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.

 

[debodine]

I too loved Far Side. My favorite was the first deer in the forest has a target rondelle on his side, and the second deer says, "Bummer of a birthmark, Hal."

 

[KirbyWan]

One of the damage mechanisms I face alot is where two pieces of metal are pressed up against each other but not bolted. The interfaces between parts with such things as finger seals where relative motion impresses the shape of one part into the other, usually to the point that both need to be replaced. When repaired on a normal schedule these do not present a problem, but I have seen some components where you know the usual preventive maintenance that would have found the damage was ignored and ended up being such severe damage that the cost of repair was far in excess of what it would have cost if they had performed consistent maintenance.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[Kwan]

I'll throw in a couple.

lighning strike. I get to see these once in awhile. I've seen chunks missig from the trailing edge.

Another issue is vibration. Acoustic cracking, excessive engine vibration.

Heat treat issues. These can lead to lower corrosion resistance, allowables, alpha case, etc.

The root cause can be a result of one (or more) of thousands (can I say millions?) of abmormalties.

One last thought about the glider. The pilot should be verifying control system direction before flying (everytime!). This would have shown the mis-rig of the rudder. It also shows that full travel occurs (imagine FOB jamming a control cable).

Those are my thoughts Wil. What kinda food do I get?

 

[MintJulep]

One last thought about the glider. The pilot should be verifying control system direction before flying (everytime!). This would have shown the mis-rig of the rudder. It also shows that full travel occurs (imagine FOB jamming a control cable).

I don't have to imagine it. It's happened to me.

And yes, I did a full control check before take-off.

Mechanic left a flashlight under the seat pan. Control function was normal during pre-flight checks. However in bumping down the runway for take-off it dislodged from its benign location and wedged itself between the fuselage and aileron bell crank.

The result was that I could not move the stick to effect any left roll. Fortunately right roll and all pitch control remained available to me.

 

[berkshire]

Kwan,
"“The pilot should be verifying control system direction before flying (every time!).""

This really applies to every aircraft not just gliders, "controls free, clear and correct is or should be part of every pilots check list."

However this thing about looking for misrigging after maintenance is not often checked, control cables, can, get crossed. I personally have been guilty of it. I assembled the trim tab mechanism backwards in a Cessna 172 and did not catch it. The pilot told me about it after test flying the plane, boy was my face red. Pilots take it on faith, that when an aircraft comes out of a maintenance facility, that everything is correct and working as it should be.
B.E.

 

[WKTaylor]

It Is positively amazing the crazy thinks that happen to acft.

How about gear retraction on the ground? I had (2) C-141s where the Mains stayed in place, but the Nose gears dutifully folded-up crushing fuselage structure and doors. I developed quick/dirty ABDR repairs to fly them back to home-station.

Pilot raising the gear handle on TO roll with the "intent" of having a crisp/quick gear retraction just after lift-off... but then settling back onto the RW as the gear fold-up [show-offs]. Also pilots forgetting the LDG on landing... sliding on the belly. T-37s were famous for crazy stuff that student pilots did during take-offs and landings!

Anyone worked tire over-pressure explosions or rolling disintegration [throwing tire shrapnel into flaps and lower wing skins/spars]? The Concorde disaster is a classic example of catastrophic tire damage penetrating the integral fuel tanks and starting a raging fire just at lift-off. I think there were also wheel fragments involved with this incident.

Anyone dealt with obvious or suspected sabotage? Amazing what a full paint can can do when thrown onto a thin skin.

Titanium engine-bay panels, attached to titanium fuselage structure [flanges] on the F-15 are notorious for loosening slightly in-service...then wear/chafing/galling under sustained engine vibration and air-load buffeting.

Speaking of F-15s... the honeycomb wing tips regularly folded-up [or down]: In-fligh buffeting at High AOA was so severe, the were experiencing close to 100-G peek amplitudes.

Honeycomb structure invaded by moisture... corroding aluminum core & skin... disbonding then literally "blowing apart" after a seam opens up. Or... exploding [steam formation] when hit by a lighting stroke. Oh yeah... found out that leaking fuel, cleaning solvents, wash-water, etc... will find it's way into honeycomb.

CAUTION!!! NEVER water-jet blast adhesive bond-lines or fillet/fay sealant... intrusive water will open the joint to further moisture intrusion and peel the primer!

