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.

 

[rb1957]

i work in a "chop shop" and whilst i haven't seen the NPRM i've got some thoughts.

WFD is more a residual strength issue. i thought Tom Swift did some work on the reduction of residual strength with WFD ... (i'll have to look it up). Anyways, i use a highly conservative maximum crack length so that even with 2deltap i'm way under Kc.

An intangable advantage we have is that typically all the affected rivets are replaced with our mods, therefore you could argue (and i'm there will be an argument) that you've removed any existing fatigue damage (this is true if you up-size the rivets, if you don't there has been a close inspection of the site, so ...)

 

[737eng]

I too am not that familiar, just starting to get involved with reading the NPRM and such. How I see it, there is basically two important steps:
1)Identifying the fatigue critical structure(s) and determining it's life limit (DSO). Determining this structure will have to account for MSD and MED.
2)Once structure is identified and time-limit is implemented determining what mods will have to be accomplished or what structure will have to be zero-timed in order to extend it past the DSO.
Continue this process for the next critical structure, etc....
This will have to be accomplished for the aircraft by the original OEM (i.e. BAC, Airbus, Lockheed, etc....). Now, where I get concerned is that the STC holders will also have to accomplish this task for the new STC structure and for the OEM structure affected by the STC. Therefore, I am currently trying to put a list together of all DER's and/or Engineering firms that will be able to accomplish this task.

 

[crackman]

737eng

First off, WFD is primarily a durability/fatigue issue where repeated details are involved. As rb1957 mentioned, it does have to do with residual strength however this is coupled with the size of the area susceptible. In support of the FAA's WFD rule, the FAA tasked the ARAC WFD working group with developing technical guidance and recommendations. This was done because the FAA realized that technical expertise was required to develop the necessary guidance and as such it tasked ARAC with providing recommendations.

The ARAC WFD group of which I was a part for several years spent a significant amount of time performing round robin analyses of sample problems of both MSD and MED. For MSD, the lap joint was evaluated. For MED, we evaluated multiple cracked fuselage frames as well as OEM specific details. We found that the approach to determining the impact to residual strength is significantly different depending on whether you are dealing with MSD versus MED and can get fairly complicated from a technical standpoint (especially in the MED case). I would recommend you obtain (it is available to the public thru the FAA) the "yellow" report entitled "Recommendations for Regulatory Action to Prevent Widespread Fatigue Damage in the Commercial Airplane Fleet" March 11, 1999. It contains the MSD round robin samples and alot of other info as well. Also, there are additional meeting summaries which contain very important guidance information such as the MED round robin analyses we performed as a group.

The WFD working group was comprised of engineering representatives from Boeing, Airbus, Lockheed, Gulfstream, FedEx, KLM, FAA, and JAA as I recall. This WFD group spent literally years developing and testing out the guidance material and providing the FAA with recommendations for defining the requirements. The working group was made up of a very qualified group of engineers who I think did an outstanding job over about a 6 year time period in helping the FAA with this very difficult issue. Anyways, I believe all of the groups' meeting minutes and recommendations are available to the public probably thru the faa website.

As for STC's, we did spend some time discussing this in the working group as well and obviously with airlines represented in our group this was a good discussion topic. I guess without getting into specifics, we looked at the impact in terms of how "big" the STC was. In other words, if the STC was a cargo door, yes, this could be a significant impact but obviously one that certainly needed addressing. Particularly when some of the cargo doors also affect baseline airframe WFD areas such as lap joints. For smaller STC's, we looked into what size would actually require an evaluation and of what type based on its potential impact to residual strength.

As an additional task, this same group with several added members including repair stations and STC holders, was also tasked with providing recommendations on requirements for Multiple Complex STC's. The recommendations provided to the FAA included how to address the interaction of multiple STC's between themselves as well as impacting WFD areas. This too should be available on the faa site.

As for qualified people to perform the evaluation, this was discussed as well. We even discussed the possibility of requiring DTA DER's to have a special WFD delegation. At present, I no longer work for the OEM's but I run a consulting firm specializing in FDT and we are currently working on WFD evaluations for certain aircraft for both an OEM and other TC holders. It is not a simple task but it is not an impossible task. It just requires engineers with the proper experience level. They are hard to find but that's the case with most specialty engineering. There are however several consulting companies that can do this work.

