Fuel Jettison — Page 212, Lesson 315

I want to walk you through the fuel jettison system. This is a critical emergency system on transport-category aeroplanes, and I want to make sure you understand exactly when it's required, how it's certified, and who makes the call to use it. Let's start with the core purpose. In the event of an emergency occurring when the aeroplane mass exceeds the maximum landing mass, and the commander has decided to land as soon as possible, a system is fitted to the aeroplane to dump fuel by a controlled process. That system is called the fuel jettison system. So the fundamental problem is this: an aeroplane may take off at a weight far above its maximum landing weight — that's normal, because the structure is designed to handle the loads of take-off at a higher mass. But if you need to land shortly after take-off due to an emergency, you're still at that high mass, and landing overweight could overstress the landing gear or the airframe. The fuel jettison system gives you a way to reduce that mass quickly and in a controlled manner. Now, not every aeroplane is required to have a fuel jettison system. The regulation says every aeroplane must have one fitted unless the maximum landing mass exceeds the maximum take-off mass less the mass of the fuel necessary to carry out a 15 minute flight consisting of a take-off, climb to safe height, go-around and landing at the aerodrome of departure, all flown in the landing configuration. Let me unpack that. If the aeroplane's maximum landing weight is already high enough that, even at maximum take-off weight, you could burn off enough fuel in a short standard pattern to get down to a safe landing weight, then you don't need a jettison system. That 15 minute flight profile is very specific: take-off, climb to safe height, go-around, and landing at the departure aerodrome, all in the landing configuration. If the landing mass is still too high after that fuel burn, then a jettison system is mandatory. There's also an important legal point here. The Rules of the Air generally prohibit the dropping or spraying of materials from aeroplanes. But the use of a fuel jettison system in an emergency is not prohibited by that rule. So you're legally allowed to jettison fuel when it's a genuine emergency procedure. Now let's look at the certification requirement for the system itself. Where a fuel jettison system is required, the system must be capable of jettisoning enough fuel in 15 minutes, starting at maximum take-off mass, to reduce the aeroplane mass to enable the aeroplane to meet the climb requirements of CS-25. CS-25 is the European certification specification for large aeroplanes. So the system has to be powerful enough to get you down to a weight where the aeroplane can still meet its climb performance requirements after the jettison. That 15 minute time limit is a key design target. Before the system can be certified, jettison trials must demonstrate four things. First, the jettison system must be free from fire hazards. Second, fuel discharges must be clear of the aeroplane — you don't want fuel spraying onto hot brakes or engines. Third, fuel or fumes must not enter any part of the aeroplane, which means no fuel getting into the cabin, cargo holds, or air conditioning packs. And fourth, the jettison operation must not affect the controllability of the aeroplane. So the system has to be proven safe in all these respects before it's approved. Finally, the jettisoning procedure itself. Pilots of aircraft in flight are permitted to jettison fuel in an emergency. But it must be borne in mind that fuel jettison is an emergency procedure to reduce aircraft mass expeditiously. The decision to jettison rests with the commander alone, but the decision to jettison must be compatible with safety and the ability of the aeroplane to continue flight. So the captain is the sole authority to initiate fuel jettison, and that decision has to be weighed against the safety of the flight and whether the aeroplane can still complete its intended operation after dumping that fuel. That covers the fundamental principles of fuel jettison — why it exists, when it's required, how the system is certified, and who decides to use it.