Repetition of limit G-factor
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Repetition of limit G-factor
I am an ASIP engineer and we track and help maintaining aircraft service life.
From the structural point of view, the limit load is a load that should occur only once per the aircraft lifetime. Thus, we assume that limit G-factors (say, +9g for F-16 @22,500 lb GW) are achieved rarely. Often repetition of the limit G-factor significantly (could I say dramatically) reduces the aircraft fatigue and service lives.
On the other hand, there are quite a few youtube clips with F-16 HUD reading of 9g. I realize that, during a mission, a pilot may need go that far. I looked at a couple of Flight Manuals and found limit maneuver G-factors but no limitations on their number of occurences.
My question is, do a pilot know that he should not get to the limit G-factor often. Is there a document for the pilot prescribing or limiting the number of limit G occurences?
Thank you.
From the structural point of view, the limit load is a load that should occur only once per the aircraft lifetime. Thus, we assume that limit G-factors (say, +9g for F-16 @22,500 lb GW) are achieved rarely. Often repetition of the limit G-factor significantly (could I say dramatically) reduces the aircraft fatigue and service lives.
On the other hand, there are quite a few youtube clips with F-16 HUD reading of 9g. I realize that, during a mission, a pilot may need go that far. I looked at a couple of Flight Manuals and found limit maneuver G-factors but no limitations on their number of occurences.
My question is, do a pilot know that he should not get to the limit G-factor often. Is there a document for the pilot prescribing or limiting the number of limit G occurences?
Thank you.
F-15 pilots (in the days of the A and C models) were trained to use max g (9) in every fight. That's 4 to 5 times per training sortie in the combat taining phases. From what my oppos on the F-16 tell me it was the same there. That's a lot of counts on 9g in each airframe's lifetime.
Well, I'm just a wee truckie but, on the Jet provost, with Pilots' Notes figures of +6/-3 we frequently took it to +6 and, less frequently, to -3 *.
The JP and, later, the Bücker Jungmann, provided me with the most fun I've ever had in an aeroplane.
*Tried an outside square loop one day. The little Viper wasn't up to the huge drag of pushing -3 and I didn't make it. One did however, push -3.5 and, upon reporting same to old Chiefy in line hut, was told: "That's what it really is but they know you young gentlemen will push things a bit so they tell you it's - 3.0."
The JP and, later, the Bücker Jungmann, provided me with the most fun I've ever had in an aeroplane.
*Tried an outside square loop one day. The little Viper wasn't up to the huge drag of pushing -3 and I didn't make it. One did however, push -3.5 and, upon reporting same to old Chiefy in line hut, was told: "That's what it really is but they know you young gentlemen will push things a bit so they tell you it's - 3.0."
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I thought the fatigue life of each type was assessed by assumed usage spectra, which took into account the frequency of reaching different g levels per flying hour. This lead to some issues when the DA granted a life to a new operator on the basis of exchange rates for existing operators. Again, surely fatigue stats for sqns are monitored to optimise usage of individual airframes, so aren’t those stats fed back to individual pilots as in "Prune, you are becoming unaffordable"
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Courtney Mil,
Yes, this should drive the number of +9g counts up immensely.
This doesn't look right from the structural point of view. About F-16 :
"the aircraft was originally designed for 1,000 6G-exceedences per 1,000 flight hours, (or 1 per flight hour). "
6g once per flight meant 8000 FH of design service life. But 9g four - five times per flight... How did they deal with the dropping fatigue life? How many sortites were in those training phases? Were there required periods of low-g flying in between?
Was fatigue tracked at all at that time?
Yes, this should drive the number of +9g counts up immensely.
This doesn't look right from the structural point of view. About F-16 :
"the aircraft was originally designed for 1,000 6G-exceedences per 1,000 flight hours, (or 1 per flight hour). "
6g once per flight meant 8000 FH of design service life. But 9g four - five times per flight... How did they deal with the dropping fatigue life? How many sortites were in those training phases? Were there required periods of low-g flying in between?
Was fatigue tracked at all at that time?
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Waddo Plumber,
This is exactly my impression. Something like a preflight briefing. Nevertheless, I think there should be some formal directives.
We do the stuff you described, only for transport planes. In our bussiness reaching the limit G-factor (+3g) is really rare. I am literally shocked hearing fighter pilots get +9g so often. If one of our planes did the limit G three times within, say, a week, there would be a lots of buzz.
This is exactly my impression. Something like a preflight briefing. Nevertheless, I think there should be some formal directives.
We do the stuff you described, only for transport planes. In our bussiness reaching the limit G-factor (+3g) is really rare. I am literally shocked hearing fighter pilots get +9g so often. If one of our planes did the limit G three times within, say, a week, there would be a lots of buzz.
