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.

 

[blakmax]

Thruthefence, I saw it on two aircraft, one with two bolts "repaired" that way. Mind you it was in the early eighties, and thankfully things have changed for the better.

 

[blakmax]

Oh I should say that we found out about it when we removed the bolts as part of a bonded composite repair program to address fatigue cracks in the adjacent skins. We did not do the bolt modification ourselves, we had to fix it.

Regards

Blakmax

 

[Kwan]

Customer bought airplane and performed inspection. Found a nacelle fairing fastener installed into lower spar cap. Someone actually tapped a hole in the spar cap for the fairing screw.

Long ago I was liaisoning a mod. We were building 16g racks. The shop asked for a shelf trim note on unit 6 or something. After much measuring and scratching my head I backed up and looked over the whole rack. The whole rack was bowed because the shear panel on the back was installed incorrectly. The mechanic installed the rivets one at a time and the panel pulled. A big clamp was used to line up the panel with the holes. Needless to say when the panel was remove it sounded like a gun going off.

 

[blakmax]

Another one. Super whiz kid engineer was tasked to address cracking in a 7049-T6 structure (thank goodness not many are in sevice these days). "There is an SRM repair for that structure, implement that!" Pity the crack was stress corrosion, which grew perpendiculer to the fatigue crack envisaged by the SRM repair. The addtional fasteners located ahead of the crack not only increased the Kt ahead of the crack, they exposed fresh grain boundaries to induce even more SCC.

 

[WKTaylor]

O god… “Taper-Loks” the ingenious fastening idea that is a devil to execute properly! I have never been more frustrated in my entire life than removing and replacing TLs. Situation ONLY gets worse if over-sizes are demanded by circumstances. Spent many hours observing blue-pin checks of TLs for reinstallation and new installation. F-15 lower skins were loaded with them.

Heavy rain wind and flooding have also taken toll of light and heavy aircraft. Recent fly-in at Sun-N-Fun had a tornado blow-thru damaging dozens of acft [overstressed in-place… plus failed tie-downs].

http://www.youtube.com/watch?v=DNt1BKqF3a0

Repaired many aircraft damage by ground-taxi collisions. The AirBus A380 and CRJ700(?) collision at JFK is a classic.

http://www.youtube.com/watch?v=uogmKMUrbT4

The saddest taxi accident was an HH-3 Jolly Green Giant [Viet Nam Era] that was taxiing “close-enough” to a “taxi-line” adjacent to the ramp edge. Low visibility and a “sense of urgency” due to deteriorating late afternoon conditions with fast-moving gray overcast skies [incoming typhoon]. The crew failed to see a gray concrete light pole off to the right side. When the rotor-tips collided with pole, the reaction actually torqued the helo toward the pole starting a cascade of massive damage and flying parts “all around the scene”. One mechanic just happened to walking to work was killed by a piece of blade-tip. The investigation uncovered a series of seemingly insignificant taxi accidents all over the base. Analysis by safety office and civil engineering uncovered the fact that taxi lines [which should include extra margins for clearance with parked aircraft and obstacles] hadn’t been re-evaluated since the late 1970s. The ramp where the mishap occurred was “striped” for F-4s… not larger-span acft such as helos and C-130s.

Anyone ever investigated flutter or severe vibration problems?

I was engineering investigator on an O-2A that flew into a thunderstorm. It entered a violent down draft and came screaming [over airspeed-limit] out of the bottom of the draft. Aileron flutter occurred during attempted pull-out, resulting in the aileron tearing in half [fore-aft at ~ aileron mid-span], then departed the wing from centrifugal force as the wing failed [upward] after massive up-down-up flapping cycle. Found the aileron had 0.023 thick paint build-up [should not have exceeded 0.004 thick without re-balance]. NOTE: the aileron skin was only 0.020 or 0.025 thick.

Worked an F-15 that had severe unknown vibration at high subsonic/transonic. Pilots were leary of it and maintenance crews were baffled. When the called me [last resort], I couldn’t find anything else to investigate. HOWEVER, I had just read the latest F-16 LMTAS bulletin that described similar severe vibrations on an F-16 caused by a loose 20-mm Vulcan gun installation. I mentioned it to the shop chief who blew-it-off… at that time. A few weeks later I saw the same shop chief and asked “what happened to ‘old-shaky’”: He said he had second-thoughts about my suggestion… especially when he remembered who was assigned that task… and decided to re-inspect it personally. Sure-as hell he found the gun, main fittings loose. Mechanics went through the entire gun system “tightening/replacing” as needed. Vibrations completely disappeared.

