Question on the definition of Limit Load and service life

[737eng]

I know that Limit Load is the maximum load anticipated on the aircraft or component during its service life. My question is what is the definition of service life and how it relates to limit load. I was taught that a limit load is only to occur once in the life of a fleet (i.e. 1 aircraft out of proposed fleet of xxxx aircraft will see the limit load 1 time), therefore, most aircraft/components never see limit load during their life. Others I have talked with feel that the limit load could be reached on every aircraft, every flight. This has spurred some arguements lately and I went looking for some sort of direction on limit load, but could not find anything beyond 25.301 in the FARs, and no orders/notices, AC, etc....

Does anyone know where I can find any info on the requiremetns of "limit load" beyond what is defined in 25.301. Would really like to see a probablility of occurence, or similar.

 

[rb1957]

generally i'd say there's no connection (between limit load and service life). i take it that your interested in deriving fatigue loads, possibly based on limit loads, and whilst there's no general rule (applicable to all) i think you can make up rules for different components.

eg, for a fusealge i think the equivalent 1/flt load is about 1.2 to 1.5 * hoop stress, depending on where on the fuselage i'm looking.

a flap, for example, would get close to limit load nearly every flight; a H. stab, probably not; a hydraulic system, probably would ...

 

[737eng]

rb,
thanks for your response. My interest is to disprove the old mentality of "it flew in that way, it will fly out that way" by proving that the aircraft most likely did not experience or more critically will not be able to sustain the limit load (and thus is not in compliance). I want to prove that even though the aircraft is capable of carrying a limit load on every flight, that the aircraft most likely does not see a limit load during it's life. Therefore just because the aircraft flew in that way, doesn't mean it is compliance or can sustain a limit load if it experiences it during the next flight.......

 

[rb1957]

i'm hearing ... "just because a design's been flying for X years, doesn't mean it's compliant" ... absolutely ! ... if it often offered by project weenies as "the design's been flying for X years, 'course it's ok".

but the flip side is "the design's been flying for X years, and you think it's non-compliant ... are you sure your analysis isn't overly conservative ?".

but i don't see the point of "I want to prove that even though the aircraft is capable of carrying a limit load on every flight, that the aircraft most likely does not see a limit load during it's life."

 

[737eng]

I am trying to prove that you have to accomplish an analysis to show that it can carry the limit and ultimate loads, even for Minor repairs or damage. To me stating that the repair or damage is minor and just stating that the "aircraft flew in that way" or "we see that damage all the time and it has never resulted in failure....." is not a valid justification for not accomplishing the analysis to show compliance. However, that is the current mentality of some others that I have to deal with.

I didn't want to bring this up on here and get into a major vs minor discussion since I have to live with it daily, but
I am preaching that every repair or disposition of damage(no matter major or minor) has to first be shown to comply with the regs and then secondly the determination of major vs minor has to be made. Both the major and minor require the same substantiation with the only difference between the two is that the major requires the FAA approval of that data. I am being portrayed as "blowing minor issues out of proportion". My question on the probability of occurence for a limit load is for a presentation that I am putting together to try and defend my stance.

 

[rb1957]

weelll, i think there is plenty of room for engineering judgement to assess that this repair is acceptable, and most times (unless you're the OEM) you don't know the limit load of a detail. and, what is the requirement of a repair ? to return the structure to it's original strength, is a reasonable answer to me. in my mind, certifying a repair means that, rather than "i've proven that the repaired structure can handle limit load". remember the original design was certified, which should be good enough as a baseline.

there are lot's of things we can "get away with" today 'cause we've had 50+ years working with the structures and materials. in my mind, that's a big concern with the new composite structures (787) ... we don't have the "body of knowledge" so we'll have to tread very carefully.

mind you, the two reasons you gave for accepting dispositions are pretty darned crappy ... recurring problems should have terminating actions (so they don't recurr).

 

[ieaz123]

A repair must restore the capability of the structure to sustain regulatory loads. Usually that is done by restoring the static strength of the damaged structure since the applied loads are generally not known. In some circumstances, if the loads are known, it may be possible to demonstrate compliance with the FAR’s for structure that has a lower capability than the original, i.e. a blendout or something like that. This must be done with extreme caution unless you know what type of analysis and criteria were used during the design of the structure. Just because you can show a MS > 0 for some type of check doesn’t demonstrate compliance.

The difference between limit and ultimate load or probabilities of occurrence of a particular load level are irrelevant to the structural adequacy of a repair. Also, major and minor classification play no role in the regulatory requirement to demonstrate compliance with the FAR’s for repaired or altered structure.

 

[737eng]

Thanks again for your responses. I think we are all in agreement that the structure has to be shown to meet limit and ultimate loads when either repaired or damage is reworked and let go. Compliance has to be shown by some sort of engineering substantiation, what that engineering substantiation consists of could vary, as long at it adequately shows compliance. This is what I have been saying all along, but I don't think I am getting through, which is why I was looking for another avenue. Based on your responses, possibly the aircraft not seeing limit load every flight is not the correct path to take.

