Third Party DTA 2

[structures282]

I'm curious if any Engineers working for Airlines use outside sources other than the OEMs for accomplishing Damage Tolerance Analysis? If so, who do you use? Are there any limitations to going this route. I understand Delta had a DT group prior to filing for bancruptcy protection, but that the group disbanded as the engineers that made up the group went to work other places. Any thoughts?

 

[rerig]

DERs can be rated for DTA.
There aren't many.

 

[737eng]

The key is to show compliance with 14CFR 25.571. You can utilize consultant DER's, OEM DER's, OEM AR's, FAA, etc.... to obtain this approved data or to obtain approval for the data that you generate. Therefore, yes it is acceptable to utilize other sources than just the OEM. As rerig stated, these outside sources are limited and due to the data required to accomplish the DTA(spectrum, fatigue stress, Beta functions, etc.....) they may not be able to complete the analysis in the time or budget alloted by you or your company.

I will not promote any particular person or company, however, I have had good luck with outside resources and have got good results and I have got a few very convervative results, based on a extremely conservative analysis, that produced a supplemental inspection interval that was unacceptable as an operator. So make sure you know what your paying for up front, sometimes the high bid with the longest lead time is worth it in the long run.

 

[prost]

If you go to the FAA's website

http://www.faa.gov/other_visit/aviation_industry/designees_delegations/designee_types/der/

You can find a PDF that lists all current DERs and their respective specialties. As 737eng said, you don't need to have a DER on staff, all you need to do is show compliance with the relevant gov't regulations--it seems to matter not to the FAA whether that DER is on your staff or independent, as long as the DER is certified to approve whatever maintenance action is taken.

While Delta may have gotten rid of staff DERs, they still have to comply with the laws, so they would have had to hire a DER every time they needed one anyway--can't fly without them.

 

[rerig]

737eng made an excellent point about supplemental inspection. We had received (from the OEM) 9 repairs to a 737 Horiz. Stab. Spar that was corroded.
The last repair ( 9 of 9) came with the DTA Authorization that had an inspection every 500 Hrs.
Pull the Upper Skin every 500 Hrs....I DON'T THINK SO!

We change the Spar Chord. $$$

 

[SWComposites]

so s282, what are you looking for: faster turnaround for DTA, longer inspection intervals, ??

 

[crackman]

S282

I totally agree with 737eng. My company specializes in this particular work and that is one of the biggest problems we have with customers. They do not seem to understand why they cant always get a great inspection interval (usually a C or D check) and get it quickly and cheaply.

Just to give you a couple examples. If the repair is a simple patch within one frame or stringer bay and is in an area where fuselage pressure loads only apply, then the analysis can be developed fairly quickly assuming there are no ADs located in the area. If instead lets say its a patch on the corner of an entry or cargo door, much more work is involved in developing loads and stresses. Similarly, if the repair or modification is located over the top of the fuselage where fuselage bending loads are predominant, then external VMTs must be developed in order to create the full fatigue spectrum.

Another example where repairs are more difficult is anything on a wing. Wings have fairly sizeable fatigue stresses which are due to wing shears, moments and torsions. In order to perform the analysis, one must develop a representative set of wing fatigue loads. These must then be turned into internal loads either thru a Unit Beam method or FEM before even doing a DTA.

Most literature on DTA focuses on the stress calculations, stress intensities, material data and crack growth equations. However, one of the MOST deciding factors in developing a representative inspection interval is developing ACCURATE fatigue spectra. Without the expertise in developing fatigue loads and spectra, one will always end up with having to make overly conservative assumptions. We spent the first two years at our company developing the expertise and tools to be able to develop fatigue loads from scratch for most commercial aircraft and went thru the FAA approval of them. This investment in up front methods development can be a bit costly in money and time but it has allowed us to be much more capable of developing reasonable inspection intervals for our customers. On a couple programs, we have even developed complete airframe external and internal loads to support a full DTA of the airplane without the customer going to the original manufacturer. Regardless, the key is that whatever company you select to support your repair efforts, make sure you chose one that has the expertise to do the work.

As a final note, here is one method which has been tossed around by the regulatory agencies in presentations as acceptable if loads are not available for fuselage structure where pressure and bending are concerned because of its conservatism and in my opinion it is one of the reasons why so many inspections are being developed too conservatively. The method basically is to take the material Ftu divide by 1.5, subtract PR/2t and divide the remainder by a limit maneuver load factor of 2.5. This is the 1g stress which then has to be used with an Nz spectrum. I would challenge anyone to come up with a decent inspection interval with this method especially when you realize limit load is set at Fty! Also, it assumes limit load is due to maneuver and not gust, which is not always true for all structure.

Anyways, did not mean to go so long but this is a major issue with our industry in understanding how the inspection interval, schedule and price is directly related to the amount of technical work involved in DTA and the capability of developing accurate fatigue loads.