On a similar “warning note”… I worked on a large jet that has thin spot-welded skin panels, lots of sheet-metal longitudinal stiffeners and relatively few ring-frames and bulkheads. The spot-welded skin-panels [skins + SW doublers, tripplers, etc] were being replaced with monolithic thick sheet skins, same alloy-temper as the originals. ‘A bit heavy… but so-what”??? Well stress and vibrations/flutter guys ran the numbers. Excessively stiff panels at certain locations drove adjacent or opposite [thin] skin-sections into potential resonance frequencies [panel and possibly the entire fuselage section. As a result of this revelation, there was an immediate push to make sure new skin panels were tailored similar to the old skin panels [thick skins were chem-milled AR]… and/or thin sheets were stacked/field-fastened.

Blakmax… Ahhhh… the infamous SCC crack that starts on the surface, runs for a short distance… then turns 90-degrees and runs laminar [sideways] down the sheet center… what a pain. The old 7XXX-T6 sheet and plate alloys [usually machined across grains] are notorious for that.

On a strange note. I had a mechanic call about a crack in a machined-frame on a HH-60. This frame supports the roof and runs past the side window sill to the floor structure. It is critical for crash-worthiness. Hmmmm: not good. The mechanics wanted a temporary repair to fly for "just a couple of weeks more". I said “OK use eddy current to find the crack-tip and I’ll work the repair”. A couple of days later they called and told me they’d traced the crack down the frame to the floor. Maintenance supervisor grounded the helo and sent it back a few weeks early for depot repair/overhaul.

How many of You have see terrible workmanship that causing multiple drilled and/or elongated holes at critical locations?? Replacing riveted nutplates thru thick members, blind is a classic YGBSM story (all because a nutplate rusted or spun-in place; but was absolutely essential to hold a flimsy fairing in-place)!!!

Anyone have their own horror stories regarding Elliptical Blind Nuts [NAS1734 or NAS1735 per NAS1736?] used at critical locations on F-16 wings? Thank-god for FTI [etc] Rivetless Nutplates with replaceable nut elements.

Gotta go back-to-work!



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.

 

[kontiki99]

fire damage that ruins heat treat

leakage of battery acid, mercury,

 

[thruthefence]

The maintenance level "bodges" alluded to here should be required reading for any maintenance training curriculum, be it Military or Part 147.

 

[FastMouse]

I sure do wish that people would stop removing installed rivet heads with chisels.

 

[MikeHalloran]

You mean there's another way? ;-)



Mike Halloran
Pembroke Pines, FL, USA

 

[WKTaylor]

When I was 17, I was hired for a summer to take a Beech-Baron wing apart. The shop was rebuilding the outboard end and leading edges that were crash-damaged. It had both solid and blind rivets. The shop owner taught me correct way(s) to remove both with absolute minimum damage and "speed". Text Book USAF T.O. process... so I found-out later.

NOT only were the correct ways precise, but they could also be very fast and easy when You got the hang-of-it and got a rhythm going. The principle problems were: (a) stripping the accumulating rivet heads from the drill-bits; (b) spinning blind rivets; and (c) hand or back fatigue due to working odd positions. Even changing drill bit sizes was quick/easy [spin chucking].

Tears me apart seeing the amateurish rivet removal methods used by so-called sheet metal technicians.

On a similar note. I work with an overhaul shop that is now building brand-new rudders and elevators for the acft I'm working. Our Assys are made from extremely thin sheet metal [0.012--0.040 thicknesses, 2024-T3/-T42, 7075-T6 & T62, etc] hand-riveted together in jigs/fixtures. In today's world of thick machined parts, aluminum honeycomb or composites, the "art" of lightweight sheet metal assembly had to be practically re-learned. I found old [WWII] sheet-metal Assy texts [theory and practice; topics ranging from handling, fixturing, drilling/reaming, dimpling, countersinking, hand riveting, etc] and sent them digital copies. The shop assembly techs were struggling and were racking-up discrepancy after discrepancy, such as: rivet gun or bucking bar marks/skids/gouges, wrinkling, oil-canning misalignment, etc. PAINFUL.

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.

 

[der8110]

WWII texts and old T.O.'s are indeed fabulous. Another medium from the same era is animated training films on sheet metal techniques done by Walt Disney:

http://www.imdb.com/title/tt0036497/

Well done, and entertaining too

 

[WKTaylor]

Anyone seen titanium heated [in air] like these samples?