Sorry I ranted on too long, just not a simple answer. I hope some of this was helpful.

Good luck

James

 

[prost]

737eng
in case you having trouble finding the "Recommendations for Regulatory Action to Prevent Widespread Fatigue Damage in the Commercial Airplane Fleet" crackman discussed, on the FAA site, here is the link

http://www.faa.gov/aircraft/air_cer...ng_Aircraft/media/ARAC_WFDFinalReport399A.pdf

 

[737eng]

Read quite a few reports and there is a lot of information that is quite hard to read and comprehend right off the bat, so I broke it down into the following:
NPRM will require:
1. All TC holders to identify IOL and to provide this IOL in the ALS.
2. All STC holders to identify IOL of the STC structure and STC affected structure and to ensure that mod does not affect original baseline IOL and to include in ALS.
3. Repairs to fatigue critical structure(made prior to reaching IOL) - will have to be evaluated to ensure they do not affect IOL. This will go beyond a standard DTA and will require a WFD evaluation.
4. Repairs (made if IOL is extended and after initial IOL) - will have to be evaluated to ensure they do not affect the new IOL and will require ACO approval.

I have many questions, but will continue reading and trying to interpret. But when it comes to repairs to fatigue critical structure made prior to reaching the IOL, I have the following question for now (this may be a question for the FAA and no one may have answer yet!!):
If a repair is accomplished which affects Fatigue critical structure (i.e. lapjoint) and a WFD evaluation has to be accomplished to ensure it does not affect the IOL, will an engineer/engineering firm be able to accomplish the DTA taking into consideration the MSD/MED and then will the consultant/company DER with damage tolerance delegation be able to buy off the repair and analysis? Or will the DER only be able to recommend approval and then submit to the FAA for approval?

Crackman,
based on your previous post, do you know if the FAA is still deliberating on coming up with a seperate Chart A delegation, just for WFD. Or will a DER with A12 or P12 be able to buy off the DTA considering the WFD?

 

[crackman]

737eng

Yes, the amount of reports and guidance is pretty voluminous and can get a bit heavy at times with respect to technical requirements. However, the overall analysis approach is fairly straightforward even though it may get involved.

With repairs which impact PSE's in the ALS which would also apply to those establishing the IOL, typically DTA DER's are only allowed to recommend approve and the ACO must do the final approval. However, (now dont quote me on this as this obviously depends on who you are dealing with), but if the repair you are applying to the PSE physically impacts it but its design meets or exceeds the IOL and it does not impede any inspections, then you may be allowed to DER approve it. The rationale is that you are not "detrimentally" impacting either the IOL or PSE inspection. This seems to be fairly accepted in most cases.

With repairs, the most difficult task may be identifying whether or not the area being repaired is prone to WFD. The OEMs are to provide the IOL but may not necessarily identify each and every area which sets the IOL. As a rule, I have always done both a DTA and a Durability/WFD evaluation for repairs and incorporated any design requirements required to exceed the DSG. Once you have had experience doing the analysis its pretty straightforward and easier to do than to guess whether or not its required.

Have not heard any more about special delegations for WFD. However, as in some special cases, the ACO's may review the DTA DER's qualifactions for WFD before allowing him to proceed on a project. They may even forward the DER's work up to TAD for review since WFD and the IOL are a new field in terms of approvals. I do know that there is lots of training planned within the FAA regarding this new rule to make sure everyone understands it.

 

[MotleyTool]

We are doing a project to duplicate a aircraft from 30 year old drawings. Our models always seems to be off 0.01 to 0.001 off in the drawing file.(We are using sketcher and primitive modeling) Problems happen to both. Do I need to change modeling tolerances? Any suggestions.

 

[rb1957]

can we copy this thread to the "FAA code issues" forum ?

 

[crackman]

Sounds good to me although that forum doesnt seem to have alot of activity. By the way, just out of curiosity, does anyone know how many consultant companies are working on WFD programs?

