Design News for Mechanical and Design Engineers

The airplane had just taken off and was over Cape Cod Bay when the pilot reported oil on the windshield. Shortly thereafter the airplane lost all power and went into the water, sinking in about two minutes. The pilot and one passenger got out of the aircraft. The other two passengers were knocked unconscious by the accident. They and the dog went down with the airplane. The airplane sank in water too deep to attempt recovery so there was no way to determine the cause of the oil on the windshield or the loss of power.

The Coast Guard arrived fairly quickly and rescued the one passenger. The pilot was not able to stay afloat and drowned. There was no flotation gear on the airplane, though such was available at the Beverly airport, where the flight originated.

The pilot was guilty of two serious errors. Not carrying flotation gear probably cost him his life. In addition, a pilot-neighbor of mine told me that flying light aircraft over water is to be avoided since it is easier to make a safe emergency landing on land. Certainly the newspapers abound with pictures of light-aircraft landing, usually without loss of life, on country lanes, cow pastures, and other improbable places.

There was no action in the case for about two years. Then a fishing boat brought the airplane wreckage up in its net. The families of the deceased men then started seeking financial recovery for their deaths. I was retained by the attorney for the family of the pilot.

Everyone involved in the case wanted to know the cause(s) for the loss of oil and power. The National Air Safety Board spearheaded the failure analysis, which took place at a plant of the company which manufactured the engine. I note here that the government people involved in the case were all highly competent.

The propeller had snapped off the engine and was never recovered. The engine side of the fracture extended part way through a graphite front oil seal. The progressing crack in turn cracked the seal, allowing oil to escape and spray over the windshield.

The engine was heavily corroded and had to be sawed apart to be disassembled. Two years exposure to sea water should have so corroded the fracture surface of the crankshaft as to have made fractography impossible. Fortunately, such was not the case. The airplane engine block was aluminum or magnesium (I forget which), and this less noble metal corroded in place of the steel of the crank shaft, acting as what is known as a sacrificial anode. The same technique is used in hot water heaters, where a sacrificial magnesium bar is connected to the inside of the steel tank. Many years earlier I proposed bolting sacrificial anodes of aluminum to the inside of the stock tank on the family farm. My father reacted as if I were daft so I quickly dropped the idea.

Scanning electron microscope study revealed the coarse beach marks characteristic of a fatigue failure. The fine individual striations had been eaten away by sea water. The fatigue crack was traced back to one of numerous corrosion pits on the inside of the crankshaft. Metallographic analysis and hardness testing showed the crankshaft to be of tempered martensite of appropriate hardness. The report was purely factual and made no attempt to say why the fatigue crack had formed and propagated.

So what happened? The corrosion pits indicate acid in the crank-case, which could come from not changing the oil often enough. But why did the fatigue crack propagate as it did? All structural alloys contain small cracks and designers must allow for their presence. Could the engine have been operated outside specifications, perhaps at too high RPM or under conditions giving excessive vibration? There was no record of such abuse.

My part in the case ended after witnessing the disassembly and studying the fractography report. My client later retained me on several other cases. So apparently my client liked my work. I never learned any of the details of the settlement.