I have: F-15 that experienced a catastrophic engine failure and had a sustained engine bay fire [~15-minutes]. The engine bay was primarily Titanium 6AL-4V. Many parts were oxidized purple to chocolate brown. Aft fuselage was removed and replaced.

NOTE: these samples were made by a HS student in the sheet metal shop on base by special permission. Her dad was in the safety office and he/I wanted samples to hold in our hands. She won a science award for the project.



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.

 

[thruthefence]

"animated training films on sheet metal techniques done by Walt Disney"

any idea if these are available online anywhere?


OT, regarding the "old texts", I have accumulated a pretty good collection of old training & technical books from the late 'teens through the early '50's. They make fascinating reading.

An example: From a '30's Aircraft engine school text; they describe how to disassemble, clean, inspect, and test "spark plugs". not "remove", but disassemble to it's various component parts! These were the days when engine overhaul periods were measured in the hundreds of hours (if not less) rather then thousands.

One paragraph describes how to build a siphon type sprayer to "clean engine components" with gasoline!! Apparently an "approved method'! And these were the days when everyone had a lucky Strike hanging out of his mouth!

A bunch of manly men, in those days!

 

[MNLiaison]

Curled blind rivet heads due to use of single action rivet gun for double action rivets.

Carbon fiber repairs installed over painted surface.

Aircraft slipping off jacks and receiving punctures in the wings / fuselage due to faulty jacking equipment and inattentive operators.

Double drilled holes at 40 locations due to improper removal technique by mechanic. Same mechanic allowed to install repaired panel onto aircraft resulting in short edge margin.

 

[blakmax]

Let's try an advanced technology one! There have been heaps of PhD studies on embedding optic fibres in a composite structure to detect impact damage. Broken fibres, no light comes out the other end. Great idea. Saves all of that ultrasonics etc.

Now look at the practical side. Once broken, these fibres can not be re-joined in any manner which provides a compatible surface profile. Further, when scarfing the structure to implement a repair, you cut even more optic fibres. Now every station outside the damage and repair is no longer monitored by the technology. The operator must revert to ultrasonics etc, and carry the cost and equipment for two inspection methods. Eventually the aircraft is weighed down by many pounds of useless fibres.

Similar comments for an alternative idea of dye-filled glass micro-spheres for impact detection in composites. Impact releases the dye and the damage is easily seen. However, when the area is sanded back, heaps of paint released over the site where you want to bond. Not good for adhesion!

Sorry to get all boffinish here, but these whiz kid ideas really bug me. Hey WKTaylor, this reminds me of the Garry Larson cartoon where a group of men in white coats are standing beside a very badly built rocket shaped object, with the caption "Let's face it. We are not rocket scientists!". Love it.

Regards

Blakmax

 

[Kwan]

Blakmax,

I can relate to the advanced technology solving the worlds problems! The fiber optics are wonderful! But I bet they don't detect delams. Delams happen more than broken fibers, at least in my world.

I've worked on WWII aircraft. Had to work out external/intenal loads. Back in the day they used a handful of loadcases. The engineers understood the corners of the flight envelope.

Now what do we do? We run ten thousand loadcases. It's not an elegant solution anymore. And I think any weight savings isn't worth it.

Anyone ever see peel ply left in the layup? Happens more than I would have guessed.

 

[blakmax]

Kwan

I have seen peel ply left in the lay-up, and the release ply on the adhesive. However, there is an even more sinister peel ply problem. For those who rely totally on peel ply removal for surface preparation, be warned. For the peel ply to be able to be removed, the peel strength of the bond to the peel ply must be lower than the peel strength of the laminate, otherwise the first ply is stripped off.

To achieve that there are two metods for treating the peel ply fibres in the process of making the peel ply material. The first involves coating the fibres with a release material (e.g. silicone). This material transfers to the bonding surface, despite the claims of the manufacturers that the material does not transfer. This contaminates the bond surface, resulting in poor bond performance in service.

The alternative is to heat scour the fibres which produces a glazed surface which does not bond to the resin. Unfortunately, the cast surface left by the fibres also does not bond well to the adhesive.

The crux of the problem is the belief that removing a peel ply leaves a clean surface for bonding. This mantra ignores the fact that surfaces must not only be clean, they must be chemically active to enable chemical reactions between the adhesive and the surface. Silicone (or teflon) treated fibres contaminate. Heat scoured fibres leave a chemically inactive surface. Conculsion? Peel plies are not an adequate surface prearation for composite bonds.

