Limiting g onset

[OATNetjets]

Hi everyone,

I am looking for some guidelines concerning the limitations that should apply in terms of g-onset for an aircraft that does +8g /-3g...

1°) For positive g's: around me some experienced designers explain they have lost aircraft in the past due to very high g onset (more than 20g/sec). Therefore it was decided to limit it to 10 for our aircraft. Anyone aware of any formal guideline / rule that I could not find myself?

2°) For negative g's: in this case we have not been given any requirement and therefore the design is such that we often go to -25 g/sec. I would tend to limit it proportionnally to the positive g onset limit (let's say around 6 g/sec in our case), or at least not give more than for the positive case, but again there does not seem to be any paper on that subject (not that I am aware of at least).

Thank you in advance for any input, Pierre

[John Farley]

around me some experienced designers explain they have lost aircraft in the past due to very high g onset

You don't say why they think that. I'd be interested to know their reasons.

[Mad (Flt) Scientist]

Ive never thought of it in terms of g-onset rate; but if you figure it in terms of alpha-dot or pitch rate then there may be concerns over, perhaps, inertia coupling effects (for the latter) and amybe some kind of downwash lage effect for the former? Of course, it may just be a consequence of gross overshoots of the target g.

Still sounds odd to try to limit the rate at which g can be built, regardless of the actual g level. A rapid change from 1g to 2g on an 8g rated a/c shouldn't be a hazard surely, whether it happens in more or less than 0.1 sec?

[OATNetjets]

2 questions to answer here apparently:
1°) For the reason to limit the g onset, apparently it was due to the fact that on a past protoype an aircraft was lost because of a pilot black-out. At the time they did not see anything wrong or strange with the aircraft, but they noted that the g-onset was very high... Thus the reason for limiting it on new projects.

Now I have to admit that I am not convince with that since I have never heard of such limitation and I can not find anything on the litterature, but maybe you guys have other experience...

2°) Now for what our aircraft response is concerned at low g demands we provide the pilot with the Nz in about 2 seconds (for 4 g that means a peak of Nz rate of about 6 g/sec), but when we go to higher g level we limit the peak in Nz rate to about 10.


Now again:

1°) Are you aware of any physiology requirements linked to g onset?

2°) Any reason to limit it in a different way for negative g's (if you revert sharply from 8 g's to - 3g's... not that it would be confortable)?

[Proof Reader]

Pilots suffer the effects of g because the heart cannot pump oxgenated blood up into the brain in sufficient quatities. However these effects take time to cause brain/sight issues - time and g. For literally a second or two we are OK so the rate of g onset is not a factor just the g you go to and the length of time you are at the high g. In the old days before g meters were fitted in cockpits pilots momentarilly pulled numbers like 10-12 g and so had the wings off in a flash without passing out.

Negative g is a very different matter. Here blood is forced into the head and you can burst small blood vessels in the eyes and brain - not good.

[peekay]

The aircraft I work on has similar authorities to yours and also has a restricted g-onset rate for positive g manoeuvres. If my memory is correct, the requirement to constrain the g-onset rate derived from some F16 accidents in the '80s, although I don't have any documentation supporting this and can't identify the accidents in question.

I believe that were no requirements to limit the g-onset rates in negative manoeuvres.

My understanding, and I am not an aeromedical expert, is that achieving high g very rapidly and then holding it for some seconds can lead to loss of conciousness without the usual warning symptoms (i.e. tunnel vision, grey out etc). This understanding seems to be supported by the report at the following link, particularly Figure 1, although that graph may apply to pilots without g-protection.

Link

I have never felt in possession of all the facts on this topic. I will be very interested to see what this thread can unearth.

[john_tullamarine]

Are you aware of any physiology requirements linked to g onset?

Now your question is a little easier to understand.

There are two situations of interest regarding G and pilot response

(a) low onset rate - typical situation where the pilot at some G (depending on individual response, technique etc) greys out and then, at a higher G, blacks out. Recovery with G reduction is rapid and the outcome (generally) fine.

(b) high onset rate - the pilot response is different and blacking out occurs rapidly. More importantly, recovery with G reduction is prolonged and the pilot, functionally, may be incapacitated for some time .. with undesirable effects. As I recall, the Blue Angels mishap in 2007 (where a member pulled very aggressively into a rejoin) was put down to this effect.

Generally referred to as G-LOC (G-induced loss of conciousness) or some similar expression.

Any number of articles on the net, one here and another here.

[OATNetjets]

Thanks for your answers.

To sum-up it is sensible to limit the g-onset to some limits to avoid sharp loss of conciousness, even if the time the pilot stays at a certain G level is a preponderant parameter of grey or black-out appearance.