What about steel or CRES nuts/nutplates mounted to aluminum structure without being installed wet with primer or sealant? the corrosion [especially with high-nickel-ferrous alloys against aluminum can be mind-boggling [especially in a seacoast atmosphere].

Anyone seen the effects of using sanding disks over a broad skin area, to clean-off corrosion and/or rust from the heads of flush steel fasteners? Grinding across the skin and fastener heads spreads/embeds steel particles in-the-skin. REALLY bad corrosion pitting in 7075-T6 or 7178-T6 sheet material... especially at/around each of the steel particles and along the grinding marks... with a fine red-rust peppering every where!

Found a production-run of titanium fittings that had been "hand-cleaned-up" using either a sanding disk or CRES wire-wheel. There were fine score-marks perfectly aligned with the tight fillet radii ... and a deep-blue discoloration within the score-marks. Fatigue-cracking failed the main-flange... and was identified in other flanges by FPI NDI.

How about abusive anodizing of end-grains on forged-aluminum, sheet or extruded parts? An engine mechanic whined at me that he "broke" the [0.25-thick] flange off of a forged [7075-T73] engine mount [C-130 truss] with a rubber mallet. Huhh? With just 30X magnification I was able to ID a checkered-crazed surface along the cracked machined fillet, where end-grains where exposed. Looking all over the E-Mount it wasn't rocket science to realize a Type III [hard] sulfuric acid anodize had been applied ILO the required [thin] Type IIB.

Speaking of -130s... The [MS20012, I think] mount bolts/washers/nuts from one engine were sent to the NDI shop for inspection... but the NDI guys "couldn't inspect the hardware". I asked Why? He said come take a look!!! Every bolt had threads that were bent/cracked, the shanks were bent and scored circumstantially... along with the underside of the heads. The nuts and washers were no better-off. Checking with the -130 operators, I found-out they were concerned because the engine was vibrating badly... so they were changing the engine/prop. The hardware NDI was mandatory because of the engine change. I hand-carried the parts to the -130 squadron maintenance chief... who was stunned when I showed him the junk hardware. He agreed with me that the engine was probably vibrating because [at least] one or more of the engine mount bolts/nuts had become OBVIOUSLY loose.

Anyone ever seen a fabrication Jig slip and miss-align a major component such as a door frame or cabin skin? A production liaison engineer's nightmare! Funniest [after all was said and done] was the "completed aircraft ready for customer delivery". The customer felt like the aircraft was leaning left. Mechanics looked it over carefully and did an alignment inspection. Everything seemed OK... but even they had a "funny feeling" about it. That when I was called in. Checking the WINDOW water line locations, left and right sides, it was obvious that the Lt side windows were ~0.5 inches below design... and the Rt side windows were ~0.5 above design. Found out that the jig that sets the window cutouts was subject to "slipping around slightly". Inexperienced mechanics had NOT checked it for perfect alignment before fixing the window position. The entire upper cabin skin had to be replaced.

Keep-up the in-put guys.. this is going to be interesting... see You again in a couple of days!

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.

 

[MintJulep]

It doesn't only happen with airframes.

Your typical stick-and-skin subway car is built up from floor, wall and roof sub-assemblies.

Found one somewhat "twisted" once.

Turns out that the roof section was rather badly out of "flat" prior to final assembly.

But hey, with enough big enough clamps they were able to get all the gaps closed and weld things together.

When it was rolled out along the bumpy rail it went "boink".

 

[Sparweb]

Yes Wil,
For every action there is an equal and opposite screw-up.

I have a structural analysis report on my desk where I've gone through and marked in red all the places there the author erred:
bad arithmetic,
missed a fitting factor,
used the wrong assumption,
used the wrong gross weight of the rotorcraft,
changed an assumption between the line it's declared and the line where it's used,
ignored advisory material on the subject,
declares potential failure modes that are not checked anywhere,
invents formulas without quoting the source (eventually I found in Bruhn),
uses strength data from a unqualified source.

All that in ONE report. Signed and approved by an engineer with supposedly higher experience and status than me.

But it does have a pretty colour FEA model in the appendix so it must be okay.

