Aircraft Fuel Systems — Page 327, Lesson 406

Right, let's pick this up with the fuel jettison system on a multi-engine aircraft. We're looking at an older four-engine design, but the principles carry straight across to the twin-engine systems you already know. First, a key difference in the fuel storage layout. This aircraft has a stabilizer tank. That's a tank located in the horizontal stabilizer, the tailplane. Its job is to feed fuel either into the transfer gallery or into the centre tank. So we have main tanks, a centre tank, and this stabilizer tank feeding into the system. Now, a critical safety feature. Where fuel pipes are routed through the pressurized rear fuselage, they are double skinned. That means each pipe has an outer sleeve around it. The purpose is to prevent fuel fumes entering the cabin in the event of a leaking pipe. If the inner pipe leaks, the outer skin contains the fuel and fumes, keeping them out of the pressurized cabin. Now, the jettison system itself. Why do we need it? Because the maximum landing mass of the aircraft is significantly less than the maximum take-off mass. If we took off heavy and had to land soon after, landing at that higher mass would compromise the structural integrity of the aircraft. Also, the aircraft might not be able to satisfy the climb requirements of CS-25, or the discontinued approach requirements of CS-25. CS-25 is the certification specification for large aeroplanes. So in an emergency, fuel can be dumped to reduce the mass down to the maximum landing mass. How is it done? Fuel is pumped out through a dump master valve. Typically there's one on each wing, at the trailing edge, well outboard. That position is deliberate — it lets the fuel be dumped safely, with no danger of it entering the aircraft or any of its systems. Who controls it? From the pilot's or flight engineer's fuel control panel. The amount of fuel to be dumped — or the amount of fuel to remain — can often be selected and automatically controlled. The dumping process stops automatically when that selected level is reached. Now, the safeguard. It would be clearly undesirable to dump all the fuel. So there's a minimum amount that must remain. CS-25 stipulates this: the fuel remaining after jettisoning must be sufficient to enable the aircraft to climb to 10,000 feet, and thereafter allow 45 minutes of cruise at a speed for maximum range. That's the legal minimum reserve. Finally, the management of this system. In older aircraft it's manual, handled by the flight engineer. In more modern two-pilot aircraft, like the 747-400, it's almost fully automatic, requiring only minimal input from the pilot. The majority of the monitoring and switching actions are done by a fuel management computer. The stabilizer and centre tank fuel would be used first — either by transferring it to the main tanks, as in the old system, or by selective fuel feed, as in the modern aircraft. And for the same reason as in the twin, fuel is kept in the outboard section of the wings as long as possible, to keep the wing structure loaded and reduce bending stress. Let me show you the layout of that wing and the dump valve position. So to tie it together: the jettison system exists to protect the airframe from an overweight landing, it dumps through master valves outboard on the trailing edge, it's controlled from the fuel panel with automatic stop at a selected level, CS-25 sets the minimum fuel remaining at 10,000 feet plus 45 minutes cruise, and modern systems automate all of this through a fuel management computer.