By Peter Conner

My customers often ask me about how to deal with cracks, dents and other damage to fiberglass parts during an annual inspection.

Lately, quite a few owners have asked why a repair that was acceptable  year ago, may not be acceptable now.

The usual way of repairing cracks in fiberglass is to simply stop-drill the part; a strategically placed hole will usually keep the crack from propagating further, assuming the rest of the part itself is sound. Take a close look at any of the aircraft parked on the flight line, and you'll surely see a lot of stop-drilled parts.

Is it safe? Yes, within reason. As long as the part is structurally sound and no crumbled and broken, stop-drilling is an acceptable fix; we've done it for years. Is it legal? Yes, but this is becoming somewhat of a gray area.

Although all mechanics are taught stop-drilling for metal, fiberglass and plastic and it's well described in advisory circulars, the FAA considers it a temporary repair. That means if a fairing or wingtip develops a crack, you can stop-drill it to keep it from worsening.

Come annual time, however, the part is supposed to be replaced or repaired, at least under current FAA guidelines. Technically, then, all those fiberglass parts you see with ancient, weathered stop-drills are unairworthy. (The exception is if the manufacturer specifically allows something less than full replacement or repair.)

Non-structural fiberglass parts are fairly easily repaired with common glass cloth and resin, although the mechanic has to be careful about building too much weight into the repaired part.

The thermoplastics (ABS) used on many aircraft such as Cessna wingtips and stabilator tips on Pipers are a different story.

Fiberglass doesn't adhere properly to ABS, so it has to be repaired with a paste made up of chips of the plastic solution, dissolved in methyethyl ketone, a strong solvent.

The paste can be worked to fill damaged areas and it will adhere well, within limits. You can't rebuild the part with paste, however. The question is, when does the cost of repair labor exceed the cost of buying a new part?

Here are a couple of examples: A horizontal stabilizer tip for a Cessna 150 costs $59 new from Cessna or less than $30 for an aftermarket version. (PMAd.) At a shop rate of $40 to $50 an hour, it doesn't take sophisticated math to realize that replacing the part is cheaper than repairing it. This is especially true if a relatively small or cheap part has a lot of cracks or damage.

Even a small amount of damage will require at least a half-hour of work, plus a bit more for painting. And if the part is cracked at the screw holes, as is common, it may not be practically repairable at all.

Consider a Piper Warrior ABS wingtip. It costs $240 new from the factory or $320 for a PMAd fiberglass replacement, say from Globe. With that kind of replacement cost, there's a little more room for considering a repair, especially if the original part is basically sound, but just needs cosmetic touch-up. It's quite possible that the repair would cost less than $100.

My personal preference is to go with original manufacturers parts whenever possible. They seem to fit well and there's never any question of PMA. However, some aftermarket companies -- Globe especially -- provide good quality parts and may have much wilder variety and availability than the original manufacturer.

This article appeared in the January 1996 issue of The Aviation Consumer, Volume XXVI, Number 1, pages 20-21.