We are currently working on WFD programs for 3 different aircraft affected by the NPRM. One of the key issues for most companies doing this type of work is the development of representative external and internal airframe fatigue loads and corresponding spectra. During the first year in business, we worked almost full time in developing our own external and internal loads and spectrum methods and got them FAA approved. I was just wondering how many other companies have done this and are prepared for this under the WFD Rule. Most engineers with good training can eventually do the WFD evaluation but developing the aircraft fatigue spectra is a different story altogether.

James

 

[rb1957]

I'm working on a DTA manual for TC delegation. My approach for a fatigue spectrum for fuselage components is to work with the hoop stress due to the maximum relief valve setting (which gives some margin over the service pressure differential, and is a number easily obtainable from the Maintenance Manual). i then factor this for different parts of the fuselage to determine a once-per-flight (G-A-G) stress, in order to account for gusts and manoeuvers and torques (shear stress has a big effect on max. principal stresses, a bigger effect than the fuselage bending moment spectrum). factors are typically less than 2.

for simplicity i also use the basic hoop stress as the GAG for the unpressurised portion of the fuselage, pretty conservative.

i have had the chance to spot check with OEM data and the approach is reasonably conservative. btw, i work mainly with commuter turbo-props.

i think too that fuselage hoop stress should also be a reasonable guide for the wing GAG stress, since both structures have similar durability.

 

[crackman]

Hate to disagree rb1957 but I have spent about 15 years developing fatigue spectra for fuselages, wings, landing gear and empennages. I have done a bunch of comparisons of different approaches from very conservative but simple approaches up to full loads development. Here's what I can say for certain.

No matter how you cut it, there is no way to develop a simple approach based on pressure or any other single load to represent the full effects without a thorough knowledge of what the damaging baseline airframe conditions are. Everyone seems to want a generic approach that applies to all aircraft that is simple. Why? Because most people dont have the expertise to do it, not because it is correct. In fact, the FAA tech center was working on funding some companies which I am in contact with regarding developing a generic approach. They have quickly and recently found out this is not achievable and therefore will be working on developing a spec to follow rather than a generic method. It is very difficult to justify a "conservative" approach unless you have the original OEM data. I have personnaly evaluated these so called "conservative" methods and found many of them to be unconservative when compared with OEM data (both while I was at the OEM and now as a consultant). And even if it does turn out to be very conservative (something the FAA is now paying close attention to and may be tough to prove), it ends up impacting the customer by resulting in overly conservative inspection methods and frequencies.

Just as a sample of some of the current simplified methods and why they dont work, the following is a current conservative approach used by many DER's (not me of course) of using the following to come up with a 1G stress: (Fty/1.5-PR/2t)/2.5. Assuming a business jet operating at 10. psi with a 50 inch radius and 0.040 2024 skin, the 1G stress would be 8.7 ksi. Now, you have to use this with a GAG delta G of 1.5g. This ends up with a once per flight stress of 19.3ksi and limit stress of 42ksi! Cant develop very nice inspection intervals with this and residual strength is minimal at best resulting in fictisiously small critical crack lengths. I cant tell you how many times I have been asked by owners/operators to redo someone else's analysis as a result of the above approach.

As an example of the correct approach (an not very technically difficult for the average DTA engineer so long as he has been trained and has an aero background), the following is the way to develop spectra for say any installation on the fuselage. First develop fuselage 1G bending moments. This can be done with weight and balance reports, etc and basic aircraft data. Then, develop internal loads using some simplified approaches. One very good way is using a unit beam approach which is essentially a 2D analysis of a section of fuselage with geometry, skin and stringer properties (FEMs take far too long). This will provide you with good 1G internal loads. Then, use the multitudes of FAA recorded load histories for gust, maneuver, landing, and taxi contained on their website to develop the spectrum:

http://aar400.tc.faa.gov/Programs/AgingAircraft/airbornedata/FAA HTML Design Template5a.htm

The FAA has spent tons of money developing these databases for both large and small aircraft as well as commuter aircraft. There is no reason why any DTA engineer should not be using them. Obviously this approach can get more complicated depending on the structure such as wings, empennages or etc. But, IMHO, if you think its took complicated then you probably should not do the work or try to oversimplify it.