See the paper at the link below.

Regards

Blakmax

 

[thruthefence]

I'd like to comment on "Paint Shop" bodges.

My fav, and I've seen it for 40 years, and continue to see it. (must be the high turnover in paint prep jobs):
Specific Aircraft, Small Beechcraft, Bonanzas, Barons, ect. After stripping the paint, Aluminum etch & alodyne is applied to the bare metal. The problem being the ailerons & elevators are magnesium sheet. No matter how many water rinses you do, it keeps cooking the magnesium, and after a year or so, you have these little "termite tunnels" creeping under the paint film.

The stripping crew carefully masks the periphery of the cabin windows with metal duct tape to protect the acrylic from stripper. They then tape over the windows themselves with crepe tape & paper. In order to do a "neat" job, they trace around the window edges with an exacto knife, or box cutter. Makes for a very workman-like masking job. Until you pull the paper off & see the kerfs the knives have made in the transparency. We're talking pressurized aircraft.

Paintshop owner mortgages his house to buy one of those new (at the time) plastic media blasting stripper rigs. Instead of waiting for the tech rep, he does some "OJT" on a Cessna 414A. Works great on the fuselage, "Shucks, who needs training, let's get this puppy stripped! Time is money!!"

Until you get aft of the pressure bulkhead, where the sheetmetal is say .020", (and the flight controls were the same). Stretched the metal so bad between structural elements it looked like a Dope & Fab airplane!

Operator brought it into my shop so we could "fix it". Every affected panel would "oil can" when pushed. Insurance Ended up totaling the aircraft.

 

[Kwan]

Thank Blakmax, that's good info, a star for you.

It's amazing how easy it is to make a mistake in this industry. Problems can occur from any direction.

Wil mentioned fastener removal. However, it's friday and I don't want to talk about that subject. I like to go into the weekend happy! But here is a link to EDM fastener removal. Anyone have any experience in this?

 

[WKTaylor]

I worked with a shop repairing boron-fiber-epoxy and carbon-fiber-epoxy control surfaces. The usual technique for repair was to grind/scarf the damage areas, clean with IPA and hot-bond preformed patches or lay-up in-place. We learned a hard lesson early-on.

Immediately after drying the parts of the IPA, the tech added the repair plies as advertised. He then set the equipment for cure... 375--400F + Vacuum pressure. Just as the repair was reaching ~350F, we ALL heard a muffled but unique "krumph"; and found the vacuum bag around the part heating-up had been blown apart. In the twisted core I found concentrated carbon-black residues, suggesting a detonation. Consulting with experts lead to an obscure report recommending vacuum bagging and low-heat [180-200F] extended drying of composite parts after they had been extensively cleaned with solvents. Light solvents, especially IPA, tend to wick into the core areas by capillary action thru the porous fiber matrix. At the perfect stochastic ratio voila... an explosion during repair heat-up, typically close to 350F.

OH yeah, the shop guys [and I] learned a bit late about the possible hazards of breathing composite material sanding dust. Hopefully the long term effects from boron & carbon fiber dust] are more benign than asbestos fiber dust.

Anyone worked over-pressurization incidents? I've worked (2) wings and (1) fuselage that were over-pressurized during testing. What a mess. The wings were repairable with great effort... the fuselage was scrapped.

I know of (2) acft that were blown apart by oxygen tank ruptures, caused by servicing low pressure gaseous O2 systems [800-PSI max] with high pressure [3000-PSI] O2 by mechanics who ingeniously figured-out how to install high pressure servicing-fitting onto the acft's low-pressure O2 servicing fitting.

Also had one bird heavily damaged by a freshly serviced LOX tank. The sequence of events went something like this: the empty LOX bottle had been stored in a open-air shed; was then serviced with LOX and placed onto the transport cart [exposed] which encountered brief/light rain. The tank was then loaded into the jet as part of pre-flight servicing, without checking the tank vent. As the LOX warmed slightly, the tank couldn't vent-off building gaseous O2 pressure, which quickly over-pressurized and ruptured the tank. Investigation revealed that the over-pressure vent was probably plugged by hard ice, due to condensation and/or rain intrusion into the vent port!?!?#$@$$^^$. It was a good thing there weren't any arcing/sparking sources near-by which could have made this mishap MUCH worse... and the same thing could have happened to the other acft serviced at the same time [but didn't, thankfully].

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.