Customer brings this to me a few months ago. He paid the guy a lot of money for that work. I have to be honest with him. If I'm going to touch it, I'm going to load test his product. And throw all the engineering work the other guy did in the garbage.

Because I like making people happy, you know.

Steven Fahey, CET

 

[evelrod]

Haven't seen too many screwups, personally. Belonging to the Planes of Fame for a few years I heard all to many scary stories. Only saw one...a Beech Stagerwing. Gear would not crank down and pilot thought it would be 'safer' to put down in the adjacent "grass". I put that in quotations for those of you who have not seen the grass next to the runways at Chino. Bad idea. Prop dug in and pulled the cowl down flipping the ac on it's back. About a year and a hundred grand and it was back in the air again. No big deal ! ??????

I was at Douglas Long Beach Bldg 55 when the contractors built the wing jig for the C-17 backwards and engineering did not catch it until it was almost finished. That was fun. It was even more fun when the rigging contractor, (rigging co. name omitted), made a big deal, televised for Douglas of course, to pick it up and could not budge it.

Rod

 

[thruthefence]

"antenna removed by sawing off screws (hacksaw between fus. and base)"

This ties my favorite:

Lower wing inspection panels on a transport category aircraft required removal,but the heads of the threaded fasteners stripped by poor technique of contract "technicians", who were told by the overseer to simply "drill 'em out!" (meaning the screw heads); This was interpreted to "chain drill", in a circular fashion, around the head of each fastener, allowing the panel to be pried off (with difficulty) with a large screw driver. Three panels were removed in this fashion before the "supervisory personnel" discovered the "work-around".

I'll leave it to your imagination what the underlying structure looked like after the panel removal.

I must add that the "panel removers" were furnished by a contract labor company, and were said to be 'experienced' mechanically minded workers. It turned out they were rounded up a few days before off of a street corner in Ft Worth.

 

[berkshire]

Thru the fence,
I had one of those,” be careful how you give instructions moments”, some years ago.
The shop was slow and I was finding make work projects for the technicians .
One of these involved breaking down a wooden wing to salvage the components.
The wing had a large number of good ribs and enough spar cap material to make a good splice on another wing if needed.
I showed the tech how to remove the ribs by prying them away from the face of the spar with a chisel.
The work was tedious and slow going, but that was ok, he had nothing else to do.
I got a phone call and had to leave the building.
When I got back the place reeked of gasoline fumes and there was a neat pile of ribs on the floor next to a large pile of sawdust.
After I had left, the tech had a brainwave, he went out to his truck and retrieved a chainsaw he kept there. He took the saw and sliced down either side of the rib through the spar cap, pried the remaining block off with his fingers, voila job done.
He just did not understand that I wanted to save the spar cap as well
B.E.

 

[FastMouse]

Some of my favorite moments over the years...

Mechanics doing engine run-up of large jet. Used chocks, but not the correct ones. Aircraft jumped chocks, rolled into a hangar, wing LE impacted a massive steel I-beam. Wing damage surprisingly light. Steel I-beam: mangled! On that day we learned that there is nothing wrong with mid-70s aircraft design philosophies.

Someone with a problem threw handfuls of carbide chips into titanium melt at a material supplier. A long way downstream, lots of expensive parts were found to contain carbide inclusions.

Someone with a problem threw acid into an empty wing box. Maybe mercury was also involved, can't recall. Significant damage.

Airline based in hot and very humid location. Plus significant industrial pollution. Plus bubbling sulfur springs upwind of main hub. Plus aircraft flying very, very short missions and "cruising" at 10.000 ft, so were never dry. Ideal environment for significant corrosion.

Fresh fish container burst in hold of widebody twin. Airline swilled out the entire compartment with salt water. Noted corrosion some years later.

Mechanic working in fuselage dropped tool between floor beams. Made loud clatter when it hit the hangar floor. Surprising, since the belly skin was between the mechanic and the floor. Corrosion had reduced skin integrity to zero.

To achieve weight target, "they" needed to reduce the anticipated loads and so justify a lighter weight structure. Did so by setting unrealistically low values of typical TOW, etc, in the fatigue missions used for analysis. Witnessed early in-service degradation due to the assumed typical TOW, etc, being too low.

