All... hope this makes sense...
Sspeed-tape, 500-Kt Tape, etc has it's purpose and validity in aircraft temporary [mostly] repairs... primarily in sealing small gaps, holes and penetrations that are NOT structurally significant; but cold get worse with environmental or pressurized air exposure. I understand Speed-Tape was a staple in crew-chief tool boxes in Viet Nam for fixing thin aluminum honeycomb skins, or fiber-glass etc… fighters, choppers, etc made little difference.
First.
Speed-tape is supposed to be L-T-80… however A-A-59258 [was MIL-T-47014] has been used interchangeably.
L-T-80 Tape is made from a dead-soft aluminum foil strip with pressure sensitive adhesive. A classic QPL part is 3M 425 or 3M 427 Aluminum Foil Tape. The aluminum foil is 0.0030+/-0.0005 thick and has a fairly high tensile strength with good ductility for stretching, compressing and penetration resistance. The [usually] acrylic adhesive can add anywhere from 0.002--0.004 in thickness to the raw tape. In combination with this mechanical strength the adhesive produces relatively high peel/shear strength. I always use the 3M performance numbers that stated useful temperature range of -65F + 300F [camouflaged USAF acft skin temps can reach +180F sitting in the desert; and +250F in high speed flight… although engine accessory ‘areas’ can get a lot hotter.
A-A-59258 Tape is made from thinner aluminum foil [0.0020-nom] which is more-easily penetrated but does have a much higher service temp limit of 500F… probably due to silicone pressure sensitive adhesive… which also makes it less-suitable for aircraft use. I might use this tape for the truly hot areas but silicone residues can be a pain to remove.
Second.
In the 1990s I ran a speed-tape test [L-T-80, 1.5” wide] on an aluminum clip-board: surfaces anodized aluminum ~0.063 thick, deburred [6061-T6 or another hard alloy]. Cleaned the surface with solvents and alcohol… then layered tape in a staggered pattern all around the Lt, Rt and bottom edges… and transitioned corners with 45-deg, strips that wrapped around in a “folded-W” pattern. I trimmed each piece with ~rounded or chamfered corners [3/16—3/8” R or C, cut with scissors] and then wiped the previously applied tape-surface dry with alcohol on a process wipe. As each layer went down, I pressed them dead flat and ‘worked each edge down flat and really hard’. A couple of pieces had trapped air bubbles which I punctured with an Exacto knife blade-tip… then smashed the trapped air bubble areas flat/tight; and in one I cut a small 0.25 X 0.25” X. It was exposed to car-heat, rain, high humidity and salty air… plus my abuse… I handled the clip board with sweaty hands and chucked it in car/dash board constantly each work-day. The first year I handled the clipboard carefully and watched it religiously. Somewhere in the second year I hardly noticed the condition of the tape and just didn’t care, since it had already passed my initial expectations. The clip-board sharp edges eventually took the lion’s share of damage due to point impacts, gouges and tears all-around; but the tape never peeled or disbonded. My abuse finally cracked the clip assembly and pulled the rivets to the point it was useless as a clip board. Test-over. I peeled [tried to peel] a few strips off and found it very tough…finally used an Exacto knife blade to cut tape into long strips which peeled off, mostly.
Third.
I came up with a few general rules for using speed tape, that would last until proper repairs can be accomplished [estimate temporary repairs to last at least 6-months to 3-years… honest-to-God].
Use 3M 425 or 427 [or DLA supplied L-T-80] tape, minimum 1.5” wide.
Use primarily for sealing panel or butt-joint edges, honeycomb damage [after trimming skin smooth], frayed edges of composites, fibeglass panels, leading edge damage, etc. . although I have used ST for repairs on engine inlet lip… and stated limitation to ‘minimize inlet heat to extent possible W/O affecting icing condition safety’… and make sure if tape fails it fails safely… out of harms way. Never use Speed-Tape when fuel, engine oil or hydraulic oil is present [leaking, etc].
Deburr structural sharp edges to prevent knife-tears to tape.
Meticulously clean the surface where tape would be applied… and all around. Abrasive and solvent clean with cotton or process wipes all surface types. Alcohol wipe tape surface in-between over-laps.
Round-off or chamfer/round-off ALL corners minimum R0.25 or 0.25 X 0.25-Chamfer (plus nick the remaining sharp corners to break the little (2) 45-deg sharp-corner-tips).
Press the tape-down center-first, then press/rub toward the edges. Press-tape-down hard around all edges, to the point the tape-edges are almost ‘crushed flat’.
Over-laps shall be ¼-to-1/3-to-1/2 tape-width [tighter over-laps generally better]. Also Over-lap from aft-to-forward [minimize air-flow peel on all layers] or from lower-to upper [minimize edge exposed to moisture/fluid drainage all layers] on outside of acft. On leading edges OK to over-lap inbd-to-outbd or vice-versus…be consistent and uniform… and take tape edges well beyond the damage to eliminate possibility of minor edge peeling causing the strip to come-off easily in peel [wider-the area longer the peel/environmental resistance]. These lap-rules apply, unless I saw a need for a specific lap orientation. … or multiple laps for increased ply strength at a certain point.
NOTE.
The photo of the USAir mechanic using one-long-strip of tape on a wide skin gap on the cowl-to-inlet-lip made me edgy. Exposed tape-edges into airflow could peel the strip-off. Also, possibility of internal cowl air pressure bleeding along the joint could lift/bubble the tape in the center, aggravating peel-adhesion. I would prefer at-least 2-layers of tape over-lapped by 50—60% in the middle; with the over-lap centered along/over the gap. The first layer would overlap the joint primarily from the rear-side of the joint; the second layer would over lap primarily from the front side of the joint. This extends the tape well forward/aft of the joint and increases the tape-thickness over the joint-gap. I think the tape will be far enough from the inlet lip heat; but if heat is a real potential [winter ops especially] that might be a good reason to use the A-A-58258 high heat tape.
NOTE.
What I loved about 10-years of field service engineering is seeing my ‘training, imagination, etc’ for repairs put into actual service… then seeing how well they actually performed over-time. Speed-Tape was a ‘temp repair’ success story [if I adhered to my rules]. Other repairs… mostly temporary… never permanent… had a bit more checkered results that I am: (a) both proud of… and (b) really humbled-from… due to some hard-hard lessons learned.
“Field service engineering can be a cruel teacher: no matter how prepared You think You are for ‘a new circumstance’ You have-to take the test… then learn the lessons. Hopefully none of the lessons [pray-to-God] are ‘wicked’ [~=irreversible/dangerous].
Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]
