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The Case of the Torrid Turbocharger
Kenneth Russell, Professor Emeritus, MIT -- Design News, February 4, 2008
A group of seven American men enjoyed some of the excellent hunting available on Anticosti Island in Northeastern Quebec, then chartered a twin-engine airplane to fly them to Bangor, ME. The engines were equipped with turbochargers which are powered by exhaust gases and turn a turbine wheel connected to an air compressor. The result is a significant boost in intake pressure and engine power.
The Scene of the Crime
Along the way, the pilot reported a rough-running engine and obtained permission to land at Charlo, New Brunswick. At 1,100 ft, the pilot reported he could not maintain altitude. At about 200 ft, the airplane entered a roll to the right and plunged nose first into the terrain. All lives were lost. The ensuing fire caused serious damage to the wreckage.
The Investigation
The question was, what caused the rough-running engine and the apparent loss of power? The Transportation Safety Board of Canada concluded the turbocharger experienced an in-flight burst-type failure associated with high-speed rotation and/or excessive temperature. They did not do a detailed metallurgical investigation of the wheel. I was retained by a plaintiff's firm to do such a study.
The figure, above right, shows part of the wheel from the right turbocharger. An undamaged blade is to the left and two broken blades are to the right. The question, now, was did the damage occur during flight, causing the crash or during the crash? The wheel is made of a nickel-based alloy that would be little affected by the ground fire.
The strength of alloys is often improved by precipitation strengthening in which fine particles of a second phase are induced to form in the matrix. The process is similar to salt crystals forming as a concentrated brine is cooled. The strength and hardness of the alloy depends on the size and number of particles, which in turn depend crucially on the cooling rate. The turbine wheel was, of course, solid and the precipitates were of a chromium and aluminum rich phase, but the principles are the same.
The Smoking Gun
The hardness of the blades of the failed turbine wheel were found to be significantly below the proper value while the hub hardness was normal. The hardness change could have only been caused by an excursion in temperature.
Polycrystalline alloys behave a bit like popcorn balls, in which the kernels are held together by candy. At room temperature the candy is hard and the ball will fracture through the grains, while at high temperatures the candy softens and the grains pull apart. A scanning electron microscope study (SEM) showed fracture of the blades occurred along the grain boundaries rather than through the grains.
A study of a polished section of the blade tip under the SEM showed the precipitates were much smaller than expected. The precipitate had been dissolved during use and re-formed very rapidly, giving the fine size. The grain boundary fracture mode, the altered hardness and fine precipitates were clear evidence failure had occurred at a very high temperature. The failure occurred during flight as concluded by the Canadian authorities.
But why did the temperature get so high? I was told the exhaust gases had probably reignited, which would give a situation rather resembling chimney fires in houses with fireplaces. I left the cause of the reignition to the engine experts to figure out.
The Canadian investigation found the aircraft was properly maintained and the pilot appropriately qualified. The airplane was estimated to be very slightly overweight at the time of the accident, but should have been flyable, even with the loss of power.
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