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In this question, a comment mentioned that if a 4-engine airliner suffers a 3-engine failure shortly after takeoff, the only way to stay aloft may be to dump fuel.

What is the minimum altitude which fuel dumping is possible? Possible here means, the fuel will not cause a hazard to the airplane or anyone on the ground.

Is the altitude MSL or AGL? (i.e., is it "it takes 2,000 feet for the fuel to fall before it evaporates", or "the atmosphere pressure at 8,000 feet makes fuel evaporate a lot quicker, so you can do that if you're at 8,000 pressure altitude even if the ground elevation is 7,000 feet"?)

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  • $\begingroup$ @mins the "physical possible" altitude, not the regulations $\endgroup$
    – kevin
    Nov 12, 2016 at 21:44
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    $\begingroup$ FWIW, I can pretty much guarantee you that if a 4-engine airliner suffers a 3-engine failure shortly after takeoff, they'll start dumping fuel immediately without regard to their altitude. $\endgroup$
    – Terry
    Nov 12, 2016 at 22:43
  • $\begingroup$ A mist of partially vaporized fuel is much better for the health of the surface dwellers than would be the burning wreckage of a heavy or super heavy. $\endgroup$
    – J W
    Nov 13, 2016 at 15:02
  • $\begingroup$ You may read a lot of interesting details on this booklet for Zurich area, where altitude (6,000 ft, but usually at 15,000 if possible) is regulated when time allows for a clean dumping. Note that fuel dumped this way is still oxidized into water and C02. $\endgroup$
    – mins
    Nov 13, 2016 at 19:28

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The altitude appears to be above ground level. From FAA Order JO 7110.65W Air Traffic Control:

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.

The wording says 'above the highest obstacle', indicating AGL. The CAA Manual of Air Traffic Services is much more explicit:

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.

This indicates that the altitude requirement of dumping fuel is from the from the first consideration i.e. it takes x feet for the fuel to fall before it evaporates. Swiss regulations go into this at a little bit of detail.

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.

Again, it says 'above ground' indicating AGL.

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