Aviation Investigation Report A98H0003

1.6.14  Fuel System

  1. - General
  2. - Fuel Status at Departure
  3. - MD-11 Fuel Dumping System  General (STI1-25)

The fuel is stored in three main tanks and two centre auxiliary tanks (upper and lower). The three main fuel tanks are located in the wings. Tank 1 (in the left wing) and Tank 3 (in the right wing) are identical, each having a main compartment, and an outboard compartment called the tip tank. Tank 2 is located in the inboard portion of each wing, and the two halves are interconnected by a large diameter fuel line to tanks 1 and 3. The two centre auxiliary tanks are located in the interspar fuselage section and are interconnected to the main tanks via a fuel manifold. The engines normally receive fuel, under pressure, from their respective main tank; the APU receives fuel from Tank 2. Because engines 1 and 3 are located below the wings, they can draw fuel from the fuel tanks even if the electric fuel pumps become inoperative. Engine 2, being tail-mounted and higher than the main fuel tanks, needs fuel to be pumped to the engine to maintain normal engine operation. In the event of a total electrical failure, fuel pressure to Engine 2 can be maintained by the Tank 2 left aft fuel pump and the tail tank alternate pump, both of which are powered by the right emergency AC bus following deployment of the ADG. The MD-11 is also equipped with a tail fuel tank, located in the horizontal stabilizer. During flight, fuel is automatically transferred in and out of this tank as required to maintain an aircraft C of G that aerodynamically provides the most economical fuel consumption. Every 30 minutes while the tail tank temperature is above 2°C, fuel is automatically transferred from the tail tank to Tank 2 or the upper auxiliary tank. On this particular flight, Tank 2 would have been the tank receiving the tail fuel.

There are seventeen, 115 V AC motor-driven boost or transfer fuel pumps interspersed among the various tanks. All of these pumps are electrically powered by one of the three generator buses; the Tank 2 left aft and the tail alternate pumps are powered from the right emergency AC bus, which receives power from the ADG if normal generator power is lost. All the pumps are automatically controlled throughout the flight by the fuel system controller (FSC) depending on the required fuel schedule, which includes fuel load, fuel distribution, phase of flight, fuel dumping, water purging, weight and balance control, and engine cross-feed operation requirements. The FSC checks and maintains the fuel schedule to satisfy structural load requirements and transfers fuel to the appropriate tanks to ensure proper distribution. The pumps can also be operated in MANUAL mode or, in certain failure conditions, the FSC may automatically revert to MANUAL mode. In the MANUAL mode, a selected set of fuel pumps will automatically turn on and can be controlled individually by a push button selection on the fuel SCP.

Three cross-feed valves can be used in the event of a fuel system delivery malfunction. In the event of an engine feed pump failure, the associated cross-feed valve can be opened to direct fuel to that engine. In the event of a main transfer-pump failure, fuel can be transferred using the engine feed boost pumps by opening the associated cross-feed valve.

The auxiliary tank fill/isolation valve works in conjunction with the tail tank fill/isolation valve when fuel is automatically transferred in and out of the tail tank for C of G control. Both valves are open when fuel is being transferred into the tail tank. When fuel is being transferred out of the tail tank, the tail tank fill/isolation valve is closed. Depending on flight conditions and the quantity of fuel in the upper auxiliary tank, the auxiliary tank fill/isolation valve is either open or closed. An open valve directs fuel to the three main tanks; a closed valve directs fuel to the upper auxiliary tank.  Fuel Status at Departure

After refuelling at JFK airport, the occurrence aircraft had a fuel load of 65 300 kg of Jet A fuel. The flight plan indicated that SR 111 would use 1 000 kg for taxi, leaving a fuel load at take-off of 64 300 kg.  MD-11 Fuel Dumping System (STI1-26)

The MD-11 has two fuel dump valves for dumping fuel overboard. There is one dump valve on the trailing edge of each wing, between the outboard aileron and outboard flap. Fuel dumping is initiated by selecting the DUMP switch on the fuel SCP in the cockpit. Selecting the DUMP switch activates the boost pumps, transfer pumps, and the cross-feed valves. The fuel dump rate is approximately 2 600 kg per minute, provided that all of the fuel pumps and both of the dump valves are functioning normally.

Fuel dumping will cease when the DUMP switch is selected again, when the aircraft gross weight reaches a weight that was pre-selected by the pilots through the FMS, or at any time the FUEL DUMP EMERGENCY STOP button is pushed. The FMS fuel dump default is set to the maximum landing weight of the aircraft: 199 580 kg. If a pre-selected weight is not set by the crew, fuel will be dumped until the aircraft weight reaches the default weight. Pilots do not normally pre-select a weight; they use the default setting as the desired dump weight. As a backup, each main fuel tank has low-level float switches that will stop fuel dumping from that tank when the fuel load in the tank reaches 5 200 kg.

Fuel dumping flow rates will be reduced if the SMOKE ELEC/AIR selector is selected while dumping is taking place. (STI1-27) Fuel dumping had not started prior to stoppage of the FDR recording, and fuel dumping was not underway at the time of impact. If the SMOKE ELEC/AIR selector was selected during the last few minutes of the flight, any associated reduction in fuel dumping rate would not have been a factor in this occurrence.

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