Fuel Dumping: The Facts Explained
Panic on board flight UA1175 from San Francisco to Hawaii ensued after a load bang was heard roughly 35 minutes before the Boeing-777 was scheduled to land in Honolulu. The reason: Engine #2 was falling apart, with distressed passengers taking photos and videos of an exposed and shaking engine as the “engine cowling” broke away in mid-flight.
Despite the commotion, one Google engineer – Erik Haddad, apparently remained composed throughout the incident, posting a series of pictures on social media and quipping: “I don’t see anything about this in the manual.”
The pilots immediately sent out a distress call to request an emergency landing. Aside from a broken engine, the pilots had another problem: the fuel-laden plane was too heavy for a safe landing.
Every aircraft has a Maximum Take-off Weight (MTOW) and a Maximum Landing Weight (MLW), which is calculated to include a plane’s Dry Operating Weight (base weight), Payload (passengers and cargo) and Fuel (for the trip + contingency fuel + taxi fuel). The idea is simple, if a plane is too heavy, it cannot take off.
However, as explained by Patrick Smith – pilot and author of Cockpit Confidential: “The maximum weight for take-off is often considerably greater than the maximum weight for landing,”.
“This is the case for a few reasons, the obvious ones being that touching down puts higher stresses on an airframe than taking off, and heavy-weight landings require a very high touchdown speed, which makes stopping more problematic.”
Landings above the MLW will cause the aircraft to suffer damage to its landing gear and airframe structure, and in some cases makes the plane too unsafe to ever fly again. To make things worse, the landing has to be light enough for the safety of the 350 passengers on board the B777.
The solution? Dump fuel. Wings on newer planes such as the B777 or A340 are fitted with rear nozzles which jettison fuel into the atmosphere at an approximate rate of 2.5 tons/min + fuel burn during the procedure. A B777 typically consumes 8 tons of fuel per hour. So with 35 minutes left to the flight, we can assume that the fuel dumping procedure took approximately 5 minutes to dump all the fuel.
However, the overall process to dump fuel may take much longer, as a pilot has to go through a three or four step process to engage the plumbing and start dumping fuel. This also includes requesting and waiting for ground approval for a fuel dump, and taking the plane to a safe location/altitude to safely complete the procedure. Air Traffic Control also has to coordinate other aircraft to stay clear of the area.
“Generally dumping happens at a high enough altitude for it to dissipate – it doesn't reach the ground in liquid form or come raining down on people,” says Smith. “It sounds terrible but one way or another that fuel is going into the atmosphere.”
Not all planes are capable of dumping fuel. Older regional models such as the B737 and the popular A320 are not fitted with the fuel jettisoning system. These smaller jets have to burn the fuel by flying holding patterns until their weight is low enough to land safely, or in some urgent cases engage an overweight landing procedure.