Bell Helicopter rotor blade corrosion fatigue failure (Brazil)

Helicopter rotor spar failed in Brazil. Initial diagnosis blamed surface sanding scratches; Tom's fracture-mechanics calculation four years later showed grinding scratches were too shallow to explain failure. Reread the photos: stress corrosion cracking under inadequate paint repair produced a 3 mm crack that did fit fracture mechanics. Tom's deposition led to the manufacturer withdrawing its alert service bulletin.

I had a helicopter rotor spar once that failed in Brazil. They initially said it was because people had put a little notch in the surface. The Brazilians didn't want to send the rotor spar back to a certified repair facility. Not everyone can — the aircraft manufacturers do not certify just anybody to start grinding on your rotor blades. Because if you lose a rotor blade, it's all over, folks — you're going to crash. Even the people from the helicopter company went out there and said, oh, these people were sanding on the rotor spar and repainted it, and they put a little notch in there. They didn't do the fracture mechanics.

Operators avoiding Bell-authorized service center costs ground down rotor dings without proper repainting. Stress-corrosion-assisted fatigue from poor paint coverage propagated cracks until the blade flew off, killing several. Tom's fracture-mechanics analysis showed the initial flaw was too small for the proposed pitting-only mechanism; stress-corrosion fatigue was required.

I once had a problem with a helicopter rotor blade down in Brazil. These guys decided Bell Helicopter was charging too much money to do the work on the rotor blades — a critical element — and Bell recommended you send it back to a Bell-authorized service center. But they're pricey. So these guys decided to do their own repair. They took off the rotors, ground down the paint and any dings in the blade. They kind of blended it smooth, figured they got rid of any stress concentrations, and then they didn't do a very good job painting it again. They went to fly it, and within a couple of weeks the blade flew off, killed a few people.