The primary Federal Aviation Administration (FAA) regulations involved in landing overweight and fuel jettison are:
- Federal Aviation Regulation (FAR) 25.1519 — Requires the maximum landing weight to be an operating limitation.
- FAR 91.9 — Requires compliance with operating limitations.
- FAR 121.557 and FAR 121.559 — Allow the pilot in command to deviate from prescribed procedures as required in an emergency situation in the interest of safety. In June 1972, the FAA issued Air Carrier Operations Bulletin No. 72-11 giving three examples of situations the FAA considered typical of those under which pilots may be expected to use their emergency authority in electing to land overweight:
- Any malfunction that would render the airplane unairworthy.
- Any condition or combination, thereof, mechanical or otherwise, in which an expeditious landing would reduce the exposure to the potential of additional problems which would result in a derogation or compromise of safety.
- Serious illness of crew or passengers which would require immediate medical attention.
- FAR 25.1001 — Requires a fuel jettison system unless it can be shown that the airplane meets the climb requirements of FAR 25.119 and 25.121(d) at maximum takeoff weight, less the actual or computed weight of fuel necessary for a 15-minute flight comprising a takeoff, go-around, and landing at the airport of departure.
To comply with FAR 24.1001, the 747 and MD-11, for example, require a fuel jettison system. Some models, such as the 777 and some 767 airplanes have a fuel jettison system installed, but it is not required by FAR. Other models such as the DC-9, 717, 737, 757, and MD-80/90 do not require, or do not have, a fuel jettison system based on compliance with FAR Part 25.119 and 25.121(d).
SAFETY AND ECOLOGICAL ASPECTS
Landing overweight and fuel jettisoning are both considered safe procedures: There are no accidents on record attributed to either cause. In the preamble to Amendment 25-18 to FAR Part 25, relative to fuel jettison, the FAA stated, “There has been no adverse service experience with airplanes certificated under Part 25 involved in overweight landings.” Furthermore, service experience indicates that damage due to overweight landing is extremely rare.
Obviously, landing at weights above the maximum design landing weight reduces the normal performance margins. An overweight landing with an engine inoperative or a system failure may be less desirable than landing below maximum landing weight. Yet, delaying the landing with a malfunctioning system or engine failure in order to reduce weight or jettison fuel may expose the airplane to additional system deterioration that can make the situation worse. The pilot in command is in the best position to assess all relevant factors and determine the best course of action.
Some operators have questioned whether fuel jettison is permissible when an engine or airframe fire exists. There is no restriction on fuel jettison during an in-flight fire, whether inside or outside the airplane. During airplane certification, Boeing demonstrates to the FAA in a variety of flight conditions that jettisoned fuel does not impinge or reattach to airplane surfaces. As fuel is jettisoned, it is rapidly broken up into small droplets, which then vaporize. Boeing does not recommend operator-improvised fuel jettison procedures, such as jettisoning fuel from only one side during an engine fire. Such procedures are not only unnecessary but also can increase jettison time and crew workload.
The ecological aspects of fuel jettison have been most closely studied by the United States Air Force (USAF). These studies have shown that, in general, fuel jettisoned above 5,000 to 6,000 feet will completely vaporize before reaching the ground. Therefore, Boeing’s general recommendation is to jettison fuel above 5,000 to 6,000 feet whenever possible, although there is no restriction on jettisoning at lower altitudes if considered necessary by the flight crew.
Fuel jettison studies have indicated that the most significant variables related to fuel vaporization are fuel type and outside air temperature. Some studies found that temperature can have a very significant effect on the altitude needed to completely vaporize fuel. For example, one USAF study found that a 36-degree Fahrenheit (20-degree Celsius) reduction in temperature can change the amount of liquid fuel reaching the ground by as much as a factor of 10. Other factors such as fuel jettison nozzle dispersion characteristics, airplane wake, and other atmospheric conditions can affect the amount of fuel that reaches the ground.
Even though fuel is vaporized, it is still suspended in the atmosphere. The odor can be pronounced, and the fuel will eventually reach the ground. Boeing is not aware of any ecological interest promoting a prohibition on fuel jettisoning. Because of the relatively small amount of fuel that is jettisoned, the infrequency of use, and the safety issues that may require a fuel jettison, such regulations are not likely to be promulgated.