Repetition of limit G-factor

[Bevo]

Unsymmetrical maneuvers (rolling maneuvers) on fighter aircraft is the one area where pilots are not a careful/aware as they might be. Since there is no “unsymmetrical g-meter” and the limits are normally listed as “over 1/2 lateral stick”. In a maneuvering air-to-air engagement most pilots do not monitor their lateral stick displacement well enough to know if the unsymmetrical limit was exceeded. Fortunately the most modern fighters have flight control computers which will keep the pilot out of trouble.

[Arm out the window]

Scene: WWI RFC maintenance tent:

Lord Flashheart enters holding bent control column.

"Right you lot, get me a new kite pronto or I'll give your wives something to hang their towels on. Woof!"

[ASIP]

The following is a quote taken from an aviation text

Brian, the text is correct. In what context did you post it?

What reference are you able to provide ASIP that operating at the limit load is a no no.

I am afraid you are missing my point. Never said I that going to the limit g-factor (limit load) "is a no-no". I did say that the number of times of reaching this load must be limited.

We all recognise that pulling "g" impacts fatigue life,

I am glad to hear this. However I have some doubts. Somehow you always point out to aircraft static strength only. And you do that incorrectly. Your V-n diagram is just a design tool and does not correspont to the actual situation with strength of the aircraft operated under the Damage Tolerance principle. You never mention fatigue and Damage Tolerance as an aircraft design criteria.

so please correct our, or I should say, mine, distorted thinking.

Under distorted thinking (again, admittedly dramatizing) I meant the conviction that the limit g-factor can be pulled up in every flight any arbitrary number of times.
Disasters are avoided not because it is intristically safe, but rather because there is a numerous army of specialists tracking the problem continuously.
As some reference I can give a link to a paper about F-16 service life problems:
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA407261

Pictures follow.

[ASIP]

This was original plans for F-16 service life.


This is comparison of the design and actual usage. Somebody pulled high g's too often.


And this is the result.


If the fleet would have been kept flying without premature part replacement up to the design 8000 FH, some aircraft could have disintegrated.

[Schnowzer]

Bevo,
The F-15E had an "overload warning system" that varied the amount of g you could pull depending on the speed the aircraft was flying to avoid serious damage whilst rolling in the transonic regime. Most "over-Gs" where called at very low g-load, 3-4g whilst transonic and rolling, pretty annoying.

ASIP,
Despite aiming at 9g in an F15, most pulls by competent pilots would be in the mid to high 8s, we used to pull, wait till the jet howled and then back stick it. In a DBFM training sortie there would probably be 5 or 6 splits each with 3 or 4 pulls to between 7 and 9g and I never saw any bits fall off.

JC was in the back of a clean jet that chucked a bit of the stab off at M2.4 and I lost a Winder in a CB but that was just careless

[Courtney Mil]

Schnowzer,

Agreed. See posts 2 and 10.

[ASIP]

Thank you Schnowzer for the objective information.

There is no much information available on actual +9g occurences. What I found, though, indicates that the real number is not large, at least far shy of "pulling 9g four-five times a flight".
These are data from a presentation on the F-15 Life Extension Program


Spectrum FTA1 is the design usage.
Spectrum FTA6 is an actual or at least close to actual usage, confirmed by the on-board Structural Data Recording System.
Spectrum FTA7 is a design spectrum for future severe usage.


It can be seen that the original design usage predicted 3 exceedances of limit load per 1000 FH. In reality, the number was 42 exceedances. This means one +9g or higher maneuver in approximately 250 - 300 flights.
Still, McAir guys consider this too many. They want you to pull 9s no often than 20 times per 1000 FH.

In a DBFM training sortie there would probably be 5 or 6 splits each with 3 or 4 pulls to between 7 and 9g and I never saw any bits fall off.

Three things here:
1. You did everything properly.
2. Guys in St. Louis new their business well. Stresses in the aircraft parts were well below FTU/1.5 plus they coldworked fastener holes and didn't accounted for that in "certification".
3. Fatigue damage is proportional to the fourth power of stress in aluminum parts (approximately). Thus 7.5g develops only one half of the fatigue damage compared to 9g.

