Aging Aircraft and WFD Issue 2

[737eng]

With the release of the WFD NPRM and the ongoing Aging Aircraft Issue there is going to be a requirement on STC holders to accomplish a WFD assesment in a relatively short amount of time. It is my feeling that for most STC holders and Engineering Firms this is going to a monumental task that they probably do not have the expertise nor staffing to accomplish. I am looking for opinions, comments, discussion on this issue. Also, Does anyone know of any Engineering Firms and/or DER's, etc... that may be able to take on the task of accomplishing a WFD assessment? I am not trying to provide a forum here for advertisement for these firms, however, I would like to 1) try and show how much of a burden this might be on STC holders to accomplish this task if only a few firms are qualified and have the expertise to accomplish the WFD assesments and 2)I could greatly use this contact info.

 

[crackman]

This came up quite a bit in the working group and we were all keenly aware of the ups and downs in the industry and that the business side is an important one. However, resetting the IOL and getting EOL is not as difficult as some want to make it sound. In fact, OEMs have already extended the service lives of several of the aircraft over the years. Particularly with the major OEMs technical might and all of their testing, resetting an IOL will be very easy for them to do "if" they chose to. The other side of the marketing thought is that if you limit the aircraft to an IOL then one gets to sell newer aircraft quicker. By making it sound like a major effort to create an EOL, companies will be able to persuade airlines to purchase newer aircraft rather than keep flying old ones. You can bet your dollar that the marketing folks are keenly aware of this and are keeping it in their hip pocket. You just wait, as soon as a newer model aircraft comes out, they will immediately set the IOL for their current aircraft. Certainly there should be no questions or qualms about setting an IOL for the 777. They tested it to 3 lifetimes!

The main point is however that the IOL is just a first start and setting requirements for re-assessing the aircraft due to its age. In the working group, we spent a considerable amount of time to define how the IOL could be reset to a newer EOL and that the IOL is NOT a stopping point for the aircraft but merely a waypoint where additional evaluation needs to take place. How in depth this evaluation needs to be is then based on available technical, test and service data. In some cases, the EOL will merely be a formality (possibly as in the case with the 777).

 

[rb1957]

it sounds like the IOL is the DSG, perhaps once the fatigue testing is completed (since in the early days of in-service, the fatigue test is probably still ahappening). it is a hugh marketing thing, but most companies set their lives like their competitors. then, as james points out, if there is extensive testing data (looks like boeing have done more than required, rather than their previous practice of testing for one lifetime) and lots of service experience it should be easy to justify an extended service life (again a marketing angle would be to charge operators for this extension, offered by way of SB or STC) ... doesn't the SSID guideline require a fatigue test to twice the operational life ?

the wrinkle is going to be how to extend the life of STCs and repairs incorporated ? at the time of initial incorporation, the STC could/would anticipate airplane retirement at the DSG, if this extended later either the original manufacturer (of the STC) or a 3rd party (like james) would do the analysis. i'd suggest that it would be very difficult for a 3rd party to extend the service life of a plane without (or without regenerating) the original type cert. data ... to the current rules (under change product rules this'd be a major?)

 

[crackman]

I think thats why Boeing went with 3 lifetimes on the 777 knowing what was coming down the road for WFD. The current FAR 25.571 Amnd 96 is what now requires a 2 lifetime fatigue test for new aircraft. With large OEM's,extended testing is definitely the way to go. Set the IOL at the 2 lifetimes then extend it to the full amount by justing taking advantage of the 3 lifetime testing which immediately gives you a 50% OEL extension.

Yeah the STC side is a bit more complicated. It is tough to do but not out of the question. We are currently working on 3 aircraft for which we are doing full up DTA and WFD assessments from scratch for the whole airplane. We have had to redevelop full external loads and internal loads as well as performing detailed stress analyses before even doing the DTA and WFD.

Obviously there can be considerable costs involved depending on the extent of the STC. Our efforts involve the full airframe. If however the STC was only on one component, ie fuselage, then the effort could be reduced significantly. Wings of course are probably the toughest due to all of the various additional loadings, from engines, gear, flight controls, and fuselage attachments involved. I've worked on several major STC mods such as cargo doors, large pods/antennas, and I can tell you its not as tough as doing the entire fuselage, wing and empennage. Actually its fairly straight forward once you have done it a few times. The key in all of this too is to have a large amount of fatigue test data. It doesnt necessary have to be for the exact aircraft so long as it is representative of the structural joints you are analyzing. Over the years I have accumulated well over a hundred reports on fatigue testing from coupons to large components and full scale fatigue tests. I can tell you that they have been invaluable and a great benefit to us in not only validating our analyses but also setting up our methods. Our entire SSF method for fatigue is based on alphas and betas established by test data.