizon, and ground rush are enormously significant.

Only turn back if you have planned to do so on the grounds that you knew it was safe. This requires a highish performance aircraft (either in terms of climb angle, glide performance, or both) and a skilled and determined pilot.

[ProfChrisReed]

cwatters wrote:

Basically the message appears to be not to float around the turn slowly trying to preserve height but the get the turn done positively. A slow turn has a larger radius, longer distance to fly therefore potentially a greater loss of height. I don't believe this contradict the advice to fly the turn "just above the stall" because the stall speed will vary with rate of turn.
The BGA training (backed up with the maths and empirical investigation, see e.g. Longland, Gliding (A&C Black 2002) pp 105-9, though this discusses turning generally and not this specific case) is that the minimum height loss in a gliding turn is at 45 degrees of bank. This is the best compromise between time in the turn and sink rate, for gliders at least, and produces the minimum loss of height in the turn.

So far as speed is concerned, such a turn would normally be flown at at least approach speed (1.3 vs), some draggy types maybe faster. This would allow a (for gliders) comfortable margin above the turning stall speed of around 5-7 kt, so it might be "close to the stall" as discussed in this thread.

I only fly gliders, so can't say how far this translates to a powered a/c with engine failure. My sink rate in such a turn is less than 3 ft per second, so at 50kt I'd expect to lose no more than 50ft if I fly it properly. My guess is that the 45 degree turn is optimal for all a/c forced to become gliders, because the maths is the same, but this is only a guess.

Glider turnbacks would be from a failed aerotow, which would likely be caused by engine failure of the tug aircraft, and this is something which we consider and in some countries train for (though not the UK). The UK briefing is to set a decision height below which you will not turn back, even if you might theoretically make it. For experienced glider pilots that height will be between 200 and 300 ft - this gives a margin of, say 100ft for a hurried turn and 100 to 200 ft to make it back to the airfield. As the climb rate on an aerotow ought to be at least twice the sink rate of a glider, this leaves an adequate margin.

I'd say do the calculations on the ground - a failed engine a/c which has a sink rate higher than its climb rate with the engine working would never be in a position to turn back, even if it could make a no loss turn!

[Kit d'Rection KG]

The Professor wrote:

a failed engine a/c which has a sink rate higher than its climb rate with the engine working would never be in a position to turn back

...and he clearly hasn't considered runway length, wind speed and direction, or the possible desireability of getting back to the airfield over, say a landing on water. It is climb angle, descent angle (both taking into account wind speed and direction and pilot actions), and airfield environment, that count. :cool:

[Knight Paladin]

Sorry Chris, but I think you're wrong. I rarely like to deal in such black and white terms, but while the idea of the aircraft moving relative to a package of air is a fine way to think about navigation and the like, you really can't forget that an aircraft's inertia comed from its groundspeed (well, technically we need to get even more complicated than that, but bear with me!). An aircraft's airspeed will determine the aerodynamic forces available for it to turn, while its groundspeed will determine its momentum, and thus rate of turn, turn radius, etc.

Take for example an aeroplane flying at 60 knots IAS with a 60 knot tailwind. Fairly obviously, it will have a groundspeed of 120 knots, and so 120 knots worth of momentum. If that same aeroplane turns into wind, with sufficient added power to compensate for the extra induced drag, then, assuming a relatively instantaneous turn, it would suddenly find itself with 120 knots groundspeed into a 60 knot headwind, resulting in 180 knots airspeed. The large resultant increase in drag would rapidly reduce the airspeed to 60 knots, resulting in zero groundspeed. Of course, this drag doesn't wait until the aircraft is pointing directly into wind to take effect; it will begin to bite the moment the aircraft turns slightly from the wind, and is therefore easily disguised by induced drag effects in the turn. It will be more noticeable in heavier aircraft, as JF's Harrier example shows.

Chris - your statement about sink/climb rates is also suspect - you've neglected to consider the effect of runway length. With a sufficiently long runway a turnback would still be viable even if your sink rate was higher than your climb rate, as you won't be aiming to arrive back at the same point you got airborne from. Feel a bit like I'm having a personal dig at you, promise I'm not, sorry!