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You are correct in that each aircraft has several usage spectra based on the mission type for multi-mission aircraft. This is then used with the fatigue article to validate the specified airframe life (i.e. 8,000 flight hours). This same spectra is use to define engine time between overhaul. Once the aircraft enters service, if the usage is different than originally forecast the airframe/engine life must be reevaluated and possible structural updates made if the original airframe life is to remain.
An example of this was the original P&W F100 engines were assumed to be used similarly to the engines in the F-4. Once the aircraft entered service it was found that unlike the F-4 where the pilot usually selected afterburner/reheat at the start of a dog fight and left them there; in the F-15, with much more thrust, the engines were cycled in and out of reheat more frequently. This resulted in change overhaul schedules for the engine.
As for ASIP's comment about limit load: if you folks have assumed that the aircraft will see limit load once during its operational life you are in for a very big shock. In most air combat training the pilot will go to limit load several times during a given sortie.
An example of this was the original P&W F100 engines were assumed to be used similarly to the engines in the F-4. Once the aircraft entered service it was found that unlike the F-4 where the pilot usually selected afterburner/reheat at the start of a dog fight and left them there; in the F-15, with much more thrust, the engines were cycled in and out of reheat more frequently. This resulted in change overhaul schedules for the engine.
As for ASIP's comment about limit load: if you folks have assumed that the aircraft will see limit load once during its operational life you are in for a very big shock. In most air combat training the pilot will go to limit load several times during a given sortie.
F-15 pilots (in the days of the A and C models) were trained to use max g (9) in every fight. That's 4 to 5 times per training sortie in the combat taining phases. From what my oppos on the F-16 tell me it was the same there. That's a lot of counts on 9g in each airframe's lifetime.
Taking the tanks off and flying to the proper limits once a year at Deci' wasn't really an acceptable alternative - but that's all we were allowed.
When the Hawk replaced the Hunter at TWU, BwoS were surprised that the aircraft was being flown to higher G values than the Hunter. "You didn't use such high G on the Hunter, why are you doing so on the Hawk?", they asked.
"Because we're fighter pilots and because we can!" was the response. Nevertheless, the Hawk G limit was soon reduced to save fatigue consumption....
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ASIP, there is a very big difference between an engineering structural limit load (where some damage to the airframe may (will) result, but it won't fall apart) and the G-limit imposed on pilots. The G-limit is imposed to allow to aircrew to operate up to it, without damaging the airframe. I don't recall seeing a structural load limit published for fighter aircraft - generally if we needed to pull so hard that we might damage the airframe there is probably good reason (such as the ground, missile inbound etc).
One of the best steps forward in fatigue and load monitoring on the F-15 was the introduction of the OWS - Overload Warning System. This measured gz at valious points and calculated stress in the wings, fuselage, mass items (engine mounts and the like), etc. It took into account fuel weight and stores, rolling g and produced an audio tone to allow the pilot to adjust the amount of pull, to get to and maintain amximum allowable g for any configuration, airpeed, altitude, etc.
If the calculated g limit was exceeded Betty would chout "Warning, Over-g" and we stopped the fight (assuming we were only training). We could then call up a display if the warning and work out if we could continue or go home. (That was due to carrying simulated weapons loads.)
Introducing the system increased the stated g-limits and reduced the number of unknown overloads. Good for everyone.
If the calculated g limit was exceeded Betty would chout "Warning, Over-g" and we stopped the fight (assuming we were only training). We could then call up a display if the warning and work out if we could continue or go home. (That was due to carrying simulated weapons loads.)
Introducing the system increased the stated g-limits and reduced the number of unknown overloads. Good for everyone.
If I may echo Burrito's point, the G limits in the Flight Manual are not the same as the structural load limit cited in the OP.
Example: back in the 80's a friend of mine was instructing in a T-34C. The G limits in the NATOPS were +4.5 / -1. Any G over 4.5 had to be reported to maintenance. His student, coming out of a split S, got a little frisky and pulled hard enough to gray/blackhimself out, and my buddy got a bit gray as he called for and took the controls.
All said and done, 7.3 G put on the aircraft. They reported it, a few bolts were replaced, some skin removed and repaired, and that bird was still flying years later. (At the time, we referred to it as "The Corsair" ... )
Did that one pull materially shorten the FLE for that airframe?
Good question. I don't have the answer to it, but I suspect an engineer at NAVAIR knows, or once knew.
Example: back in the 80's a friend of mine was instructing in a T-34C. The G limits in the NATOPS were +4.5 / -1. Any G over 4.5 had to be reported to maintenance. His student, coming out of a split S, got a little frisky and pulled hard enough to gray/blackhimself out, and my buddy got a bit gray as he called for and took the controls.
