?It Must Be??

1292008162854.jpg

After I wrote my article last month about some of the mindsets that can get us into trouble, I found an excellent example of one of those situations. Air Transat Flight TS236 was an Airbus A330 that was on its way from Toronto to Lisbon. The discovery of metallic particles in the oil system about a week earlier had led the airline to do a precautionary engine change. A manufacturer's service bulletin recommended a change to how a hydraulic line was connected in order to provide more clearance to a nearby fuel line. All of Air Transat's engines had this modification, but the replacement engine had not been modified.

The lead mechanic tried to find the service bulletin but was unsuccessful due to computer problems. Finally a supervisor came up with a way to complete the installation by spreading the lines apart to create more space, and the installation was completed. Over the next few days normal vibration and flexing caused this clearance to be lost, and the hydraulic line started rubbing against the fuel line, eventually causing a leak during the flight to Lisbon. Initially the leak was a relatively moderate 1,000 pounds per hour, and the crew didn't notice anything wrong.

About 25 minutes after the leak started, the crew did notice a problem, but it wasn't with the fuel system. As is common on most jet engines, the fuel on an A330 goes through a fuel/oil heat exchanger on its way to the engine. The heat exchanger uses the cold fuel to cool the engine oil, which in turn warms the fuel. Because the crack in the fuel line was located downstream of the heat exchanger, fuel was flowing through the exchanger much faster than normal. This led to the fuel cooling more than normal, which in turn led to lower than normal oil temperature indications coupled with much higher than normal oil pressure on that engine.

The crew first discovered these anomalies during routine flight log notations as they were crossing the Atlantic. These were very unusual indications that defied any attempts to discover what the problem might be. A few minutes later, the Electronic Centralized Aircraft Monitoring system (ECAM) informed the crew that there was an automatic transfer of fuel occurring from the trim tank in the aircraft's horizontal stabilizer to the right wing tanks. There was nothing particularly unusual about this, as there are normally a series of small transfers throughout the flight, however it occurred much earlier than normal in the flight, as the Fuel Control and Management Computer (FCMC) sensed the reduced fuel quantity in the right fuel tanks and attempted to maintain fuel balance by pumping fuel from the trim tank to the right fuel tanks.

While the first transfer was only 600 pounds, as the fuel leak increased the FCMC sent the entire 6,000 pounds of fuel in the trim tank to the right wing. The ECAM let the crew know this was happening, but they were focused on trying to figure out the strange oil system indications, and the timing and amount of the transfer or the possible connection to the oil indications did not register. With no more fuel available in the trim tank, the aircraft now began to develop a fuel imbalance, leading to an ECAM advisory message when there was a 3,000-pound imbalance. Still focused on the oil anomalies, the captain thought this was due to uneven fuel burn between the left and right sides, so he initiated fuel crossfeed from the left to the right side without consulting the checklist. If he had used the checklist, he would have seen a caution that said, "Do not apply this procedure if a fuel leak is suspected. Rather refer to FUEL LEAK procedure." This procedure would have instructed the crew to shutdown the right engine if they suspected a leak in an engine, and ensure the crossfeed remained closed.

By initiating the crossfeed, the captain was transferring all his remaining fuel to the side with the leak, which was now losing fuel at the rate of about 27,000 pounds per hour. The crew finally noticed that the fuel remaining was about 15,000 pounds lower than expected, leaving much less fuel than they had planned for their arrival at Lisbon. The crew contacted Air Transat maintenance to get some help trying to discover the cause of the fuel loss. Because of the unusual oil system readings and the sudden nature of the fuel loss, the crew became convinced they were dealing with a computer problem that was leading to inaccurate indications. However, they decided to be conservative and divert to Lajes, in the Azores.

The crew's confusion increased as they continued to talk with the maintenance department and at the same time prepared to land at Lajes. They actually tried reversing the crossfeed, transferring from the right to the left side for a few minutes before again crossfeeding from the left to the right side. As they continued their descent towards Lajes, an ECAM came on advising them of minimum fuel in both tanks. At that point they finally began to consider whether they actually had a fuel leak and the fuel really was as low as was indicated, but they still leaned towards the idea that the whole thing was just erroneous indications. Even when the right engine flamed out they still suspected computer malfunctions were causing the problems.

As they were descending through FL345 about 78 miles from Lajes the left engine also quit. With the loss of all generators, they were now relying on only emergency electrical power and the sophisticated glass cockpit went dark, leaving them with only a few standby instruments with which to fly the airplane. They were very fortunate that it was a clear night and they could see the lights of the airport ahead. The captain had to deal with heavier than normal hydraulic controls as he visually headed towards the airport.

As often happens in a situation like this, the airplane arrived at the airport way too high (about 13,000 feet only 8 miles from the runway), so the captain did a left 360 followed by S-turns to lose the extra altitude. They crossed the runway threshold with excessive speed but still managed to achieve a hard landing in the touchdown zone. With no anti-skid available and maximum braking, tires blew out and caught fire, but the captain managed to come to a stop on the runway.

One of the most difficult situations to handle is when a system failure occurs, but the symptoms seem to indicate a very different problem. It can be easy to develop tunnel vision, focusing on the first indications and failing to keep an open mind about other possible causes. In this case, the crew was further mislead when they decided that "it must be" a computer malfunction and got so locked onto that idea that they continued to blame the computer even after an engine flamed out. As pilots, we like clear cut situations and solutions, so we tend to lock onto a possible explanation for something we can't explain.

When you are dealing with an ambiguous situation that doesn't fit the normal failures you were trained to deal with, try to keep an open mind. Rather than saying "it must be," think "it might be" as you explore the various possibilities. Keep exploring other possibilities, especially when there is new information. Occasionally ask what else might be able to cause those indications. Use your checklists even for "simple" procedures because they may contain additional information that could help solve the problem.

Ultimately the crew of Air Transat Flight TS236 did the right thing by using the most conservative response rule and diverting to Lajes, and they handled the loss of all engines over the ocean about as well as anyone could, but by keeping an open mind and using their checklists they might have been able to figure out what was really causing the various anomalies they were experiencing, leading them to keep the fuel crossfeed closed or even to crossfeed the remaining fuel in the right wing to the left side so it would not be lost.