A altitude parece estar acima do nível do solo. De Ordem FAA JO 7110.65W Controle de Tráfego Aéreo :
Section 4. Fuel Dumping
9−4−3. ALTITUDE ASSIGNMENT
If an aircraft is dumping fuel in IFR conditions, assign an altitude at least 2,000 feet above the highest obstacle within 5 miles of the route or pattern being flown.
O texto diz "acima do obstáculo mais alto", indicando AGL. O Manual de Serviços de Tráfego Aéreo da CAA é muito mais explícito:
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.
Isto indica que o requisito de altitude de despejar combustível é a partir da primeira consideração, isto é, leva x pés para o combustível cair antes de evaporar. regulamentos suíços abordam isso com um pouco de detalhes.
Generally, air masses are only moved vertically very slowly, usually at a settling rate of approximately 60 centimetres per second. In addition the upper limit of the planetary boundary layer (PBL) at the altitude of approximately 3000 ft over ground renders the air flow from the upper to the lower layers more difficult. Therefore the chance that even a small quantity of fuel reaches the ground is estimated to be unlikely.
Mais uma vez, diz "acima do solo", indicando AGL.