All of which is fairly well off topic, apologies!

Turnbacks are still a currency item for RAF single engine FW QFIs. Tucano turnbacks have already been thoroughly described here. Hawk turnbacks are only routinely practiced to runways other than the reciprocal of the departure runway, with an additional consideration being that the approach end barrier of the runway in use (ie the one likely to be needed to stop a heavyweight aeroplane landing downwind at high speed on the reciprocal of the departure runway) is generally electrically isolated, and would therefore likely be unavailable. Tutor turnbacks are very similar to the Bulldog variant described - 80 knots and an immediate 45 deg AoB turn into any crosswind being the key parameters.

For my money I'd always get the 180 done first, rather than worry about jinking away from the crosswind for separation from the centreline. I'd rather sort out the line up after at least pointing myself in the general direction of the airfield crash services. That is, howver, just my thinking, and jinking first may well be a better plan in aeroplanes with steep climb angles, to leave more runway available for the downwind landing by tracking away from the runway initially. So many variables!

[MadamBreakneck]

Most definitely a potential 'killer', one of the major problems being the loss of airspeed going from a headwind to a tail and the tendency to stall in the turn therefore.

I always thought it was an optical illusion at low levels affecting the pilot's judgement and causing them to fly unbalanced, draggy turns - that's what I was taught and it's what I teach ("check yer ball, check yer speed, and do something about it - now!").

I've not got any instrument rating so haven't been able to experiment... does this loss-of-airspeed effect manifest itself above cloud and out of sight of the surface? I look forward to reading responses from people who've tried it.

MB
:hmm:

[Islander2]

Take for example an aeroplane flying at 60 knots IAS with a 60 knot tailwind. Fairly obviously, it will have a groundspeed of 120 knots, and so 120 knots worth of momentum. If that same aeroplane turns into wind, with sufficient added power to compensate for the extra induced drag, then, assuming a relatively instantaneous turn, it would suddenly find itself with 120 knots groundspeed into a 60 knot headwind, resulting in 180 knots airspeed.That simply isn't true. You can measure inertia with respect to whichever frame of reference you wish to choose. The result is the same. The aeroplane cannot instantaneously change its momentum with respect to either the ground or the air mass.

In your example, for a hypothetical instantaneous turn the airplane would, as you say, initially continue to have 120 kts groundspeed. However, the point I think you may be missing is that it will initially maintain its same path across the ground, in other words it will also maintain its 60 kts with respect to the air mass, but it will now be pointed in the opposite direction, thus flying backwards and having a negative airspeed of 60 kts, not the positive airspeed of 180 kts you claim. Of course in the real world (everyone knows Harriers and helicopters are Alice in Wonderland!), we turn anything but instantaneously!

[Milt]

Knight Paladin
You are wrong wrong wrong and misleading many aviators.

Once you leave the earth the ONLY relativity your aircraft has with the earth is gravity. The aircraft performs relative to the air supporting it and to gravitation attraction. Groundspeed becomes entirely irrelevant to manoeuvering and the earth only becomes significant in the vertical plane to earth.

Your momentum becomes relative to the air molecules or to whatever you run into be it another aircraft or a rock filled cloud.

[Flying Binghi]

You are correct Milt.
What the Earth does add to the wind facter is Wind Shear and turbulance.
An EFATO turn back is a desending manuver with wind shear a facter in high winds.

[Knight Paladin]

Milt - Sorry chum, A-level physics wins. Yes, the force produced by the air that acts on the aeroplane comes from the difference in momentum between the aeroplane and its surroundings, and yes, the only direct link between the Earth and the aeroplane is gravity, BUT, in just the same way that the Earth has it's own momentum travelling through space, so does the aeroplane. This momentum can most easily be referenced to that of the Earth itself, hence groundspeed is relevant. Of course, strictly speaking you need to consider the momentum of the aeroplane through 3-D space, probably referenced to orthoganal axes, but I fear that would be getting far too complicated.
At least, that's the way I've always understood and been taught it, the various poor buggers who've had to teach me could all have been clowns. Or... physics could work completely differently when you're on the wrong side of the world, surely you guys learn this from having to coordinate with opposite rudder? ;)

Islander - true, instantaneous turns are indeed impossible, was just using a rather contrived example. As I said, the actual change is very progressive throughout the turn.

