Safety Investigation Report 2018:1 Factual Information/1.6/1.6.8 Aircraft Systems/6
SAFETY INVESTIGATION REPORT MH370 (9M-MRO)
6) Fuel System
The fuel system has three fuel tanks, two integral wing tanks and one centre tank. The tanks are part of the wing structure and have many fuel system components located inside the tanks and on the rear spar. The fuel tanks are vented through channels in the wing to allow near ambient pressure during all phases of flight.
An integrated refuel panel (IRP) on the lower left wing and two refuel receptacles on each wing allows rapid pressure refueling of the aircraft. The refueling operation is automatic with fuel load selection on the IRP. Fuel quantity indicating system (FQIS) processor unit controls all fueling operations and measuring of fuel quantity.
Several enhanced features were incorporated in the design to include the following:
- Ultrasonic Fuel Quantity Indicating system
- Automatic centre tank scavenge system
- Ultrasonic water detection system
- Densitometers
- Jettison system
Fuel quantity is displayed on the fuel synoptic page and the upper EICAS fuel block.
- a) Engine Fuel Feed System
There are two boost pumps for each main tank and two override/ jettison pumps in the center tank to supply fuel to the engines. The fuel flows through the crossfeed manifold to the engines. Redundant crossfeed valves isolate the left and right sides of the manifold.
At the start of a flight, when all the tanks are full, the normal procedure is to turn on all the fuel pumps. The override/jettison pumps supply center tank fuel to both engines. This occurs because the override/jettison pumps have a higher output pressure than the main tank boost pumps. When the override/jettison pump output pressure decreases because of low fuel quantity in the center tank, the boost pumps automatically supply fuel to both engines from the main tanks.
- b) Auxiliary Power Unit Fuel Feed System
The Auxiliary Power Unit (APU) can receive fuel from any tank. A DC pump supplies fuel from the left main tank if no AC power is available. APU fuel is supplied from the left fuel manifold. APU fuel can be provided by any AC fuel pump supplying fuel to the left fuel manifold or by the left main tank DC fuel pump. On the ground, with the APU switch ON and no AC power available, the DC pump runs automatically. With AC power available, the left forward AC fuel pump operates automatically, regardless of fuel pump switch position, and the DC fuel pump turns off. In flight, the DC fuel pump operates automatically for quick left engine relight with the loss of both engines and all AC power. Figure 1.6I (below) shows the Engine and APU Fuel Feed System.
Figure 1.6I - Engine and APU Fuel Feed
(Copyright © Boeing. Reprinted with permission of The Boeing Company )
- c) Fuel Inlets
The fuel intake inlet for the APU (in the left main tank) is located lower than that for the engine. As the fuel level drops below the engine fuel intake level the engine will be starved of fuel, however fuel will still be available for the APU as its fuel intake is lower. This difference in level between the engine and APU fuel intakes, accounts for approximately 30 pounds of fuel in a standard flight attitude (1° pitch). The APU is estimated to consume (when electrically loaded) approximately 2 pounds of fuel in 55 seconds which will amount to a maximum APU run time of 13 minutes and 45 seconds. The pitch attitude and in-flight accelerations can affect the actual amount available for the APU.