It's ironic, but crashes are making flying safer, and this book again demonstrates [a:Henry Petroski|10019|Henry Petroski|http://www.goodreads.com/images/nophoto/nophoto-M-50x66.jpg]'s (my favorite civil engineer) aphorism that we learn more from failure than we do from success. Adair has painstakingly reconstructed the investigation into the crash of a Boeing 737, one of the world's safest airplanes. It was Boeing engineers who discovered the peculiar flaw in the special and unique design of the power control unit (PCU) that caused the rudder to go into hard over - a full application of the rudder, the large movable section in the plane's tail that in a 737 is used mostly during crosswind landings. The problem was rudder movement in the wrong direction, opposite to what was indicated by the rudder pedals. In the end, this investigation also resulted in computer data that lead to a determination that something similar happened in the crash of a United 737 over Colorado Springs.

Crash investigations throw together a multitude of people, each with a particular agenda or point of view, and places them under extreme pressure. One of the criticisms of the NTSB had been the "party" system that permitted each agency or group its bias: Boeing (which had a desire to implicate the pilots and clear the airplane), ALPA (the Airline Pilots Association that had a special interest in clearing the pilots), USAir (that would have been happy to blame anyone other than itself), and the FAA (an agency often accused of having a contradictory role: promoting aviation yet making it as safe as possible).

The NTSB is charged with overseeing investigations and making a final determination as to the cause of transportation accidents. It could be a demanding and uncomfortable job, but most investigators knew that the results of the investigation would make things safer, and they considered it a failure when they could not determine a cause, as in the case of the Colorado Springs 737 crash.

There was a multitude of clues, but one of the key points that led to an ultimate resolution was a radar track that showed flight 427 crossing the wake of a 727 at the exact moment when things started to go wrong. The exact sequence was duplicated under similar weather conditions and the slight bounce was easily handled by the pilots, but they suspected some relationship. The other key finding came several years into the investigation. Several PCU units, including one from an Eastwind Airlines flight that had suffered a rudder hard-over, and the one from flight 427, were sent to a lab to be tested. In the Eastwind Airlines flight, the pilot, who had many hours of experience in turboprops, was able to counteract the influence of the rudder forcing the plane in the wrong direction, by applying asymmetric thrust, i.e., speeding up the engine on the opposite side, counteracting the thrust of the rudder. Using the engines in this manner was common in small turboprops but would rarely be used in a jet. His quick thinking saved many lives. A misconnected yaw damper was at first suspected, but when the pilot reported the unusual behavior to the NTSB, they decided to take a close look at the PCU. The factory unit tested out fine under all conditions, and so did the Eastwind and 427 units until they simulated moving the plane from a high altitude where it would be -300 and then pump 170 degree hydraulic fluid through it. A Boeing analysis of the data revealed that in rare instances the double valve construction would line up incorrectly, and the rudder would move in the direction opposite to what the pilot intended. Even more remarkable, the malfunction left no marks on the unit at all, so there was no evidence of a malfunction when examined after the fact.

After many years of painstaking work by the NTSB, the pilots' union and Boeing, they were able to come to a decisive conclusion as to what happened on flight 427 and what most likely happened at Colorado Springs. Boeing, even though they stood to lose millions, redesigned the valve and paid to have the PCU valves replaced on 737's despite obvious evidence that the plane had the safest record of any aircraft. ( )

Very good rundown of the two 737 uncommanded reversals. ( )