Edited to add lame Aussie banter

[Islander2]

Knight Paladin said:
Milt - Sorry chum, A-level physics wins. Yes, the force produced by the air that acts on the aeroplane comes from the difference in momentum between the aeroplane and its surroundings, and yes, the only direct link between the Earth and the aeroplane is gravity, BUT, in just the same way that the Earth has it's own momentum travelling through space, so does the aeroplane. This momentum can most easily be referenced to that of the Earth itself, hence groundspeed is relevant. Of course, strictly speaking you need to consider the momentum of the aeroplane through 3-D space, probably referenced to orthoganal axes, but I fear that would be getting far too complicated.Oh well, there goes the last vestige of any notion that A-level is the gold standard! :hmm:

And also:
As I said, the actual change is very progressive throughout the turn.That's as may be, but how do you account for the IAS you quoted for an instantaneous turn being 240kts in error (I assume by 'airspeed' you meant forward airspeed, i.e. IAS)?

[Knight Paladin]

Sadly my idea of "A-level" refers to the UK qualification, and is therefore, particularly in this day and age, far from being a gold standard.

I'm fairly sure that speed in the inertial plane is of (minor, relatively) importance in the general physics of flight, but we're deviating massively from the point of the thread, and I really don't think it's a critical issue when considering turnbacks. In addition, I'll be honest, I'm primarily an operator rather than a boffin, so we're getting well away from my Stick Monkey comfort zone.

Bit of a cop-out I know, but will gladly hear any more techniques/advice about the flying of turnbacks, have very much enjoyed the more practical discussion in this thread.

[Islander2]

I'm fairly sure that speed in the inertial plane is of (minor, relatively) importance in the general physics of flightErm, absolutely no! Speed with respect to the airmass inertial frame of reference is entirely what it's about.

I really don't think it's a critical issue when considering turnbacks.Erm, absolutely no again! Significant loss of (positive) airspeed in the turn, 'cos of induced drag for relatively slower-rate turns by conventional fixed wing aeroplanes or inertial effects for rapidly-turned Harriers/helicopters moving slowly with respect to the ground, is one of the major factors going against the attempted turn-back and which, for conventional fixed wings, can result in the stall/spin fatality. In response to earlier posts, however, this is not a 'turning downwind' phenomenon. Exactly the same occurs turning upwind.

[Contacttower]

I can't believe downwind turns seem to have come up again...:ugh:

I'm supprised that no one has mentioned what effect the length of runway has to do with whether or not a turnback can be done or not....If I take off from say 2000m of tarmac in my Super Cub and climb away....I'll be pretty high by the end of the runway and while I admit I've never tried it for fear of some instructor telling me off I'm pretty sure that I could land back again. There was an article in Flyer recently that covered them and actually it was quite positive. Just try finding an instructor who is prepared to teach them though:ugh:.

[cwatters]

I suspect the reality is somewhere between the two positions being taken on inertia.

Consider the example of wind shear. As a glider descends to land it experiences reduced headwinds due to wind shear. According to one camp a glider shouldn't notice this reduction in headwind - it's ground speed should "automatically" increase to maintain a constant airspeed (afterall it's lift drag and weight and trim hasn't changed so it's airspeed shouldn't change). In practice due to it's mass and inertia a glider cannot accelerate fast enough so it does experience a loss of air speed if it descends rapidly through wind shear. Glider pilots typically add half the wind speed to their landing speed to allow for this effect and for other reasons.

[ProfChrisReed]

cwatters wrote:

I suspect the reality is somewhere between the two positions being taken on inertia.

