Let me pose a general fud-4-thot question. 6

[WKTaylor]

OK Guys...

Let me pose a general fud-4-thot [I loved "the Farside"] question.

Note. This thread has the intent of raising the hairs on Your neck... and stimulating discussion. Hopefully it will also raise general awareness as to how amazingly vulnerable airframe structures are to a variety of damaging factors. Relatively new engineers... or those not involved with mishap investigations and damage repairs [depot liaison, field support, etc]... will gain some insights from those of us "gray-hairs" who have "been-there, seen-that, done-that...

For all airframe structures only... NOT including engines, landing gear, instruments/electronics, mechanical/electrical/environmental systems, etc [maybe we'll try this same question for each of these later]...

Identify obvious, and NOT-so-obvious mechanisms, for structural damage.

Sub points to carefully consider:

There ARE substantial differences regarding the aircraft Type, IE: para-sails; Ultra-lights; GA; Commuters/corporate; Medium and heavy transports; Cargo; Military [USAF, USN, USCG, USA, IE: Trainers, Fighters, Bombers, Helos, etc]; Light helos; Heavy helos; LTAs, etc...

There are substantial differences regarding the aircraft construction Type, IE: all fabric; Tube-fabric; Wood; Sheet metal; Machined-metal; Composites-metal mixes; all composite; etc...

There are differences regarding Mission type, IE: training, commuting, airlines, cargo, tactical, etc...

I'll lead the parade…

Examples of damage mechanisms.

Some environmental factors. Ice/slush, "Arizona dusty road", rain, hail, airborne volcanic dust, bird/critter-nests...

Operational factors: overstress [g, airspeed], hard landings, taxi collisions, severe turbulence, bird-strikes, lightning strikes...

Assembly and Maintenance factors: force-fitting parts, poorly drilled holes, loose/incorrect fastener installations, sealant adhesion failures...

Exposure to corrosive or abusive fluids/materials: urine, salt water, detergent wash-water, jet-fuel additives, hydrazine, deicing compounds...

GOT the concept??? Your comments appreciated.


Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[rb1957]

other than covering every occurrence of murphy's law, is this thread going anywhere ... it's more like a pub discussion

my 2c ...

 

[debodine]

rb1957 the next round is yours, we look forward to your generosity in setting up the pints for us!

 

[WKTaylor]

rb1957...

Amazing bad things happen, subtle and not-so-subtle... daily... when working as an aero engineer on REAL aircraft.

Lots of "office-bound" engrs are blissfully unaware how obscure and/or brutal these events and be. This thread was started with the intent of highlighting real-world examples of the "bad things that can happen"... as fud-4-thot.

Let me know if this thread should be put-out-of-it's-misery. Otherwise... get Your experiences in writing!!!

PS: I prefer Foster's in an Oil Can.

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[debodine]

wktaylor has a very good point. It always amazes me that most engineers in our company have rarely ever walked an aircraft, much less watched mechanics try to comply with a drawing or turned a wrench themselves beefore becoming an engineer.

I have found my designs ALWAYS work better when I get an experienced lead to walk through my pre-release design with me to see if it can be accomplished with available tools and skills and access points. I am often shocked at how difficult my design would be to accomplish per the drawing, and how easy it usually is to make my design better with a few small changes. And I had 15 years working on aircraft before I became an engineer. I certainly have not seen it all yet!

So a forum like this helps me see things I have not yet personally experienced.

 

[berkshire]

The post today by Black Max on peel plies confirms thoughts that I have had for years,
about the adequacy of just ripping off a peelply and bonding straight to the spot.

I have only ever used the un coated polyester peel plies. Since I mostly work with room or low heat cure resins
The feeling in the Company where I used to work, building fiberglass sailplanes, was that the primary use of the peel ply was to remove the amine blush scum left by the room temperature curing resins. The technique used, was to peel off the ply then hand sand to get a bonding surface. Prior to the use of peel ply the bonding surface had been scraped with a hook scraper to remove the amine scum, then hand sanded to expose the fibers for bonding.
After we started using peel ply, after stripping the surface would be sanded to remove the visible weave, then to be bonded to surfaces were blown off, with dry filtered air, but not wiped down because it was believed that solvents like acetone carried more contaminants into the bond line than they removed.
Now the report Max produced, is saying that even that may not be adequate to get a good bond line. That hand sanding to remove the visible weave, may still leave un bondable sites in the joint.
B.E.



The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[blakmax]

Berkshire

We found that the best way to treat composite surfaces was to use a heat scoured peel ply, and after removcing that we grit blasted using 50 micron aluminium oxide in a dry nitrogen gas. Only needs a light blast to remove the surface molecules so you get a chemically active surface. No need to blast the impression of the ply off. Dust was blown off the surface with nitrogen.

Never solvent degrease after the grit blast. Any small pockets of contamination are dissolved by the solvent and spread over the entire surface. Besides, it is impossible to get the solvent resdiue off the surface.

Regards

Blakmax

 

[berkshire]

Only needs a light blast to remove the surface molecules so you get a chemically active surface. No need to blast the impression of the ply off.

Duly noted Max.

B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[MikeHalloran]

Uh, Max, could you write that up as a FAQ and put it in the "Composite engineering" section? That kind of hard-won knowledge never finds its way into books.





Mike Halloran
Pembroke Pines, FL, USA

 

[blakmax]

Mike Halloran

This information is in the FAA Tech Center Library in DOT/FAA/AR – TN06/57, Apr 2007 BEST PRACTICE IN ADHESIVE BONDED STRUCTURES AND REPAIRS. I also added a lot more information on adhesive bonding technology, especially in-situ repairs.

