Took me 3 days to find a tiny crack in a bleed air duct on a 737. I was sure it was the valve.

I mean, the symptoms were textbook for a bad precooler valve. High duct temp warnings, slow pack response. I swapped the valve, did the ops check, everything looked good. Next flight, same issue. I spent a day and a half chasing wiring and sensors. My lead finally came over, took a mirror and a really bright light, and found a hairline crack about an inch long on the underside of the duct itself, right near a bracket. It was so small you couldn't see it unless the light hit it just right. I felt like an idiot for not doing a more thorough visual first. Anyone else had a bleed air leak hide like that? What's your go-to method for finding them?

3 comments

3 Comments

the_laura7d ago

Just be careful with soapy water on hot ducts, it can leave a residue. A dry paper strip test is safer for those areas.

blair_torres707d ago

Yeah, that "mirror and a really bright light" combo is key. I've seen guys use a piece of paper or a thin plastic bag taped near a suspect area to see if the air stream makes it move. Sometimes you can hear a hiss, but on a busy ramp? Forget it. You almost need to get creative when the leak is hiding in plain sight.

ben_fisher7d ago

A soapy water spray in a spray bottle works wonders for finding those tiny, silent leaks.