Twenty airline crews, two pilots each, came to a NASA simulator to fly a twin-jet on a trip modeled on their own airline’s operation. Half of them arrived within two or three hours of finishing a real three-day, high-density, short-haul duty cycle. The other half had usually been off duty for three days, a few of them for only two.
The researchers, from NASA’s Ames Research Center, had built the flight to go wrong. The weather was poor and getting worse. At the destination, the crosswind rose above the legal limit for the approach, the runway stayed hidden in cloud, and every crew ended up abandoning the landing and climbing away. During that missed approach, one of the jet’s hydraulic systems failed. The landing gear and flaps had to be lowered by hand, braking was weaker, and once the gear was down it couldn’t be raised again. The other airports a crew might divert to had visibility of a quarter mile or less, and some were closed. The only reasonable choice was a short, wet runway, 5,800 feet long, sloping downhill next to mountains.
The crews coming off the trip were, not surprisingly, significantly more fatigued. Yet their performance was, in general, significantly better. An expert observer rated them better on a number of dimensions relevant to flight safety, and they talked to each other more. The authors called the pattern “somewhat counter-intuitive.”
The variable nobody had set
The researchers went looking for a reason. Crews at the end of a trip had usually flown the whole trip together. Crews at the start often had not. So they set aside “tired” and “rested” and sorted the crews again by one question: had these two pilots flown together on their last duty cycle?
The gap was larger than the one they had been studying. Crews who had flown together averaged 5.0 errors in the simulation. Crews who hadn’t averaged 11.67. On the errors the study classed as operationally significant, the kind with a direct negative impact on flight safety, the averages were 1.30 against 5.67. The authors concluded that recent operating experience together seemed to help crews coordinate, and that this could offset the fatigue that builds up by the end of a trip.
The re-sorting came after the fact. Nobody had assigned crews to fly together or not, and the comparison of tired with rested crews on errors alone had not been statistically significant. It was twenty crews in a simulator.
The accident record
Eight years later, the National Transportation Safety Board looked at the major U.S. airline accidents from 1978 to 1990 in which the crew’s performance was cited. For 15 of them, it could tell whether the captain and first officer had flown together before that day. In 11, or 73 percent, the accident happened on their first day together. In 7 of 16 with data, it was their first flight together. The Board estimated that at four airlines, a pilot pair would be on its first flight together only 2.8 to 10.5 percent of the time. It said its findings lent support to the NASA study.
The Harvard psychologist J. Richard Hackman later retold both results in Harvard Business Review, in rounder terms. Airlines keep re-pairing pilots, he said, because using each plane and each pilot as a separate unit gets the most out of them financially. He once asked an airline’s operations researcher how long it would be before the two of them, once paired on a trip, could expect to work together again. The answer was 5.6 years.