From what was posted above, it also seems to me that when it comes to negative g's it is important to be able to release the G level quickly. Am I right? In other words no G loc can occure there and therefore we can (and should) give the pilot the ability to go to 1 g as quickly as possible.

Do I miss anything?

Thanks again for the help

[BOAC]

I would suspect that 'rapid onset' has caused problems in disorientation and physical limb/'head+neck' motion due to whatever movement caused the onset of the g rather than in any g related area. As MFS has said, even at 25g/second (gulp) it is 'over' in 1/3 second in your a/c in which time no part of the circulatory system will have responded. Movement towards negative g at a high rate would be particularly disorientating and uncomfortable.

A rapid onset of any g would cause significant disorientation and delay any remedial action. I would suspect that even 10g/second if unexpected would have confused me in my 'younger' days (said one who nows 'greys out' if he levels out from a descent while turning). If in the positve direction, unless the head is restrained, neck injury would be likely. Certainly my ride on a Martin Baker (admittedly at a higher rate) slammed my head down painfully hard into my chest in the brief time the seat accelerated. I cannot recall the g rate on a rocket seat but I was eyes open and looking at the world (albeit through my size 10 boots) within no more than 1 second, in time to see (and react to) the seat separating during a low level ejection.

Regarding 'return' to normal g, it is certainly not in my book of favourite manoeuvres, but I cannot see any need to 'return' rapidly to +1 if the pilot is used to -3?. For positive, it depends on the physique and protection systems - a rapid and unexpected onset will give no time for precautionary preparation such as pilots are trained to do, and therefore back to +1 g in a couple of seconds would enable a dazed pilot to recover over the next few seconds. I don't think the body would react that quickly in shutting off the brain supply.

[Genghis the Engineer]

I don't have any direct knowledge of the topic, but recall being told that this was a major consideration in the design of the dynamic (centrifuge) flight simulator at Linkoping - somebody there might have some useful insights if you go and talk to them.

G

[John Farley]

At last a topic I know something about and for the first time ever I find myself in disagreement with John T.

In my opinion the rate of onset of positive g has no effect on G-LOC which I believe depends only on the g you go to and the time you are there – indeed as Proof Reader said earlier – with but one exception that I will discuss at the end.

John T quotes two articles to support his post. The first does not mention rate of onset and even includes the following error “a G-suit squeezes the abs and legs, forcing the blood up toward your cranium to help maintain vision and consciousness”. A g-suit of course does not force the blood up it merely stops the blood descending into the parts of the body that the g-suit covers. This minor error is all the more surprising because in an earlier sentence the author states (correctly) “G-suits work by transferring a force via air-inflated bladders to the lower limbs to increase peripheral vascular resistance, thereby reducing venous pooling.”

The second article quoted by John T does discuss rates of onset (as high as 15 g/sec) but goes on to say that rates of onset are not relevant as there is a delay of 3 to 5 secs before the brain is affected by g. The relevant para being: “Very high G levels can be tolerated if they are not sustained for very long. The brain has a small reserve supply of oxygen dissolved within it. In the absence of blood flow, this will give something like three to five seconds of ongoing brain operation”.

Personally I would have thought that those times were at the top end and I have always thought in terms of 1-2 seconds before I am affected. The trouble with all this is that the effects of rate of g onset on aircrew can only be measured in flight as centrifuges cannot respond sufficiently quickly.

Now the exception I mentioned earlier. Modern aircraft can SUSTAIN levels of 6+g (which was not the case in say the Hunter era for reasons that we don’t need to get sidetracked into here). Indeed in an early Hawk you could hold 6g until you run out of fuel. Such sustained levels of g, if maintained for say 20 secs or more, allow the body chemistry to respond and it does this by increasing one's blood pressure – which of course helps. HOWEVER if following such a period of adaption you suddenly reduce the g to less than one the body kicks in at once and drops the blood pressure off a cliff. So you faint (or G-LOC for the macho). This fainting is not preceded by the normal g-induced effects of loss of peripheral vision, grey out and black out (the eyes being particularly susceptible to lack of the optimum levels of oxygen) that all occur before actual loss of consciousness in the situation where you start a pull to high g levels from normal flight. Clearly passing out without the normal warnings is very dangerous for a pilot.

If readers feel I am getting in to the nit-picking/pedantic level of comment with all this I apologise, but I do hate half truths or even 90% truths for that matter - especially on this forum.

JF

[john_tullamarine]

for the first time ever I find myself in disagreement with John T

.. I defer to your account, good sir.

My in-the-flesh knowledge of high G is limited. While I have read moderately on the topic, perhaps I now need to read up some more to fill in the apparent gaps in my knowledge ? As we often observe, PPRuNe is wonderful learning resource.

[stressmerchant]

HOWEVER if following such a period of adaption you suddenly reduce the g to less than one the body kicks in at once and drops the blood pressure off a cliff.

