I've heard that in a lot of instances a jet that is making an emergency landing is required to dump excess fuel. In a lot of instances this would make a lot of sense. For example, if the gear cannot be lowered and the plane must land on it's belly. The last thing you want is a hundred tons of jet fuel involved.
But, I have to assume there are also emergencies where a plane isn't required to dump it's fuel. I just am not sure what they would be?
So, my question is: When are aircraft required to dump fuel before landing (or, at very least, when is it advisable to dump fuel before landing.)
The reason they dump fuel is to keep the landing gear intact during the landing, a plane at max takeoff weight will not be kind to its gear. Max takeoff weight has to be equal or higher is also higher than max landing weight. A plane where max takeoff weight is within 5 % of max landing weight doesn't need to be able to dump fuel.
When there isn't a designated fuel dumping site (typically a nearby lake or ocean) or they plane can't make it there so they can't dump fuel, They will instead circle around until the fuel level has dropped sufficiently. This is not possible when the emergency is of the kind that needs to be on the ground NOW.
For many medium and large sized jets the maximum gross takeoff weight is higher than the maximum landing weight. If the airplane has an emergency that requires an air return or other landing in the early part of flight, it is very likely overweight for landing. The plane has 3 options at this point:
For some airplanes, option 2 is not available. This includes airplanes that are not equipped to dump fuel (not all airplanes have this capability or have the capability under MEL) or are not approved to dump fuel at their current location and altitude. For these airplanes, only option 1 and 3 are available.
The choice between option 1 and 2/3 comes down to what kind of emergency it is. If you have to be on the ground now, you won't care about being overweight, and will just put it down. If you have time and landing now isn't a priority, then you would choose to dump or burn fuel.
If an airplane chooses to dump, they will generally choose one of two values of remaining fuel:
This choice comes down to how likely the landing will end in an accident. If you are sure the landing will be a non-event (e.g. air return for anti-ice failure) then you'll likely just dump until landing wont be overweight. Conversely, if you have a control issue or something that may end badly, you want minimum fuel to lessen the potential for fire. In these cases the ARFF crew will spray the runway with foam to assist in reducing the fire potential.
The primary reason is to reduce the weight of an aircraft. Aircraft are designed to land at a maximum weight lower than their maximum weight. In order to be under that maximum landing weight, they must either burn that fuel during flight or dump it.
There is no requirement for them to dump fuel. This decision is up to the pilot. They may choose to land the aircraft "overweight", meaning over the maximum landing weight. In this case the aircraft will have to undergo an inspection to ensure that nothing was damaged. On some aircraft, an inspection is only required if the touchdown acceleration was over a certain amount (like 1.7G).
Dumping fuel will generally be an issue with a plane that has taken off for a long flight and has to divert for some issue. The pilot must decide how urgent the emergency is. If it is an urgent problem, such as engine failure, medical emergency, hydraulic issues, or fire, their priority is to land ASAP regardless of weight. They may dump fuel while headed to a diversion airport if they can.
If the emergency is not as urgent, the pilot may decide that they do not want to continue the flight, especially if it involves long stretches over water without diversion airports. In this case they can hold somewhere near a diversion airport to burn off or dump fuel until they are comfortable with their landing weight.
For example, the 777-200ER can take off at 656,000 lb, but maximum landing weight is 470,000 lb. This means they could have to dump 150,000 lb of fuel if they encounter an issue at the beginning of a long flight.
The amount they leave after dumping will be up to the pilot. For serious gear issues, they may want to be as light as possible and only leave a minimum amount of fuel in the tanks (on a 737, this is about 4,000 lb). For other instances, they may be satisfied with the maximum landing weight and not want to spend any more time dumping or burning fuel. No pilot wants to dump a bunch of fuel and then end up needing it later.
In reference to how much fuel is retained, in the 747-100 and -200 aircraft it was not possible to run yourself out of fuel by dumping. There were stand pipes in the the system that prevented that. A common call to the flight engineer in the simulator for certain emergencies was to "dump down to the stand pipes." Offhand I don't remember how much fuel was left when that was done.
The most fuel I ever dumped to get down to max landing weight (635,000 lbs as I remember) was around 100,000 lbs. We did it at 4,000 feet over the ocean off Lima, Peru after an engine blew (it literally disintegrated inside) on takeoff. As I remember, we believed that dumped fuel evaporates in 2 to 3 thousand feet of fall, so we did not believe it would reach the water. Not that that was any consideration at the moment.
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