Structural pressure test after big repair in fuselage

[Ruag]

Hi everyone,
Does anybody have experience in performing a structural pressure test (not a simple leakage test) after fuselage skin repair (500x500 mm doubler)?
Shall I really take care of it? Is it sufficient to reach the pressure relief valve setting? Does anybody have guidelines or procedures about that to share?
Thanks!

 

[rb1957]

you're supporting your installation with some stress analysis, including damage tolerance, yes?

it should be enough to go to max relief valve pressure. when we replaced a type III exit with a window, we went to 1.5*, but that was some 20 years ago and we haven't done it recently when we put a large window into a fuselage.

a tonne of analysis, some predictions, some ground/flight test, GTG



Quando Omni Flunkus Moritati

 

[Sparweb]

I'm a little surprised by the question: There are few reasons why a strength analysis shouldn't be adequate to substantiate a fuselage skin repair, as far as static strength goes. The regulations are set up to make it possible to do this. If you've never substantiated a skin repair before, there is advisory material and, of course, the regulatory agency in your country, to help you. And for fatigue, that's what DTA is for. The doubler is quite small, so there can't be a large opening in the middle of it. With any luck, the person who installed it knew what they were doing, and has done a proper job.

STF

 

[Ruag]

Actually this has not to do with the proof of any strength analysis.
According to our procedures the pressure test is required after the fuselage assembly during production and after big repairs or additional window/viewport cut outs.
Unfortunately this test requires special equipment (compressor and door panels) that the operator has not and the pressure value is greater then the relief valve setting.
Well the only thing he can do is to pump air in by means of the A/C conditioning system up to the relief valve setting. My question is: is this enough to get the same benefit achieved by the proper test?
)The aim of this pressure test is to pre stress the structure in or to make the repair/modification able to set (especially the joints) and get benefit for the fatigue and DT)

 

[rb1957]

confusing .. are you an OEM ? "According to our procedures the pressure test is required after the fuselage assembly during production"

"The aim of this pressure test is to pre stress the structure in or to make the repair/modification able to set (especially the joints) and get benefit for the fatigue and DT" ? really? i mean i understand what you're saying (beneficial effects of an overload) but to claim this benefit the overlaod needs to be part of the analysis spectrum.

but with either of these statements the question becomes "why are you asking ?" ... it sounds like it's your common practice.

if the operator is using standard a/c equipment to do the test, i doubt he'll get much above operating pressures (even if he disables the outflow valve) and so won't achieve the result anticipated (a "healthy" overload).

if you are the OEM and you recommend this overload after large repairs, it'd make sense to me that you'd have the necessary equipment available to your customers (to rent).

btw, i assume we're talking about Al alloy fuselage (ie no composites, which react negatively to overloads)

Quando Omni Flunkus Moritati

 

[WKTaylor]

Generally speaking, a proof-pressurization test after major pressure vessel mods/repairs, and/or during every depot maintenance cycle, is an excellent requirement.

Reasons are varied.

Verifies static load capacity of the mod/repair and of the entire pressurized structure; and confirms there is no noticible deformation or yield... especially around mods involving the skin. Any weaknesses are likely to show-up at this point. Be prepared to inspect around the mods/repairs using straight edge and fuselage contour gages to help find bulges or distortion.

Proof [overload] pressure also tends to stress [strain] relieve minor defects. When I worked on T-38s, there was an expectation of an occasional over-load [+over-G, every 500--1000-Hr, student pilot induced]. The nature of the structure was such that this was considered stress/strain relieving [cold-working] for small cracks... especially useful for extending life of the wing skins and body longerons.

Proof-pressure loading also ensures proper sealing has been accomplished and the structure is "tight" within leak-rate limits.

There are a couple of other benefits, such as...

Ensures that doors and panels seal properly and continue to operate [open-close] normally.

CAUTION. make SURE doors and large panels have protective strapping/webbing loosely covering over them... just in-case one comes loose [will make a bad day for everyone]. For certain mods/repairs this might be a good idea also.

Also... these tests tend to uncover the odd [1-off] system of structures "surprises" that may be hidden, such as loose or leaking [bleed or ECS] air ducts; pressurization-induced binding of controls, etc..

OH yeaaaa if You hear a loud BANG expect the unexpected.

A few years ago a crew at Tinker AFB was proof-pressure testing a KC-135R using engine bleed air and with the out-flow valves blocked [on the ramp]. Equipment problems and operator errors [buffoonery] lead to a gross over pressurization event [~8PSI above proof], which resulted in catastropic failure of the aft upper-deck pressure bulkhead. Photos are on the web IF You're interested.

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[hamblin100]

You should be able to bypass the cabin relieve valve by capping the cabin static pressure port which I remember with citation jets was located under the pilots arm rest.