[Brian Abraham]

Your V-n diagram is just a design tool

Beg to differ, as much as it is a design tool, it is also promulgated to pilots as to the limits in which they are obliged to operate. They only cautionary note I've ever seen as to approaching any limit is that between Vno and Vne, which states "in calm air only and then with caution".

I did say that the number of times of reaching this load must be limited

There is no mention in any text as to how often a limit load may be applied, and certainly not in any flight manual. Nor does any flight manual or regulation require reporting a of limit load being reached. Not that the pilot of most non military aircraft would know in any event, non having the necessary instrumentation.

The FAA was obliged to issue an AD on the S-2 series of aircraft due to cracking in the upper longerons due to overstressing. The AD required the fitting of an accelerometer marked with the limit loads, so that pilots did not exceed those limits. And absolutely no mention of recording the number of times those limits were reached.

You never mention fatigue and Damage Tolerance as an aircraft design criteria.

It's not a subject pilots are interested in. They fly the aircraft in accordance with the rules, regulations, and standard operating procedures laid down. Should untoward fatigue be detected it may be because of any number things,

1. Design - King Air forward spar, modification of strap on lower spar cap available
2. King Air - Pilor performed aerobatics in non aerobatic aircraft. Repaired and returned to service.
727 - Loss of control. Pulled 6 g (limit 2.5). Repaired and returned to service.
747 - Loss of control. Data dropout precluded record of max g but 5.1 recorded. Repaired and returned to service.

Should the evidence of excessive fatigue make its self known the operator will institute procedures to contain the problem.

A 727 is used for Zero g flights in which they operate between 1.8 and 0 g (limit g 2.5 to -1) and those g are pulled multiple times per hour. Are they exceeding fatigue limits? Certainly no airline aircraft ever was subjected to those limits. What limits do you suggest they should be applying?

[Waddo Plumber]

It's a while now since I was in fatigue monitoring, but IIRC, the normal SN curve is almost asymtotic to the N axis, and as long as S (or a combination of major and minor cycles) is below the asymptotic value, it can be repeated an infinite number of times. So, the question is what are the maximum values of + or - G that can be pulled without exceeding that stress. Up to those levels, there is no need to tell the aircrew to back off. As aircraft age, it's normal for the G limits to reduce specifically for that purpose.

[ASIP]

There is no mention in any text as to how often a limit load may be applied,
and certainly not in any flight manual.

We already know this. This is a fact.
However, if you use it as a proof that the number of very high g-maneuvers is unlimited, you are not right.
I hope we have established that it is simply not the pilot's business to maintain this number within acceptable limits.
If you are not aware of a problem, this doesn't mean the problem does not exist. Maintaining the aircarft structural integrity, which can be compromized by frequent high-g maneuvers, is performed by technical personnel and relevant documentation flow through technical channels.

Nor does any flight manual or regulation require reporting a of limit load
being reached. Not that the pilot of most non military aircraft would know in any event, non having the necessary instrumentation.

See, you are now trying to deny a known procedure for mandatory inspection of an aircraft that achieved its limit g-factor. Your claim about absence of such inspection in a FM is absolutely correct. Regulations? I am not sure. Are you familiar with the documentation required for certification for FAR25.571 and FAR25.1529? All those design spectra, numbers of occurences, inspections must be there. A similar stuff exists for military aircraft.
I hardly doubt the pilot caused a post-limit load inspection remains uninformed after the buzz accompanying such an inspection and an engineering report.

It's not a subject pilots are interested in. They fly the aircraft in
accordance with the rules, regulations, and standard operating procedures laid down.

Completely agree. However this is not a reason for cancellation of natural processes in the airframe material.

Should the evidence of excessive fatigue make its self known the operator will institute procedures to contain the problem.

Great! We are talking about the same things. But this still doesn't make the number of high-g maneuvers that the aircraft can sustain unlimited.