 

[SWComposites]

On a typical commercial aircraft flight, the only parts that see loads anywhere near limit are flaps, spoilers, thrust reversers and the pressurized part of the fuselage. And even these see probably only 50-70% of limit load.

Limit load is rarely if ever seen.

All repairs must restore regulatory load capability (ultimate), and repairs to PSEs must restore DT capability.

stating that the "aircraft flew in that way" or "we see that damage all the time and it has never resulted in failure....." is wrong, idiotic, does not comply with regulations, and demonstrates a complete disregard for safety.

what is the requirement of a repair ? to return the structure to it's original strength. No, the requirement is to restore regulatory load capability, not original capability which may be higher than regulatory requirements.

 

[IRstuff]

The icing problem on the Shuttle never resulted in failure until it did.

TTFN

FAQ731-376

 

[graemew]

Some interesting views expressed here, not all of which I could agree with.
All repairs on large aircraft should comply with FAR25.601
"The airplane may not have design features or details that experience has shown to be hazardous or unreliable. The suitability of each questionable design detail and part must be established by tests."

If you are repairing something, it means that it is in some way "broken". "Experience" has just shown you that the original configuration is in some way unreliable and perhaps hazardous. If it is a single instance you may be able to argue that that an isolated event doesn't constitute unreliability, but that is an aguement that needs to be made at the time. It follows that restoration of the design to its original configuration (and I am including any required maintenance here) may not be adequate to enable certification of the repair design to FAR25.601 (which will be required).
It is possible that the damage which is being repaired is an indication that the aircraft doesn't meet its intended usage or that the usage has changed and the design hasn't been changed to reflect it (and I include the required maintenance in this)
Got to go. See you

 

[thruthefence]

"recurring problems should have terminating actions (so they don't recurr)."


If you design a 'repair' that exceeds original design strength,( to prevent problems from recurring ) doesn't that now become an "alteration" ?

Not saying that's a bad thing, just thinking of the regulatory aspects of it.

 

[KirbyWan]

There are areas where the structure can end up less strong then original and still be in compliance. The CMMs and SRMs has acceptable repairs such as blending nicks and minor cracks which do not actually restore the original strenght. However each of those repairs is qualified to each part of an airplane based on the manfacturers knowledge of the actual vs. design loads. So while the structure may be weaker then original they should still perform within limit loads. So if they say "it flew in that way" you can offer the standard as provided by the CMM and SRM "this much thinning or this much damage is allowed per the SRM." Outside those limits it becomes a candidate for a repair/replacement. As a general rule for minor damage I would suggest if the original material is thinned less then 10% to correct a minor surface flaw it is acceptable. However we generally work on secondary structure. I would be more conservative with primary strucuture.

I would like to amplify IRstuff's point. On the Shuttle they had experienced O-ring degradation before, but became complacent about it because it had flown that way before. The problem with this is that it does not account for the variability of service conditions. That flap might have worked fine on the way in, but at some later time it is affected by a stronger gust and will fail then. Saying it flew is no proof of meeting limit loads. As a second Shuttle example, foam and ice have fallen off the shuttle external tank before, but it had flown that way before so it probably wasn't a problem. That complacency that it flew that way before was the reason for both shuttle disasters.

-Kirby

 

[jetmaker]

Interesting discussions. Not sure I follow where the OP was going with this, but the original question about limit and service load work this way for me.

A limit load is a once per lifetime load. Aircraft must not suffer permanent detrimental deformation.

Service load is not a FAR requirement, but an economical one. Service load may be as high as limit load and may occur several thousands of times during a flight. Of course this is a very conservative approach. Service load is often not more than 1/3 of the limit, but unless you have the data from the OEM, all you are doing is guessing. When I do a repair, I restore ultimate and limit capability, and then ensure that my repair does not have any details worse than those currently existing on the aircraft in the immediate area of my repair. This ensures that my repair will not be the first thing to fail next.

jetmaker

 

[rb1957]

i think you need to be concerned with the inspectability of the repair, and whether it (and the primary structure it repairs) can be inspected with the existing inspection program. if it can't, (eg an external doubler added to a fusealge that is inspected externally visually), then you need to apply a new program.

 

[737eng]

Well, I finally was able to pull quotes directly out of AC 120-77 and think that I at least got someone's attention on the documentation and substantiation requirements for a repair. In lieu of trying to "dumb" down my presentation, I just stuck with the Regs and stated that all repairs, minor or major, have to be substantiated to meet the Regs, period.

jetmaker,
I agree entirely, thanks for the response.

Where was I originally trying to go with the original post --- I was trying to come up with something to prove to other engineers that you can't utilize previous non-substantiated repairs as justification or substantiation for disposition of the same damage on another aircraft. I was told that this was acceptable since the "five or so" other aircraft had no probems, I stated that the other "5 or so" may have never even experienced a limit load and therefore couldn't be utilized as justification, this spurred the conversation described in my original post. I should of just originally stated that the original 5 of so repairs were never shown to be in compliance with the regulations and left it at that.