Good luck

 

[structures282]

SWcomposites: I'm actually just looking for someone to accomplish DTA on repairs that have been developed within the group. In the past when I've sent repairs off for DT I've had a few come back as "structurally unsatisfactory". Although I never agree I am really bothered by the fact that I can actually get them passed off as interim until a permanent repair can be accomplished. If they were not statically sound how can I get interim approval for 500 - 2000 FC?? Statically unacceptable is not the only excuse I've gotten, but it's common.

Well after chatting with some of the guys at the OEMs it would appear that DTA is somewhat overloading the stress groups. The overload has led the OEMs to develop repairs for problem areas on the AC. This makes sense and is great for them, but the repairs are not always the most economical or practical for the operators. For example, we had an AC ferry flown in from MIA for corrosion in the NLG upper bulkhead that went through the panel. The corrosion was trimmed out and an external patch was applied. We wanted to do an internal patch but the Frame was going to have to be cut and modified to accomplish this. I decided to go with the external patch, which involved trimming the stiffeners of the bulkhead (basically a 0.625in plate milled down to 0.100in thick with stiffeners making up 0.625in thickness) and restore them with extruded angle. When it came time to send the repair out for DT it was immediately shot down followed with a, "You should have came to us first we already have repairs for this area." Well this new repair naturally consisted of removing the 6 foot long by 4 foot section of FR and modifying it by extending the legs 0.100in to accommodate an internal doubler. Okay now our repair took the AC out of service for 5 days, this repair (which is statically unacceptable although approved interim for 6 months until we can obtain parts from the OEM) will take the AC out of service for 4 weeks easily (probably more). This is just one of my biggest examples, I have others. There are also a slew of smaller repairs but I usually can just push them to Heavy Maintenance and accomplish the alternate repair. I finally got fed up after I got the comment, "We have already developed "this" repair we are not going to accommodate your repair."

I apologize for the prologue, SW. I'm just wanting to see DT actually accomplished on these repairs.

 

[MNLiaison]

structures282,

For curiosity reasons can you divulge the aircraft type (OEM)?

Regards

 

[rerig]

"We don't have to do that."
"We are Boeing!"

Yeah. Heard it all before.

I once asked for a hole dimension on a Detail Part. It appeared oversized.
B Engineering gave me the Drawing location for the dimension, but wouldn't give the drawing or the dimension.
You can't have a detail drawing, you might make the part.
??? I just wanted a hole size!
Yeah...thanks for nothing.

Wow, what a Monday today has been.

 

[der8110]

One fundamental issue is that DER's cannot approve (directly) inspection intervals. That is an Airworthiness Limitation, which is an FAA-reserved function.

Assuming that a really short inspection interval is economical (maybe for you), is it the smart thing to do? Damage Tolerance is a house of cards, built on dozens of input variables, some very sensitive to small changes. Then, when all is said and done, you are counting on somebody being able to detect damage. Expecting the same regime that created the crappy repair design, imposed the impossible schedule, and (in many cases) caused the damage in the first place to find the damage is a bit much to expect. I cannot imagine many third parties wanting to get into the middle of that.

Damage Tolerance is just another tool. Being more expensive does not make it the answer to all problems, or the blame for all woes. Quick, good, or cheap - pick any two.

 

[structures282]

MNLiaison: No disrespect towards you, I just rather not.

 

[structures282]

Crackman: What's the name of your company and where are you located, if you don't mind me asking.

 

[crackman]

structures282

You can find us at

http://www.avengeraircraft.com.

With respect to the earlier statements regarding approval authorization for DTA DERs, some clarification is needed. DTA DERs can approve inspection intervals so long as they do not impact PSE items in the Airworthiness Limitations. Repairs or mods outside the areas listed in the ALS can be approved. In addition, if the mod or repair does not detrimentally (either by frequency and/or inspection method) affect the PSE area, a DER could get approval from his ACO to approve it with their concurrence.

I've been a DTA DER for about 10 years now and I can say the basic trend from most of the OEMs is to basically be overly-conservative with respect to repairs because its cheaper and they are short on engineers. Unfortunately even the OEMs are "critically" short of experienced DTA engineers let alone ones who have "real world" experience with analyzing and developing "reasonable" inspection intervals for modifications and repairs. When I worked for the OEMs as well as now with some of my young engineers in training, I always make them actually go out to the aircraft to analyze the repair as well as work with the maintenance folks to develop a "workable" solution with an inspection which fits into the customers maintenance program. In addition, I make them get involved in at least one full prototyping program to actually see the inspections being done and determine if they will work. I have met dozens of DTA engineers in my career who never even set foot in a repair station let alone developed and prototyped the inspection procedures.

In summary, its a common problem that is getting larger as the FAA enforces the DTA requirements and the size of our experienced industry shrinks.