To accelerate the full-scale fatigue test, smaller "non-damaging" load cycles were neglected from test. In-service fatigue lives turned out to be approx 1/3rd to 2/3rds of lives measured on test.

Probe mounted on long, springy arm was subjected to a big, inertial thud every time the system was operated, exactly as anticipated and catered for. Subsequent dynamic response of long, springy arm not anticipated and not catered for. Additional stress cycles caused premature fatigue failure.

Mechanic observed surface pitting corrosion and blended it away. Had actually blended away a region of shot-peened material.

It is easy to laught as some of this stuff, but as a tip-of-the-hat to the industry, I am happy to say that on every occasion, the subsequent response of those involved to overcome the issues was rational and appropriate. Thank heavens for people who still care about professionalism!

 

[blakmax]

Don't know if this is an urban myth, but here goes.

Some Australian Army pilots were having a similar discussion to this thread, and boasted that they had experienced a kangaroo strike on a Nomad while landing at a remote airfield in outback Australia. "Beat that."
A Canadian P3 crew immediately claimed bragging rights by claiming a salmon strike which damaged the windshield. Evidently they flew under a large eagle which had just caught the fish and they scared it so it released the fish which hit the aircraft.

Canadians 1, Aussies 0.

Regards

Blakmax

 

[KENAT]

Impact with the ocean (or possibly some flotsum etc.) when going low level in a Buccaneer. By low level I mean wave top height.

The nail used to hang a copy of the Koran from the fuselage bulkhead in an Arab VIP chopper.

Posting guidelines faq731-376

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

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

 

[berkshire]

I got a good one today,
a wing skin with a persistant crack on a fiberglass aircraft.
It had been repaired several times before, per the SRM, according to the logbook, by several different facilities.
On stripping back the gelcoat and paint to the fiberglass, what should show up, but a perfect 50/1 scarf per the SRM with no glass in it. Somebody had cleaned and scarfed the repaired area then puttied and gelcoated over it.
Repairs are now under way including the required fiberglass.

I guess the other repairmen Did not answer the following questions:
Did I perform the job task without pressures, stress and distractions?
Did I reinspect my work or have someone inspect my work before return to service?
Did I make the proper record entries for the work performed?
B.E.

 

[WKTaylor]

I worked on an overseas USAF installation... the C-130 guys always surprised me...

C130 ramp skin and sub structure was over-heated with strips of black rubber fused to the surface. A special ops team had trouble getting their ATV back on board during an "urgent" situation. Ramp was replaced [sent back for depot "overhaul"].

Note. Same aircraft was also getting a fair number of lightning-strike holes patched [30-caliber size].

Urine soaked substructure around the urinals on the aft cargo-door longerons. positively the worst stink and corrosion, I have ever seen/smelled on critical primary structure. I suggested that try lining the area under/around the urinal with puppy-pee-pads. Never did.

Old C-130 landed late PM after a really long mission. The crew chief was trying to do a post flight inspection in the dark. Kept hearing snap/crackle/pop along one area of upper wing-skin as he walked across it. Finally found a few loose/pulled-through fasteners. Mechanics opened the wing and pulled-out the ballistic foam: found SEVERAL broken/bent truss ribs and gussets and many more broken loose fasteners. I was called. NOT GOOD. "Lets look at the other wing!" Better condition outside... but a LOT worse inside. Called engrs stateside. This acft had old wings that were subject to damaging ground resonance [flapping effect] during fast taxi on rough surfaces with a certain waviness contour. The resonance could over-stress the wings under certain conditions. Funny thing.... the acft had encountered exactly those conditions just prior to takeoff at the foreign Air Base where it had operated earlier. Double outer-wing change was mandatory before next flight.

Wing commander was demonstrating aggressive off-field landings. Drove the acft at above max sink-rate into loose gravel. Dragged the aft belly for ~150-yds [very nose high], before lifting-off. found. The aft MLG pair and belly structure trashed. NOT counted as a mishap... just operational damage [major depot repair required].

Other fun stuff...

FA-18 lost a wing-tip missile launcher rail. Forward bolts holding it to the spar came loose [probably under-torqued]. The bolts simply loosened [spinning in place], until one failed... then so did the others.

