San Francisco Inquiry Looks for Similarities to Other Crashes SAN FRANCISCO - Every air crash differs in its details, but safety experts look for similarities that will help them prevent calamitous repetitions. In the crash of Asiana Flight 214 here on Saturday, investigators are considering at least two themes: a dangerous practice called unstabilized approach, and circumstances that led to two previous crashes during approach and landing after uneventful flights. An unstabilized approach is one in which the cockpit crew is adjusting the plane's position - left and right, up and down - or adjusting flaps, the throttle or other controls while close to the ground. Airlines now routinely capture reams of data from the digital flight data recorders of advanced aircraft to record such setting changes in the last minutes of flight, as a tool to identify approaches with hidden difficulties that could contribute to a crash. Boeing, for example, lists unstabilized approaches as a leading cause of tail strikes, in which a plane lands with its nose too high, so that not only do the wheels hit the runway, but the tail also does. Saturday's crash was an extreme example of a tail strike, with the landing gear, tail and rear portion of the fuselage being torn off by impact with the sea wall beyond the end of the runway. "An unstabilized approach appears in one form or another in virtually every landing tail strike event," according to the company's advice to pilots. "Digital flight recorder data show that flight crews who continue through an unstabilized condition below 500 feet will likely never get the approach stabilized," the company advises. And when the pilot lifts the nose just before touchdown, to dissipate energy, the plane "invariably has either excessive or insufficient airspeed." In the Asiana case, the Boeing 777 had far too little airspeed, and the crew was trying to add engine power in the last few seconds. But on a plane with the engines under the wings, doing so always causes the nose to pitch upward. That could be countered by using the elevators, the movable parts on the horizontal tail, but that control has less effectiveness at slow speed, when the flow of air across the tail is lower. The precise definition of unstabilized approach - the altitude at which everything must be set up for landing - differs by airline. One question for the investigators is whether Asiana's procedures are too lax in allowing such approaches, or whether the procedures were followed. Investigators are also interested in possible similarities to the crash of a British Airways 777 short of the runway at Heathrow in London in January 2008 and of Turkish Boeing 737 at Schiphol airport in Amsterdam in early 2009. Deborah A. P. Hersman, the chairman of the National Transportation Safety Board, said investigators were interested in those cases because they involved technologically advanced aircraft that crashed on approach. The British Airways crash was traced to ice in the fuel system, but that 777 had different engines than the ones used by Asiana. The Turkish crash appears to have more in common. That case involved the malfunction of an altimeter that measures height in the air by sending a pulse of radio energy to the ground and timing how long it takes to bounce back. The altimeter sent an erroneous reading to the autopilot and the plane automatically cut the throttles. The Asiana crash is also giving the investigators an unusual view into what they call "survival factors." Investigators were seeking to determine why at least one of the emergency slides deployed inside the body of the Boeing 777, reportedly trapping a flight attendant. And they were extensively documenting the conditions inside the 300-seat cabin. They would also like to know if the passengers were wearing seat belts. Another question is what the passengers took with them when they evacuated the plane. Photos show many carrying shoulder bags and even roller bags. In some crashes, the Federal Aviation Administration believes, taking belongings could slow evacuation enough to pose added risk for the last people in line. Though the Asiana crash is likely to produce changes in years to come, by coincidence the Federal Aviation Administration issued an important new rule on Wednesday growing out of the last domestic crash, of a twin-engine turboprop on approach to Buffalo in February 2009. That accident resulted from an error by the captain that was not corrected by a very junior first officer. The new rule, mandated by a law passed by Congress in 2010, will require that the first officers on commuter carriers - the second-in-command in a two-person cockpit - have 1,500 hours of flight experience. The current requirement is 250 hours. The rule sets various other requirements, including that first officers be qualified on the specific type of airplane they are flying. The new rule was announced by Anthony Foxx, who was recently sworn in as transportation secretary. "I am especially pleased to mark my first week by announcing a rule that will help us maintain our unparalleled safety record," he said in a statement.