What sort of length would the period of adaptation be?

I'm intrigued because last year I had a blackout doing aerobatics, without any of the usual grey-out symptoms. The interesting thing was that I recall coming back to one-G level flight before the lights went out. Now this was at the end of a loop in a Yak-52, so there was no way the time period was as long as the 20s you mentioned. At the time I wrote it off to a general lack of fitness and practice.

[Brian Abraham]

The loss of BAE's Deputy Chief test pilot Jim Hawkins in the Hawk 200 prototype was thought initially to be the result of G-LOC. JF may be able to comment further.

1986 | 1585 | Flight Archive

[peekay]

The following is taken from an article in the British Medical Journal, Vol 286, May 1983. It was written by John Mills & Richard Harding of the RAF Institute of Aviation Medicine.

"In an unprotected relaxed subject unconsciousness usually occurs between +5 Gz and +6 Gz, the exact level depending on the rate of onset of G. If this is less than 2 G/s then the subject invariably experiences a sequence of visual symptoms before losing consciousness. At about +3 to +4 Gz the visual fields become darker, and at +0.5 to +1 Gz later peripheral vision is lost. From this condition of "greyout" the fields contract further to a state of “blackout”, when vision is lost completely. Hearing and mental orientation are, however, adequate for intelligent conversation until consciousness is finally lost about 0.5 Gz later. At higher rates of onset loss of consciousness is too rapid for these characteristic visual symptoms to develop."

The full article is available on the BMJ website but registration is required to view it.

This article seems to support the view that loss of conciousness can occur with little or no warning in high g manoeuvres with high onset rates. Unfortunately it doesn't provide any guidance on what the limit on g-onset rate should be.

[BOAC]

stress - any idea how many g in the pull-out?

[John Farley]

Stress

Docs have told me you need to sustain g for a "considerable time - say 20 secs or more" before the body chemistry kicks in as I described. However I have no idea if the 20 secs thrown in to the conversation represented a minimum, a mean or what. I have never seen any written reports on this research.

Brian

I watched Jim through many rehearsals and the accident flight. There is no doubt from aircraft flight test instrumentation records up to impact (which were perfectly readable) that at the end he was conscious and flying very accurately as the AOA he flew was the optimum and extremely well controlled for the last 70 degrees or so of the failed pull out. However his immediately preceding manoeuvres to my mind showed completely abnormal confusion and indecision which made me feel he had been affected by g but not to the point of passing out. In my opinion the part of his sequence that led to the accident started with a very prolonged high g low level max rate turn through some 420 deg followed by snapping the wings level and a yank to a vertical climb. As soon as the vertical was achieved he abruptly checked the pull (as one does) before rolling and pulling hard to the inverted which he maintained with a push into level flight. All this was quite normal and his usual sequence.

The inverted flight section was away from the display/crowd line and used to build distance so that he could roll through 80deg to a slightly overbanked turn where he yanked it round through 270 deg, diving the while, for a high speed low level dash along the display line.

Being the expert and perfectionist that Jim was he always wanted to do the high speed run downwind for max effect. This meant that following the inverted phase he had to do either a diving left or diving right turn depending on the wind direction that day. If the wind was strongly on crowd (as it was on the accident flight) he had to extend the inverted phase to ensure he had enough distance out for the turn back down the runway and he did this by putting in a quick roll and stopping this with about 100 deg bank one way or the other for the subsequent diving turn.

On the accident flight he stopped the distance making roll inverted and immediately did the expected hard pull, but of course this was not round the horizon but straight down. The aircraft then paused in the vertical, rolled a few degrees (say 10) right then back to the left before starting the unsuccessful dive recovery hitting the ground with about 10-20 deg still to go.

My interpretation of all this was that because he was mentally confused from the various preceding g effects he stopped the roll at the wrong bank angle, pulled as usual at that stage of the sequence before he realised that all he could see was the ground and not the expected horizon. Hence the pause in the near vertical and 'thinks bubble' type look left and right (?? where the f is the nearest horizon??) by which time the end result was inevitable.

Peekay

Thank you very interesting.

[stressmerchant]

stress - any idea how many g in the pull-out?

Not unusually high - sportsman class PPL aerobatics. No more than 5-6G, held for 3-5 seconds.

The thing that confused me was that it seemed to happen after I had returned to 1G. Checked to level flight prior to the next manoeuvre and .. wham.

[BOAC]

Stress merchnat - I cannot explain that unless it was some 'bug' you had which might have caused severe dizziness?

[John Farley]

BOAC

That episode was another Hawk not a Strikemaster and was not as simple as it appeared to the audience. If you don't mind (because it involves people other than Jim) I will leave discussion of that to the evening of 17th Dec. See you then!

JF