All said and done, 7.3 G put on the aircraft. They reported it, a few bolts were replaced, some skin removed and repaired, and that bird was still flying years later. (At the time, we referred to it as "The Corsair" ... )
Did that one pull materially shorten the FLE for that airframe?
Good question. I don't have the answer to it, but I suspect an engineer at NAVAIR knows, or once knew.
Last edited by Lonewolf_50; 11th Mar 2013 at 14:57.
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Thank you gentlemen,
I've got a flavour of the problem.
Still I have difficulties to match the reports of getting to +9g several times in a sortie to the design spectrum (F-15) that shows just 4 times per 1000 hours, and even a severe spectrum, which was obtained or at least matches Structural Data Recording System data, and shows about 40 times per 1000 FH.
It is interesting that there are occurences of larger than +9g (100%) acceleration. It seems like there are different definitions of the limit load factor in structural standards (like MIL-A-83444, JSSG-2006 or FAR-25) and in Flight Manuals where "limit" may refer to a human factor.
I've got a flavour of the problem.
Still I have difficulties to match the reports of getting to +9g several times in a sortie to the design spectrum (F-15) that shows just 4 times per 1000 hours, and even a severe spectrum, which was obtained or at least matches Structural Data Recording System data, and shows about 40 times per 1000 FH.
It is interesting that there are occurences of larger than +9g (100%) acceleration. It seems like there are different definitions of the limit load factor in structural standards (like MIL-A-83444, JSSG-2006 or FAR-25) and in Flight Manuals where "limit" may refer to a human factor.
Cold War: Probably because I had an engineering degree, I was nominated as Sqn "Fatigue Officer" when a new set of sortie/role restrictions linked to the Fatigue Index of the jet came in. The idea was to find the limits of the sortie FI logging procedure so we could effectively carry on flying the way we were doing. It took me about 3 weeks to come up with a few simple rules, e.g add a couple of GCAs at the end of an ACT sortie to make the sortie length over xx minutes. No lying necessary.
From my inexact understanding of the FI calcs, I reckoned this would knock about a third off the real lifespan of the jet.
The reason this was done was because the squadron was tasked with being Operational, and it would not (in the estimation of my seniors and betters) have been possible to do this and follow the life-extending procedures.
It's pretty simple really. You can't pull 9 'g' for the first time in a real war. Buy jets more often or accept they are less capable because of the restrictions. You can't have less FI usage and keep full operational capability. Same applies to flying hours.
If the politicians and REMFs aren't prepared to accept this, the operational crews will find a way around your "rules". Always have, always will. It just suits us to appear to be knuckle-dragging banana munchers.
p.s. I have a massive 1 hour in the mighty F-15B, and personally pulled 9'g' five times. Oh boy, was it fun!
p.p.s. The Bulldog entered service in 1972 with an official release to service of 4.75'g'. For some unknown reason, the Aircrew Manual and FRCs permitted 5.25 'g' until 1984. Student doing aerobatics were taught to fly most manoeuvres to 5.25 'g' until that year. That was fun too.
From my inexact understanding of the FI calcs, I reckoned this would knock about a third off the real lifespan of the jet.
The reason this was done was because the squadron was tasked with being Operational, and it would not (in the estimation of my seniors and betters) have been possible to do this and follow the life-extending procedures.
It's pretty simple really. You can't pull 9 'g' for the first time in a real war. Buy jets more often or accept they are less capable because of the restrictions. You can't have less FI usage and keep full operational capability. Same applies to flying hours.
If the politicians and REMFs aren't prepared to accept this, the operational crews will find a way around your "rules". Always have, always will. It just suits us to appear to be knuckle-dragging banana munchers.
p.s. I have a massive 1 hour in the mighty F-15B, and personally pulled 9'g' five times. Oh boy, was it fun!
p.p.s. The Bulldog entered service in 1972 with an official release to service of 4.75'g'. For some unknown reason, the Aircrew Manual and FRCs permitted 5.25 'g' until 1984. Student doing aerobatics were taught to fly most manoeuvres to 5.25 'g' until that year. That was fun too.
Last edited by Fox3WheresMyBanana; 12th Mar 2013 at 00:37.
I've always been under the (possibly misguided) impression that the published flight manual g-limits are around two-thirds of the calculated g at which structural failure would occur.
In my experience of military flying, flight manual g-limits are regularly flown up to as many times as are required for the training or task being done. You don't go out and go crazy with the g for no good reason (well, mostly...) but nor do you hold back when required.