I spent the last few years developing both our own overall methods as well as the individual methods required to develop fatigue spectra for all sorts of aircraft from large jet transports such as 737, 747, 757, 777, A319, A320, A321, to business jets CL600, Lear35, Gulfstreams, Beech 1900, to props and turboprops L188, P3, P2V, KingAirs, etc. I can certainly tell you it was a bit of work but it was also not overly difficult, just took some time. And, once the methods are in place, you dont have to spend time redeveloping them. This is an area which is sorely lacking in the consulting business: methods development. I understand it costs money and its a tough market to compete it but providing customers with more realistic inspections will get you more business over the long run. Trust me, I know this for a fact.

Sorry to ramble on so long, but this is a favorite topic of mine if you didnt notice. Good luck.

James

 

[rb1957]

james,
granted your approach is much more accurate and mine is simplistic. i've seen that formula for 1g stress before and just shook my head ... so many assumptions, it seemed a very poor foundation for generating a stress spectrum.

my work is mainly is small mods (like antennas). i'd hate to have to generate 1g stresses, tho' your method is very rational. i find that my approach generates (in my opinion) reasonable inspections, being mostly interested in crack growth under an external doubler.

for my work, WFD would be link up of cracks at all the rivets, the doubler being frame-to-frame, this would be a 1 bay crack which most cabins probably can't arrest (would become a 3 bay crack). without thinking too much about this (yet), i'd probably propose that the doubler be removed at time in the future, get the rivet holes HFEC inspected, and re-install with a new threshold.

russell

 

[rb1957]

btw james,
just did a quick calc on your example, (R=50", t =0.04", p=10psi) ... hoop stress = 12.5ksi; that's about double my experience, if that's typical, then i can see that you don't have much room to be conservative.

russell

 

[crackman]

Yeah, the example I was using is typical for all of the new high altitude business jets, ie Gulfstream, Canadair Global Express, etc. They are all certified to 50,000 feet and normally have 0.040 skins. They do operate at higher stress but of course the DSG's arent anywhere near those of commercial airlines and are generally around 20,000 hours / 15,000 cycles. The kicker is the residual strength of course.

You are right that for very small private aircraft, ie twin Cessnas, KingAirs, (although only with respect to fuselage as wing is an entirely different story), the stresses are fairly low as well as the usage so conservative inspections may not be seen as detrimental to the operator.

What I was referring to though was any aircraft in regurlar service, an HFEC or worse yet a BHEC inspection every 1500 hours would be a killer to them. They normally rack up about 3000 hours a year. The basic request I get from all of my customers is an initial inspection threshold at DSG/2 and a repeat of a C check if possible. Obviously this is dependent on the design and I try to provide them the necessary design input up front to achieve this. I have actually done alot of business redoing DTA analyses of simple GPS, TCAS, VHF, etc 1 bay antenna installations for various customers just for this very reason. Sure its a bit of work the first time around, but once you have it set up, then its just clockwork.

Main reason for going on about this topic is that the FAA is taking a much closer look at how DER's/Consulting companies are developing fatigue spectra and this will probably get more detailed with all of the WFD work since it is far more critical structure than just simple antenna installations.

James

 

[737eng]

Crackman,
In my search, I have only found a few firms who are working on WFD programs, yours being one of them. I do know of a few consultants who currently are in the process of trying to have there mods certified to 25.571 ammendment 96 which is a big deal in itself. However, very few consultants are focusing or gearing up for WFD evaluations yet. In addition, I spoke with several ACO engineers who I am sure you are very familiar, and it sounds like there will eventually be a new delegation for DER's qualified to approve a WFD evaluation, prior to developing this delegation there will only be a few DER's in each region who will be given a special delegation to crutch the system. Sounds like there will be an FAA oversight committee who will monitor all ACO's to ensure that they are all following the same procedures and methodology and I am sure that all WFD's which affect Transport Category will have SACO involved in some way.

