An aircraft flying over a densely populated area–far away from the sea and mountains–declares an emergency, request fuel dump, and a landing as soon as possible. How does an air traffic controller know where it is safe/possible to dump fuel?
Fuel dumping was regulated by FAA Order 7110.65P - Air Traffic Control, which was canceled in 2006. FAA has the following to say regarding fuel dumping:
...If the need arises, aircraft are instructed by Air Traffic Control to dump fuel at high altitudes as far away from the airport or any populated areas as possible.
At present, there are no hard and fast rules (at-least in US, for other countries, see below) regarding fuel dumping and it is left to the air-traffic controllers to guide the aircraft.
The main criteria in determining an are for fuel dumping are:
-Altitude In general, fuel jettisoned above 5000 ft will vaporize before reaching the ground. Due to this, some manufacturers recommend fuel dumping above 5000 ft. In general, the ATC will guide the aircraft to an altitude where it can dump fuel so that it vaporises before reaching the ground.
-Atmospheric conditions The evaporative characteristics of the fuel dumped depends on the altitude and the atmospheric temperature. The aircraft fuel tends to vaporise faster in a warm day.
-Aircraft separation The ATC will try to keep a minimum lateral separation (~5 miles) between the aircraft dumping fuel and other aircraft and also some vertical separation (~2000 ft, according to Order 7110.65P).
Except in case of safety emergency, the fuel dumping is carried out at high altitudes.
Note: Some countries do have fuel dumping regulations, like Switzerland, which says,
The choice of the dumping area is determined by the urgency of the incident, the actual flight path, meteorological conditions and the general air traffic situation. The aircraft is led to a selected area by the air traffic control to dump fuel. At the minimum speed of 250 knots at an altitude of 6,000 ft, but usually with 350 knots at 14,000-16,000 ft fuel is dumped ... There exist no predefined dumping areas ...
In an emergency, pilots can dump fuel where and when they need.
It is preferred that dumping occur
- Fuel Jettisoning
13.1 Pilots of aircraft in flight are permitted to jettison fuel in an emergency. The decision to jettison rests solely with the pilot but he may request guidance from ATC.
13.2 When an aircraft in controlled airspace needs to dump fuel, ATC should co-ordinate with the flight crew:
(1) the route to be flown which, if possible, should be clear of cities and towns, preferably over water and away from areas where thunderstorms have been reported or are expected; (2) the level to be used; (3) the estimated duration of the fuel dumping; and (4) the frequency to be monitored whilst the aircraft is dumping fuel.13.3 Controllers are to recommend to flight crew that jettisoning of fuel should be carried out above 10,000 feet agl. Exceptionally, if fuel dumping at this level, or over water, is operationally impracticable or inconsistent with safety, fuel may be jettisoned above 7000 feet agl in winter and above 4000 feet agl in summer. For fuel to be jettisoned below these levels the situation must be unavoidable.
13.4 A vertical separation of at least 1000 feet between aircraft should be maintained.
13.5 Adjacent ATC units and control sectors should be informed of the fuel dumping taking place, including co-ordination with units providing services outside controlled airspace where the aircraft’s track is near to the boundary of controlled airspace (both laterally and vertically).
