Rule of thumb: Numbers of Doublers i 6

[SteveAero]

Hi,

Is there a general "rule of thumb" that addresses the amount of doubler repairs that are allowed on a fuselage skin, specifically for FAR23 a/c? Something like a density requirement. I know that some OEMs (FAR25) mandate a full stringer frame bay surrounding a 'significant' dent must be damage free. I wonder if the same would apply for doublers...?

 

[rb1957]

I'd be interested in see Schijve's calc. I've done my own and they're too conservative to use. but then my local regulator says "secondary bending … secondary effect, no problem".

another day in paradise, or is paradise one day closer ?

 

[WKTaylor]

Tom Swift's document FAA-AIR-90-01 'Repairs to Damage Tolerant Aircraft' has a LOT of important info/concepts/philosophy based on REAL WORLD experience.

As I understand his career... Tom Swift had long career with Douglas Long Beach [DAC] as a 'fatigue and durability' engineer that cross-over/integrated best principles/practices to aircraft structural design at DAC. This included the commercial and military aircraft [A-4 Skyhawk aka Scooter, aka Heinemann's Hot Rod, etc]. I think he also evolved thinking on impact-damage and battle-damage affects/durability [I think].

As far as I can tell, Dr Lincoln [AF Material Command, WPAFB] and Tom Swift [DAC] were the 'notables of the 1970s thru 1990s in the emerging field of Fatigue, Fracture, Durability and Best Design-practices.

Theodore Von Karman said... "Some fear flutter because they do not understand it. And some fear it because they do."

I'd like to propose... "Some fear fatigue-cracking because they do not understand it. And some fear it because they do."

Also... just because... as I explain to young liaison engineers... and as I experienced 'the-hard-way' as an FSE ... "There is nothing more permanent than a temporary fix.” --Russian proverb

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[Sparweb]

I use Schijve's secondary bending and I had to learn it out of the textbook. Haven't had the opportunity to meet Dr. Safarian yet.
Secondary bending is expected and Schijve is "preferred" by my local regulators, who still sign off my DTA's. Working on the authority to sign off myself but the "mutual" training process is not yet complete.

Being dependent on the local gov't authorities for DTA signoff is tricky. When a new guy shows up, they invent something "better" for everyone's safety. You end up rewriting your methods but get the same result. On the other hand, getting authority to do it yourself takes time and finesse to build up the knowledge base and procedures that are acceptable. Depends somewhat on your pedigree, and getting authority for this today is harder than it was 20 years ago.

http://www.sparweb.ca

 

[SteveAero]

I did an investigation, and the Schijve method outlined in his 2009 paper for both doubler and skin of same thickness (0.04" and then 0.05") yielded bending factors k = 0.626 and 0.553 respectively. When I increased the thickness of the doubler to 1-gauge up it shot up as expected.
Safarian's k factors for the same configurations yield k = 0.480 and 0.505 respectively. The trend is in the opposite direction! So unless I have made a mistake in my calcs, I have concluded that Safarian yields consistently lower k factors than Schijve.

 

[der8110]

SparWeb, I would partially disagree with the assertion that getting DT authority is harder now than 20 years ago. Current requirements are in Order 8100.8D (10/28/2011) Table 4-1, page 4-7:

This reads the same as Order 8110.37C (9/20/98), before appointment requirements were moved out at 8110.37D (8/10/06) into Order 8100.8. Order 8110.37B (11/12/96) changed the damage tolerance analysis experience requirement down to two years, from seven. My expansion of authority was done when Notice 8110.49 was in effect, in 1994, which had the highest requirements of all: eight years analysis experience and specialized course required, not ‘desirable’. Before Notice 8110.49 (which is NOT software Order 8110.49), Damage Tolerance DER’s were appointed/expanded on Tom Swift’s say-so. That would certainly have the appearance of being ‘easier’ than now. As far as other appearances, it only extended to two individuals, one of whom was his mentee/my mentor. But he was still consulted for my expansion, ACO’s continued to contact him for several years before switching over to doing DT DER appointments and expansions by the Order. It was never an official part of his job.

More noteworthy than his years at Douglas was Tom Swift’s tenure with the FAA as the NRS (National Resource Specialist, now called CSTA) in Damage Tolerance. He’s got some excellent historical papers especially on the DC-10. These may have contained more details than envisioned in the clearance to publish, but no harm no foul. More important than his published legacy was his kindness and patience. When passing through O’Hare he’d stop by the office, more of a social call with his former co-worker. But he always was gracious to spend some time with me, I recall him doing a derivation of why hoop/circumferential/tangential loads are examined separately from longitudinal loads, rather than a vector sum or Mohr’s Circle. Also I got some insight into his work on rivet stiffnesses (for joint analyses): it was more or less ‘goofing off in the lab’ decades before it was published, but has proven useful nevertheless. The finger doubler was just a suggestion, perhaps tongue-in cheek, Tom was very aware of the fabrication difficulties (and technician backlash).

Bob Eastin had some big shoes to fill after Tom retired (to a thatch-roofed house in the Lakes Country) but did admirably well as NRS in his own right. More Douglas connections.

Amen to relying on the government for approvals - hard to have a robust schedule, though I have frequently been impressed on turn time for Airworthiness Limitations and AMOC’s resulting from approved DT analyses. Double Amen to the new guy inventing something ‘better’. I don’t know if that is worse than “not invented here”. Same behavior, I guess.

 

[Sparweb]

@DER8110

Apologies for causing a slight misunderstanding. You are fortunate to work in the USA where these evaluations have finally been codified in a manner that can be justified. In Canada, these things are less formal, and criteria to meet still vary from regional office to office. I haven't yet seen an initiative from Ottawa that would make this more consistent. Last year I was present during an "open mike" session with Transport Canada where a number of these inconsistencies were highlighted. Maybe something will come of it, but this was before the 737Max and the virus disrupted just about everything.

But thank you very much for the round-up of the FAA policy to qualify for adding DTA to your delegation. It would be worth my time to look up the orders/notices for background and understand a separate system of evaluation. It sometimes helps to point out the FAA method when the TCCA guy goes on a tangent (depending on how yankee-fascinated or -averse the particular person is).

I've proposed some finger-doublers in the past. Never popular! Usually asked to re-design. Sort-of expected it, but I was too young to be sure at the time.

http://www.sparweb.ca

 

[Sparweb]

@SteveAero,
Purely a gut feel, since I have no way to check your math, but when doing calculations like this, and getting one method that trends one way, and another that goes the opposite, my first guess would be to check which of these methods is using gross section stress and which is using net section stress.

It's the kind of mistake I catch when checking peoples' fatigue calcs and they pull a Ktn value off a chart and put it into a Ktg formula...

http://www.sparweb.ca