 

[crackman]

737eng

Sounds like the same story I have heard thru my contacts as well. Probably not a bad approach but they (the FAA that is) really needs to initiate some training very quickly. With the currentl schedule in the NPRM, its going to be a tight fit to get even the basic efforts done. Boeing and Airbus will be fine as they have been very proactive in the whole WFD process and should have no problem complying. The problem will be as you have stated with the STC holders and out of production aircraft without OEM support. With a limited number of DER approved to do WFD, timing will be crucial. But, as you know, many companies really dont want to do anything until they have to. Just a month ago I spoke to a company whose response was "why should I do anything right now? You know how the FAA is, this may turn out to be either watered down significantly or cancelled altogether." Tough to answer such statements. I do hope the industry does smarten up, the last thing we need is more bad pr at this point.

James

 

[737eng]

I seen BAC's take on the NPRM today and they have developed and have approved their WFD methodology and do have the IOL's for all pre-ammendment 45 aircraft identified and to the FAA, however, they are wanting to put off development of the IOL's for all post-ammend 45 aircraft until they reach their original DSO. They are working with industry on putting together comments for the NPRM right now.

I do hope that the final rule is somewhat different than the current NPRM in terms of the deadlines they have set, however, I do not forsee it being watered down too much due to the issue being years and years overdue.

One of the biggest problems right now for the STC holders is that the whole NPRM states that the TC holders are to develop the methodology and the IOL's and then provide this info along with guidelines to the industry. The STC holders will then utilize this provided info to evaluate their STC and provide WFD operating limits and maintanence as req'd. This pretty much puts us at on hold until the OEM's provide us with the data and guidelines. This is going to force a huge amount of workload on the few DER's and the ACO's at one time!!

 

[crackman]

I did know that both Boeing and Airbus has IOL's for the pre-amendment 45 aircraft since I was on the WFD ARAC working group with them. Although we called it LOV at the time. I really dont see Boeing's reluctance however on the new aircraft. They can basically set the IOL at the DSG. They may just not like it from a marketing standpoint since it might be construed that having the IOL at the DSG limits the aircraft which it does not.

At the time I was on ARAC, we discussed the deadlines for a long time and discuss proposal to push out the deadlines. But, it was really hard to decide on pushing it out when we reflect on the fact that all of this started with Aloha in 1988. 18 is already way too long. The problem with the schedule is that if you dont lay a deadline that is nearby in front of companies then they just put it off and then end up screaming about it in the end anyways.

I am not sure that in the end the FAA will saddle the STC holders with waiting on the TC holders. I think that if STC holders assumed the TC holders DSG as the IOL and went from they that there should be no problem. I dont know of any TC holder setting IOL's below DSG. I certainly think that some of the major STC holders could start their plan now and get a few specialized consultants to help them. I think the ACO's would probably look favorably on any STC holder that was trying to be pro-active. Remember that the ACO's are going to bear the brunt of the burden in oversight and they already overworked and understaffed now.

 

[737eng]

BAC's point on not originally setting the IOL to DSG, and waiting to DSG to evaluate the IOL is two fold:
1) once you set an IOL, it is set and it will take an extension or an Extended Operating Life (EOL) to move it past that original IOL. It will be much more time consuming and more intensive to get the EOL then the original IOL. In addition, the EOL is much more restrictive on the operators due to the restrictions and requirements on any mod or repair accomplished on any aircraft past the EOL. This as you stated, is not good marketing.
2) Part of the WFD evaluation is based on service history. BAC is stating that there is not enough service history to support setting IOL on newer aircraft. I feel that this point is somewhat valid and concur, however, I feel that this data could also be obtained from further full scale fatigue testing. In my opinion this is also a marketing issue, if you were an operator and were in the market to purchase a new aircraft, would you want to know the operating life limit of that aircraft or find out what it is 15 years from now when the lead operator reaches it DSG? From a marketing standpoint, I feel that it would be better to spend the time and money to do the full scale fatigue testing up front and sell the aircraft with the actual IOL.