I think your summation is accurate, Tom.
..., but maybe one or all. Stuff just doesn't always behave like you think it might - that's for sure.
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Your mention of stress risers in connection with prop blades reminded me of a case I had. A Bonanza shed a prop blade in flight and several people got killed. ...attitude when I asked them why they had not used the same warning system for the Bonanza's prop.
"Oh, we only had that problem with a completely different type of prop, a D34xxxxx."
...Think about the attitude of these engineers the next time you are flying a plane with a McCauley prop bolted on the nose.
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Wow, Gary. I remember the red dye. Hate to say it, but what you describe is the classic "silo" type of behavior. Inexcusable.
I hope that attitude is rare, but as talented as engineers can be, much depends on who's running the programs to ensure the silo walls are kept knocked down. It takes strong management discipline to keep the doors open and make sure people are cross-talking and applying lessons learned - always a challenge.
One of my pet peeves is prop maintenance. There have been too many blade failures even from simple corrosion / minute nicks that turned into stress risers and cracks over time. I hate to walk the line and see a neglected prop.
It's pretty creepy examining a brittle fracture to count the striations from inititation to that final disconnect, isn't it? Sad in a case like that. I hate to think of innocents injured or killed by such things - awful.
LTM -
I handled another case resulting in three deaths from a failure of a blade in flight. This involved a separation of a rotor blade on a
Sikorsky Skycrane, S-64/CH-54, a BIG helicopter. We represented Sikorsky, Pratt & Whitney and United Technologies, their parent company, in many cases. This was also caused by a stress riser. The rotor blades are manufactured out of a long very thick aluminum extrusion with a cross section like a very big letter "D." This makes up the spar of the blade and the curved part of the "D" makes the leading edge of the blade. The trailing edge of the blade is made out of thin aluminum boxes or "pockets" (they are lightly loaded compared to the spar) that taper to the trailing edge. The wide, open side of the "pockets" are overlapped on the spar and glued into place. Since a failure of the spar would be catastrophic, Sikorsky provided a warning system to alert the crew of any developing cracks. Each end on the blade was sealed and the hollow inside the "D" was filled with low pressure nitrogen. A "blade integrity monitor" (BIM) was mounted on the inboard end of the blade where it is visible from the cockpit and on pre-flight. The BIM is a pop-out indicator that is held out, where it is visible, by the pressure inside the blade. If there is a leak of nitrogen then there must be a crack that has already progressed all the way through the web of the spar. If the pressure leaks out then a spring withdraws the BIM pin and the pilot knows he has a problem. They are exactly like the valve caps I have on my car tires that turn from green to white when the pressure gets too low.
The helicopter was returning from Florida to California to start a new fire suppression contract and they were in a hurry, they make a lot of money per day. They landed in Texan and the maintenance truck was a day behind them.
After they landed for the night in Texas the pilot noticed a bad BIM. The pilot borrowed a nitrogen bottle from the FBO (since they didn't have their own maintenance van which carried spare blades and a special low pressure, very precise, pressure testing setup) and added nitrogen to the blade, which popped the BIM indicator back out to the normal position, and then went to the motel. The next morning he checked the BIM, it still looked O.K. so they took off. Several hours later the blade broke in the middle and an S-64 doesn't fly too good with only four out of five rotor blades, three dead guys.
Of course, the widows and orphans sued Sikorsky.
A bad BIM requires a lot more than putting a shot of nitrogen in the blade and seeing if it holds pressure overnight before you can return the helicopter to service. The Sikorsky maintenance manual requires that the blade be removed and placed on two saw horses with a fifty pound weight in the center between the two supporting saw horses. The ends of the blades, the BIM, and the nitrogen valve are checked or replaced. Low pressure nitrogen is added using the special equipment. Then you wait a
week and then check the pressure again. If the pressure is still good then you turn the blade over, replace the weight, and wait another
week before making the final pressure check. The reason for the saw horses and the weight is that the blade will droop and if the crack is on the bottom the drooping will close the crack. The weight bends the blade the other way to open up any crack so the nitrogen can escape and the failure can be found. The failed blade had a crack on the bottom of the blade and the blade drooped over night in Texas sealing the crack and keeping the nitrogen in so they had a good BIM in the morning.
So what caused the failure? Well, there was a small crack in one of the pockets which was not of any importance, these are common. The crew chief/ A&P had stopped drilled that crack in the pocket, which would have been O.K, the pockets are not critical and it is a fairly common occurence to lose pockets in flight and you don't even notice until after you land. The problem was that he drilled too far forward on the pocket, where the pocket overlapped the spar and put a slight ding in the spar with the drill bit. The spar is heavily stressed and that stress riser eventually resulted in the crack and the three dead people, including the A&P.
As an aside, we tend to think that NTSB investigators are very careful and knowledgeable. The six rotor blades are color coded with a spot of paint on the butt of the blade for maintenance record keeping purposes, white, red, black, yellow, green and blue. The NTSB investigator concluded that the black blade had failed in flight so we spent over a year tracking down the history of that blade. It was manufactured in the '60s, sold to the Army, overhauled by the Army in the '70s, etc. Then we all got together one day with all the parties' experts, laid all the parts of the five blades out on the hanger floor (the other blades had also come apart after the first one failed) and lo and behold it turned out it was actually the green blade that had failed first and precipitated the accident! So much for the competence of the NTSB, if they couldn't get something as big and obvious as this right what else did they get wrong. (I had many other cases with NTSB incompetence but this was probably the most egregious example.) (And we had to pay to fly this investigator back from Brazil for his deposition, he had left the NTSB and was in charge on the safety program at Embraer!)
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We had fun on another case for Sikorsky that involved a crash of an H-3 helicopter that had formerly belonged to the Navy. Plaintiffs contended that it was a dangerously designed helicopter, "give us ten million dollars please." The judge allowed us to show some pictures to the jury so that they could see what that type of helicopter looked like and we used pictures of Marine Corps One at the Reagan Library (four miles from where I am sitting) which flew Reagan around. Must be a really dangerous helicopter design that they would use to fly the President. We had a good laugh about this one.
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