I suspect this is right, too. However, I am sure from empirical means that the effects of intertia are very small. I have flown thousands of 360 degree circles at 45 degrees of bank and an IAS of 45kt, in airmasses which are moving at 30kt or more. It is possible to maintain an apparently steady airspeed doing this. If head/tailwind had any appreciable effect on airspeed I'd stall once on each orbit!

I can't believe that proximity to the ground makes any difference, and I have enough circles at 600 ft agl to know that, again, there are no detectable speed differences related to the wind direction and my heading.

My guess is that the force of the aircraft's intertia (is it a force? Why is there never a physicist on hand when you need one?) operates at a tangent to the circle you are flying. Thus as you turn through 180 degrees the inertial force is at all times nearly aligned with your heading, so that for practical purposes you can assume no headwind/tailwind difference.

Losing airspeed because of the higher induced drag in the turn is an entirely different matter - this would equally be a problem turning from downwind to headwind.

[Kit d'Rection KG]

Regarding

I can't believe that proximity to the ground makes any difference

There are two areas where it does:

First, the visual cues change dramatically at low heights; if you fly competent aerobatics at altitude, it will take a good while to reduce your operating height down to a couple of hundred feet for displaying. :bored:

Second, you may be introducing a fixed reference point on the earth into the equation, for example you may be flying an approach to a runway, following the glideslope. Thus, you are attempting to follow a path which remains fixed in a flowing medium (the air). You could liken this to trying to thermal in 30 kts of wind, following a path exactly over a large circle marked out on the ground beneath you... :cool:

On very windy days (the worst I've landed in was 65 gusting 75 straight down the runway), it is very noticeable that as windspeed decreases, thrust must be added, and vice versa. This is absolutely crucial to windshear recovery. The seat of the pants is also valuable; certain feelings in the nether regions on the approach have me adding 5 or 10% N1 before the instruments register any change. ;)

On the topic of this thread in general...

Ahem...

QRH, please, internet discussions, thread drift checklist, recall actions! :eek:

[Kit d'Rection KG]

(Perhaps I should have added that uncertain feelings have me adding at least 15%!) :oh:

[Knight Paladin]

Islander - I wasn't saying that losing speed in a turnback wasn't important! My grasp of the physics behind the whole inertial airspeed/groundspeed issue may need some work, but I have flown many simulated turnbacks (luckily no real ones, yet), and am very well aware of the vital need to maintain suficient airspeed for the turn. I was saying that I'm not sure the inertia issue is really key in the consideration of turnbacks. You might be well advised to do people the justice of reading what they actually write before you are so keen to rubbish what they say!

[Wwyvern]

I don't remember ever experiencing loss of airspeed when turning downwind. I was taught to maintain airspeed throughout the turn. In shallow angle of bank turns, airspeed usually reduces slightly because of the increased back pressure on the stick required to maintain altitude. In steep turns, the airspeed is maintained by applying power to overcome the back pressure on the stick.

I understand the loss of airspeed in an aircraft like a Harrier (which I have not flown) or helicopter (which I have) in the hover, but I believe that in other aircraft, in forward flight, the pilot should control the airspeed by using the stick. I don't recall, in another life, having airspeed changes imposed on me when flying holding patterns into and out of wind.

I also note in my old copy of "Flying Instructor's Handbook - Piston Provost" when teaching low flying, we were to "Fly into wind and turn accurately through 180 degrees. Fly downwind then turn accurately through 180 degrees. Maintain constant airspeed. Demonstrate in moderate wind conditions."

Have I the wrong end of the stick in this discussion? Is the loss of airspeed/downwind turn a theoretical rather than practical question?

[cwatters]

> I am sure from empirical means that the effects of intertia are very small.

In the case of circling in wind I agree. The rate of change of heading is too low.

..but it's worth remembering that if inertia didn't exist you could make a flat turn using rudder only with no skidding.

..or to put it another way that 45 degree bank is only needed because of inertia. That's half the lifting force of the wing needed to counteract it. Not so small eh?

[Flying Binghi]

"or to put it another way that 45 degree bank is only needed because of inertia. That's half the lifting force of the wing needed to counteract it"

Good point