Due to the Extreme humidity, fiberglass radomes [fighter and transport] absorbed moisture like crazy if the coatings were allowed to deteriorate. When this happened radar signals notably degraded... and static electricity arcing marks would start appearing all-over the interior, which lead to disbonding and soft spots.

I worked on a small tactical jet. We started getting urgent calls from one unit that the pylon-attach [3/8"] nutplates were breaking. Some broke just after torquing... others a few days later. Lab evaluation confirmed that the nutplates had been overheated during the post cadmium plating embrittlement relief bake [lab estimated exposure up to 525F]. Cadmium infusion, and possible "blue-brittle" embrittlement were to blame. Found-out the NP manufacturer had already been disbarred from government work and the entire production lot was "suspect"... and the company had been out of business for a year.

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]

Sorry... been jammed-up recently.

Anyone every dealt with biological contamination/damage in integral fuel tanks? Nasty stuff.

During depot maintenance of CH-53 we found the entire Bilge area coated with mold, slime, fungus, urine, fuel, hydralic oil and sea water. Muck, corrosion and stink every where.

Anoyone found loose/missing nuts on spinning bolts in class 1 holes?

What about loose/migrating bushings or plain bearings?

Elongated (worn-out) or cracked-out holes in piano hinge nodes?

Shear [small-] head hi-loks or small-head bolts or blind-fasteners [rivets or bolts] that have been over-stressed [tension] and are dished or circumfrentially cracked and pulling through?

Unsealed airframe electrical grounding points [fasteners, studs, etc] that are corroded, loose and/or arced.

Crazed transparent plastics, such as windshields, canopies, windows, light lenses, etc.

Cracked, torn, spalled fiberglass fairings and cowlings.

Fuel and pressurization leaks due to loose [non-adhereing], torn or overheated fay, fillet and cap-sealant.

Titanium structure or parts [ducts, etc], that have been heat or fire damaged from straw-yellow to blue-purple to chocholate brown?

Or epoxy primer that has sustained High heat? and has turned from clean yellow/yellow green [etc] to tan-green to tan to tan-brown to tan-brown-scorched black?

Magnesium castings riddled with pitting corrosion. PS: don't even attempt to weld on these old cast-mag parts without 48-hour bake at 180--200F to drive out moisture. Pockets of trapped moisture in the casting matrix may "explode" [steam-burst] when hit with temperatures over 220F.

Parts overloaded in bending that exhibited anticlastic behavior? F-15 speed brake [carbon-fiber-epoxy skins with titanium backbone/hinge beams] that bent/deformed >>forward [upward when laid on back of jet]<< as it was over-loaded around the (2) length-wise hinge beams [speed-brake failed to load-relieve during a massive over-G and over-speed emergency pull-out so it "saw" the full load].

Small bolts/screws over-torqued and sheared or distorted during installation.

Hi-G aircraft flexing so severely that butt-gaps between skin edges compress together crushinhg the environemntal butt/fillet sealant.

Exposed edges on panels, skins, etc of extremely high performance acft that erode [paint and metal] due to wind/sand/dust/rain blast/impingement at high speeds?

Oppps... Gotta go... more later.


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]

"Crazed transparent plastics" we used to make money at my last place repairing the laser seeker dome on Paveways, so yeah I've seen it.

Posting guidelines faq731-376

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

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

 

[blakmax]

Main spar bolts on a Mirage III which tightened into nuts in a captive nut strip. Bolts has snapped off in the nut strip, so the bolts were ground flat in installed with Locktite.

Taperlock fasteners on a steel carry through box that were fitted into reamed tapered holes. Some when removed has about 0.020 in. of bearing blue on one side. Some reaming job. huh?

An A4 destined for a museum was transported down a highway, but was too wide to fit under a bridge. Someone "trimmed" the wing tips off with a gas axe. He didn't realise that the wings tips could be folded up.

Two seater jet trainer experienced problems with the canopy locking mechanism, which resulted in the canopy departing the aircraft in flight. Forward pilot saw the canopy go and assumed that the aft pilot had ejected, so he ejected. Aft pilot followed soon after. Perfect aircraft apart from the missing canopy soon hit the deck.

 

[thruthefence]

"Main spar bolts on a Mirage III which tightened into nuts in a captive nut strip. Bolts has snapped off in the nut strip, so the bolts were ground flat in installed with Locktite."

Surely you jest?