At 35,000 feet, an Air Canada jet en route from Montreal to Edmonton suddenly ran out of fuel. The pilot glided to a rough, but safe, emergency landing, and none of the 65 passengers on board was injured.
In the probe afterward, investigators found that at least four separate events led to the emergency - including a mistake by the ground crew in converting metric measurements to standard when reading a dipstick.
The lesson from the 1983 Air Canada flight is one the airline industry is increasingly drawing on today: that airplane accidents, such as last month's ValuJet crash, usually don't have one simple cause. Instead, they occur as a result of a chain of events - often including human error. Thus, if one link in the chain can be circumvented, accidents can often be prevented.
As the industry comes under renewed pressure to improve safety, it is studying past mistakes to try to make changes in current operations.
True, airlines are one of the safest and most heavily monitored industries in transportation. But as the number of flights increases, experts say, so will accidents. Travel on US-based carriers is expected to jump 50 percent in the next 10 years, and growth worldwide may be even greater.
Moreover, even though accident rates in the United States are near zero when the volume of travel is considered, the federal government - and public - is prodding the industry to become as accident free as possible.
In response, US airlines - following the lead of their European counterparts - are implementing new measures to identify the links responsible for creating an accident-producing chain.
"What we are focusing on is the prime causes of accidents," says Stuart Matthews of the Flight Safety Foundation in Washington, an international organization funded by the aviation industry. "Other parts of the world have been doing this for years."
The US industry last winter took three steps toward this goal. Boeing Company, the Seattle-based aircraft manufacturing giant, started analyzing decades' worth of data it has collected about accidents to identify contributing factors. In addition, top aviation-safety experts decided to test two emerging technologies: an enhanced flight-data recorder and an in-flight system that warns of changes in terrain.
Boeing this spring issued its first report on factors that lead to accidents. "We made a commitment to go back through our older data to see what the contributing factors were, and how we could apply a positive strategy in its place to prevent accidents," says Boeing spokesman Russell Young.
Boeing found that an average of 4.39 contributing factors led to each accident and that human error often plays a key role.
In the case of the Air Canada flight, for example, the Transportation Safety Board of Canada enumerated the following events that led to the mid-flight fuel emergency.
To begin with, the plane's two fuel gauges weren't working. So the ground crew used a manual back-up system - a stick that is dipped in the tank to measure fuel. But the stick measured liters and, in converting the measurement to a weight, the crew used pounds instead of kilograms. They filled the tank with about half the fuel needed for the trip.
The safety board found that the maintenance and flight crews were at fault for not complying with safety requirements. The airline, too, did not have a simple system in place to deal with the metric-conversion problem.
If a strategy had been in place that removed one of these links in the chain - the proper measuring stick, for example - the emergency would not have occurred.
In the effort to eliminate accidents, aviation experts say, the industry has also launched the Flight Operations Quality Assurance program. FOQA is different from the flight data recorder, or "black box," a crash-proof unit located in the tail of an airplane that is used only after a crash, to determine possible causes.
FOQA, on the other hand, employs a number of computers that are already on board most aircraft. It measures as many as 150 parameters as often as eight times per second, including flight controls, air speed, and engine settings. FOQA downloads this information via cassette tape so that experts can analyze the data.
"This information can be quickly accessed, analyzed, and used," says Al Prest of the Air Transport Association in Washington, an organization of airlines. "It will validate what we are doing correctly or identify trends that we need to take action on to correct. We won't need to wait for an accident to happen."
British Airways has used this system for 25 years. Two US carriers, United Airlines and USAir, are voluntarily testing FOQA now.
Critical to the program's success are trust and cooperation between pilots and airlines. The system records every move a pilot makes during a flight, and experts warn that it should not be used to penalize pilots for their actions.
"We built our program on trust between management, the pilots' union, and the pilots themselves," says Capt. Roger Whitefield, chief air-safety investigator for British Air. "If something is detected that we don't understand, we ask the pilots' union representative to contact the pilot. We never know who the pilot is. The union talks to him and amplifies what we need to know. This way we never use the output of this system for disciplinary purposes."
The program is "a tremendous trend indicator," Captain Whitefield says. For example, British Air pilots, who have not had an accident in the past 10 years, did not routinely perform nonprecision approaches, but they needed to use these types of landings after the airline began service to Antigua a few years ago. The flight data British Air collected indicated varying levels of pilot performance, so the airline improved it via a training program.
British Air and United Airlines also are experimenting with Controlled Flight in Terrain. It is intended to prevent accidents such as the American Airlines crash in December, in which a plane approaching an airport in Cali, Colombia, inexplicably ran into a mountaintop.
Jet cockpits now have electronic signals that work off the radar altimeters. The signal screams "pull up" if the plane approaches a steep mountain. But the pilot doesn't have much time to adjust - about 10 seconds. In the experimental program, a computer memory chip can display all the terrain of the world on a television screen in the cockpit.