A 727 is used for Zero g flights in which they operate between 1.8 and 0 g (limit g 2.5 to -1) and those g are pulled multiple times per hour. Are they exceeding fatigue limits? Certainly no airline aircraft ever was subjected to those limits. What limits do you suggest they should be applying?

I am not sure I understand you correctly. What do you mean under the fatigue limit? If it is a material property, then yes, at a level flight the main structural components of a transport aircraft experience stresses above the material fatigue limit. Transport aircraft can do maneuvers up to +2 g much many more times than fighters can pull to their +9g. The reason is that the transport A/C are mostly affected by an enormous number of turbulence cycles, and they are designed for that. For fighters, the turbulence normally is not a factor. Until they (mostly strike aircraft) fly low and fast.

[ASIP]

It's a while now since I was in fatigue monitoring, but IIRC, the normal SN curve is almost asymtotic to the N axis, and as long as S (or a combination of major and minor cycles) is below the asymptotic value, it can be repeated an infinite number of times.

Oh! I glad to hear this kind of words. That's correct. Such a design is called "boiler-proof." Normally it cannot be used for airplanes because the structure is way too heavy. Nevertheless, for fighters in a level flight in many components the stresses are below the material fatigue or endurance limit.

So, the question is what are the maximum values of + or - G that can be
pulled without exceeding that stress.

Absolutely! The devil is in details.
And we know the answer. The local stresses in fastener holes or notches will be many times higher than "that stress". They will be way above the material yield limit, approaching or even exceeding FTU.

Up to those levels, there is no need to tell the aircrew to back off. As
aircraft age, it's normal for the G limits to reduce specifically for that
purpose.

Unfortunately, "those levels" are exceeded in every flight. This thread was started because Flight Manual are silent about this problem. To give you an idea about stress levels I show the picture for approximately F-15 situation. I hope you know the difference between "nominal" (reference, remote) stress and "local" (peak) stress.

[Waddo Plumber]

ASIP (Aircraft Structural Integrity Programme?). I've woken with a heavy cold. I'll. reply when my brain answers to the helm.

[E L Whisty]

I dimly recall a tale told by an US Marine Corps friend about a fatigue survey performed by the US Navy on their fleet of F 14s, sometime late 70's or early 80's. The story was that they put strain gauges on a representative fraction of the fleet and had some very disturbing results.

It is important to understand the difference between stress and strain in the engineering definitions.

As I understand it, there was a great deal of fatigue being accumulated under rolling G conditions that was never appreciated with simple G counters then in use.

It is also important to appreciate that metals have been characterised as those which have fatigue limits and those that have fatigue lives. As I retired from the relevant research some 10 years ago, I do not know what the current thinking is, however, there was a growing body of evidence that even steel alloys have limited fatigue lives.

A fatigue limit is, simplistically, a loading limit which, if not exceeded, will give no accumulating damage.

A fatigue life is a description of how a material (thence structure) will become progressively damaged as particular loads are applied, cyclically, over time. As an simple illustration, a single 8G pull might cause as much damage as several hundred applications of 2G. Thus, accumulation of damage is complex to predict and calculate.

Aluminium alloys, however, have always been considered to accumulate damage at even the lowest loadings. So, an in service G limit for aircrew is imposed in order to preserve the service life of an airframe. It is acknowledged that damage will be accumulated and airframe integrity issues will arise as it ages. Older aviators will recall the sorry state of our F4s after they had had 15 years of enthusiastic aerial gymnastics.

As a back seater in F4s, I can recall very few 'dogfights' when G was not applied whilst rolling. Indeed, doing so was a necessity for an aggressive pilot. The G counter did not betray the angel faced pretence of cunning pilots that they never rolled whilst pulling!

It is my opinion that we operated aircraft for many years with naivety about fatigue that probably resulted in loss of aeroplanes and of friends.

The future looks good, though, as strain gauges can be incorporated quite cheaply into composite structures and data gathered constantly will make us better able to monitor the health of airframes.