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.
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