Regards

Blakmax

 

[PHovnanian]

Structural damage due to arcing electrical faults as a result of the omission of differential bus/feeder protection.

 

[Sparweb]

Betting back to "critter" stories...

The boss was looking at a Mooney parked in the grass at the end of a hangar one day. For Sale sign in the windshield. I was there for some other reason, but he's looking for a new "project". He was starting to get excited about the condition of the engine, and instruments that the owner had added.

I was paying more attention to the outside. When the walk-around got me to the tail I noticed a brown streak coming from behind an inspection panel in the tail. I got a screwdriver from somewhere. Just putting the screwdriver against the screw was enough pressure to "squeeze" another brown drip out from behind the panel. Three screws later the panel was off, and the bay inside was completely packed with straw and grass. The bird had made its home here for a while because the bay was saturated with poop and water too, hence the brownich goo.

Some kind of bad karma came to haunt me, when about a year later we found ourselves re-skinning a Beech Baron wing, where some squirrels had taken up resisdence.


Steven Fahey, CET

 

[berkshire]

Sparweb,
I had one of those, I had a pair of wings off a Schweizer 1-26 that had been stored in a barn, I had one of my guys removing feathers and straw from them when the Feds walked in to inspect my station.
They came back 3 times in the following week to check on my progess with those wings.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[WKTaylor]

blackmax: great Post! Thanks for the link to a fine document.

I was tasked to re-qualify a very expensive adhesive for our shop. It was just over the out-of date limit.

Working with a repair technician and pre-primed coupons, we set-up and cooked several Test lots.

The goal was to attain 3500-psi lap-shear, minimum. The tests revealed 2200, 3100, 2950, 3350, etc... The scatter made me concerned about configuration issues during setup: so I carefully specified the set-up and witness the technician doing work. That's I began to feel less comfortable about the tech: he rushed certain steps, misaligned a few and was a little careless in cleaning and handling processes. There was relatively little change in the scatter... but there was some overall improvement in the lap-shear values. I re-evaluated coupon set-up with advice from some other bonding engineers... and went back to the shop [yes this adhesive was really expensive and hard to get in-theater... we were working the issue hard to re-Qual it]. The original tech had already started another job and couldn't be pulled-off to help... so another tech, who I was unfamiliar with, took over. He worked in a totally different style: methodical, clean precise lay-up measured work, etc. The test coupons failed between 3250--3400-psi lap-shear. never did qualify the adhesive... but I qualified the new technician for all future testing and recommended the original technician keep hands-off.

NOTE. Later I found the original tech had a reputation for cutting corners. When I discovered he had applied a corrosion protective finish after assembly, I asked him in front of his lead to disassemble the bolted together pieces. He and I got in-to-it. When all was said and done the lead forced the issue and the assembly was taken apart: there was NO alodine or primer in the fay surfaces of 7075-T6 parts bolted together that would be used on a NAVY HELO [hoist system fitting] over the ocean. He was a proud guy... but was absolutely un-trustworthy and had consistently low workmanship standards. He was finally forced to leave by the shop chief.

Another quirky guy was our welder. Didn't take me long to realize that if it was weldable and there was an established good practice, he knew every detail of the technology and "had the touch" [exceptional workmanship]. Literally the finest aerospace welder I've ever met: fully trustworthy and filled with knowledge and "the hands-on gift". However, as an employee, he was a eccentric hand-full. Played by this own rules one-to-many-times and was "fired". Another Company doing aircraft overhauls on contract across the base hired him immediately [OK, I put in a "good-word or two for him"]. We eventually lost a LOT of work he used to do for us [he/I continued our friendship till I left].

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[WKTaylor]

Just released to the web... a frightening display of flight control system failure...

Tu-154 "wallowing" [roll-pitch-yaw] around the sky due to major controllability issues [first flight after long-term storage?]

I'm sure this flight took everything the crew had just to maintain mental control of themselves… much less the acft… which they eventually get back on the ground.

Unfortunately this may have been what happened to [as I recall] a few F-15s, F-16s, DC-10 [Sioux City], -135 [Germany] and a DC-9 [California] with serious controllability issues... just before they eventually lost total control and crashed. The Fighter guys had the "luxury" of ejection seats... everyone else was along for the ride strapped in their seats.

Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

VIDEO: Tu-154 struggles against in-flight oscillation
By David Kaminski-Morrow
01/05/11

Extraordinary video images have emerged of a Tupolev Tu-154 apparently suffering serious in-flight oscillations shortly after departing from an air base in Moscow.

Text accompanying the images indicates that the aircraft - whose registration is given as RA-85563 - was flying from the Moscow Chkalovsky base on 29 April.

Details of the incident remain sketchy and unconfirmed but a series of video clips shows the aircraft departing, before it appears to encounter problems in lateral and longitudinal control.

It eventually returns to the airfield to land. Unconfirmed reports suggest the aircraft was conducting a maiden flight after long-term storage.

[Two videos on the following webpage…]

http://www.flightglobal.com/article...-struggles-against-in-flight-oscillation.html

NOTE: the ground personnel standing around (second video, when the aircraft is on final approach from the right) appear to be watching [raptured] in disbelief... waiting for the fireball on the other side of the tree-line.... which [thankfully] never came... aircraft is finally seen rolling-out in the distance.


Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[KENAT]

rb1957, I actually think this thread is potentially pretty informative. Seems that a number of folks have a real depth of knowledge to share.

Sure some of them are 'Murphy's' but some of them show patterns etc. that could be avoided.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[cloa]

Who wants to distill a summary and post as FAQ otherwise it will be surely hidden by new threads?