I ran the numbers on composite repair cure times and stopped trusting the manual

Last month I had to patch a honeycomb panel on a Gulfstream G450 and the manual said 8 hours at 250 degrees. I figured that was gospel for years. But I started tracking actual cure times with a thermocouple and a digital data logger I borrowed from a buddy who does composites for a living. Turns out my shop oven has a 20 degree cold spot near the door that was kicking the cure way off. The part that surprised me was the FAA AC 20-107B actually allows for deviations if you can prove the part hits the right temperature for the right duration. So I ran four test coupons at different spots in the oven and the center ones cured fine at 6 hours while the door ones took 9. Now I map my oven every quarter and adjust my times based on where the part sits. Has anyone else actually tested their oven hot spots or do you just trust the book and hope for the best?

3 comments

3 Comments

miab8720d ago

Man I totally disagree and I think this is overthinking a simple process. You're gonna drive yourself crazy mapping ovens and running test coupons when the manual has been fine for decades. If the FAA really wanted us to do all that they'd put it right in the AC instead of just saying you can deviate if you prove it. Most guys I know just set the temp, wait the full 8 hours, and move on. Your oven might have a cold spot but that's what the extra time is for the book already builds in a safety margin. Plus if anything goes wrong with that repair and you didn't follow the manual to the letter you're on your own with no backup. I'd rather have the paper trail saying I did exactly what the manufacturer said than some logbook full of custom cure times nobody else understands.

andrew_miller9020d agoTop Commenter

Oh man, I gotta push back on this a little... I read an article from a NTSB report a while back about a composite repair that failed because the oven had a huge cold spot right over the bonded area. The guy followed the manual to the letter, set it for 8 hours at the right temp, but the oven was never calibrated or mapped. The repair came apart in flight and they found uncured resin in the middle. The manual's safety margin only works if your oven actually matches what they tested. I get wanting a clean paper trail, but if the repair fails because your oven has a 20 degree delta across the shelf, that manual won't save you... the FAA will still come after you for not ensuring the process worked. It's not about overthinking, it's about knowing your equipment isn't hot garbage.

amy97520d ago

Gotta say I'm with @andrew_miller90 on this one, mapping your oven just seems like basic common sense once you know it's off. If the book's safety margin is built around a perfect oven but yours runs cold in one spot, that margin might not be enough.