longest inspection cycles 1

[uGlay]

i am tasked with creating an external antenna mounting structure for b757 (pressurized fus) and they are asking me to make a design that will yield the longest inspection cycles possible.

how would one implement this? and is there a reference to inspection cycles on srm or amm? any help is appreciated. thanks.

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[YoungTurk]

I can't speak to Boeing specifics; but the inspection cycles on the installation will be driven by the FDT characteristics of your installation. Keeping stress concentration factors low with good attention to fatigue details and minimizing rivet loads by using multiple rows of well distributed rivets will be key. Look at Niu for factors contributing to stress severity factor (SSF).

Those design details in combination with the existing loads at the location will factor into a fatigue and crack growth analysis. Inspection intervals are then specified based on the speed of the crack growth for your particular design and how long of a crack the manufacturer has defined to be critical.

If you're just doing design, you should have an analyst taking care of the FDT analysis. Focus on the stress severity details and let them determine what inspection intervals to apply. General goal is to design such that inspection intervals required for the new installation are longer than existing inspection intervals for the skin/stringers/frames your dealing with, thereby preventing the imposition of any "new" inspections on the operator.

 

[GBor]

I've had analyses result in 45,000 cycles for threshold and 25,000 for follow-on. I've also ended up with 1500 cycles. There are many factors: fastener size and spacing, pressure cycle, aircraft bending, skin material, etc. I recommend a bit of a parametric study before you actually design the doubler. Start with fastener spacing at 6D and move wider from there. You have to make sure your fasteners can carry the load, you don't run in to a chem-mill or other structural feature, etc.

Are you working for an OEM? Or do you have a DTA DER to approve your work?

Garland E. Borowski, PE
Engineering Manager
Star Aviation

 

[rb1957]

not at the OP, but at the people directing him ... don't you love it when people make open ended requirements !? and usually don't know what they mean !!

i would look in the area of the fuselage your working on. what is the OEM inspection program ? you should be aware that you are probably changing the inspection technique (probably to an NDI. LFEC) to inspect the fuselage under the doubler, thus even if your inspection program produces longer intervals than the existing program, you're still doing an ICA.

 

[GBor]

(insert sarcastic voice) What? You don't like the "make the inspections as long as possible"?

I've actually had this exact same direction. I generally respond with something like, "I will try for a minimum of 6,000 cycles to get you to a c-check". That usually appeases them.

uGlay,

What they are really probably saying is that they don't want to inspect more often than a heavy check, but rb is smartly suggesting that you ask them about existing inspection cycles in the area of interest.

 

[uGlay]

Yes the open ended requirements are usually because of the poorly trained sales staff and our notoriously ROOKIE Project Managers that are running the programs. I love when they ask each other for advice, the blind leading the blind...

Thanks for the help guys. Yes we have DTA DER, not in-house but he does get a good 2 weeks with the design after completion. Thanks again.

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[uGlay]

RB1957: i would look in the area of the fuselage your working on. what is the OEM inspection program?

where would one find the OEM inspection program for this A/C? I am at STA800, between S1 and S2R.

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[bf109g]

The operator should have a copy of the OEM document for fuselage skin inspection in that area.

 

[uGlay]

What is the specific name of this document, does anyone know.

Thanks.

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[GBor]

757 Maintenance Review Board Report, I believe

 

[uGlay]

Thanks a lot good sir.

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[serker]

Maximum inspection interval?!?

Last time I got one of those requirements it involved cold working the holes.

 

[WKTaylor]

uGlay...

A really valuable "how-to" document for damage tolerant repairs was written by FAA guru Tom Swift: lots of practical design concepts for reducing severity of stress concentrations, design for improved in-place inspect-ability, negative and positive affects of proper assembly, etc...

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA225742&Location=U2&doc=GetTRDoc.pdf

Regards, Wil Taylor