This then leads to possible mismatches between the designers' assumptions and what is actually done, therefore sometimes reducing the service life of the airframe. Reduced g-limits are sometimes introduced to try and extend service life when problems (eg fatigue cracking etc) are detected.
One case in point is the Pilatus PC-9 in service with the RAAF - as an advanced trainer it naturally copped a flogging, and needed to to get the job done. However, it was soon realised that this would quickly mean the limited fatigue data available from the manufacturer wasn't sufficient to predict what might happen to the airframes long-term (bearing in mind that we Aussies keep types in service for a long time, historically).
A solution was to take a representative airframe off line, stick it on a fatigue rig which basically pushed and pulled at it repetitively for months/years to rapidly simulate the effects of the above-mentioned flogging, thus generating useful data that could then be used to work out a more accurate estimate of airframe life under real-life working conditions.
So - designers design them to a set of assumptions, then they are used within flight manual limits (barring occasional stuff-ups) and fatigue occurs at a rate governed by same.
In my experience of military flying, flight manual g-limits are regularly flown up to as many times as are required for the training or task being done. You don't go out and go crazy with the g for no good reason (well, mostly...) but nor do you hold back when required.
This then leads to possible mismatches between the designers' assumptions and what is actually done, therefore sometimes reducing the service life of the airframe. Reduced g-limits are sometimes introduced to try and extend service life when problems (eg fatigue cracking etc) are detected.
One case in point is the Pilatus PC-9 in service with the RAAF - as an advanced trainer it naturally copped a flogging, and needed to to get the job done. However, it was soon realised that this would quickly mean the limited fatigue data available from the manufacturer wasn't sufficient to predict what might happen to the airframes long-term (bearing in mind that we Aussies keep types in service for a long time, historically).
A solution was to take a representative airframe off line, stick it on a fatigue rig which basically pushed and pulled at it repetitively for months/years to rapidly simulate the effects of the above-mentioned flogging, thus generating useful data that could then be used to work out a more accurate estimate of airframe life under real-life working conditions.
So - designers design them to a set of assumptions, then they are used within flight manual limits (barring occasional stuff-ups) and fatigue occurs at a rate governed by same.
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Is there a document for the pilot prescribing or limiting the number of limit G occurences?
Each Hornet aircraft has a Maintenance Status Data Recording System (MSDRS), which provides information on the aircraft’s structural - fatigue consumption and the aircrew’s flying characteristics during each flight. This data is then processed by the Mission Severity Monitoring Program No. 2 (MSMP2), to provide FLEI data relevant to each mission, and to update the fatigue - life records of each aircraft. Tactical Fighter SPO uses this information to manage individual aircraft against their structural ‐ life limits and to identify aircraft requiring close structural ‐ fatigue consumption management and structural refurbishments. The information is also provided to ACG’s 81 Wing for the purpose of structural ‐ fatigue management, through mission planning and g ‐ force management by pilots. This is to ensure that Fatigue Life Accrual Rates in the Hornet fleet are maintained within agreed limits while maintaining operational effectiveness.
Hornet aircraft have over 70 structures—such as the centre barrel—which are subjected to accumulated stress. Once their accumulated stress exceeds the structure’s specified FLEI or airframe hours flown (AFHRS) limit, they require refurbishment, or the aircraft needs to be withdrawn from service or transitioned to a safety ‐ by ‐ inspection regime. At the time of the audit, the RAAF’s classic Hornets have over 20 structures that are subject to a safety ‐ by ‐ inspection regime. A register of these structures, their accumulated FLEI limits and their airframe hours flown limit is maintained in the Hornet Service Life Limit (SLL) register.
http://www.anao.gov.au/~/media/Files...05%20OCRed.pdf
Brian,
Thanks for the link. I found this paragraph particularly interesting:
Thanks for the link. I found this paragraph particularly interesting:
4.24 In RAAF service, however, the rate of fatigue accrual has been higher than that of the US Navy. This is due to the RAAF aircraft experiencing higher g and sustained‐g loadings, which have consumed the fatigue life ....
Any idea why that would be?
Last edited by India Four Two; 12th Mar 2013 at 05:27.
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One could only assume that the aircraft is being used in a way that the US Navy/Manufacturers never envisaged. Spending more time in nap of the earth, or air combat manoeuvring for example.
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Also it is not just longitudinal load factors that have to be considered. One bad example was the MiG-23 which had a tendency to depart at high angles of attack. On departure the aircraft would rapidly yaw which imposed high lateral loading on the engine for which it was not designed. This often resulted in an aircraft that recovered from departure but with an engine which had failed due to turbine blades having failed as the engine flexed on departure.
It is interesting to note that it’s not just the wings but all mounting that need to be designed to this limit.