What Happens When Airplanes Dump Fuel?
You’ve probably seen it in the news, or maybe even on a TV show or a film feature: an airline pilot requesting to dump fuel just before attempting an emergency landing.
Which now begs the question: what happens when airplanes dump fuel? And why do pilots have to do so when trying to land the plane?
The real reason why planes dump fuel is that the plane is too heavy to land and dumping fuel is the quickest way to lose weight.
See, aircraft have two major types of weight limits:
- The maximum takeoff weight, which is the heaviest weight at which the pilot is allowed to attempt to take off,
- and the maximum structural landing weight, which is the heaviest weight at which the pilot is allowed to attempt to land the plane.
The maximum takeoff weight is considerably greater than the maximum weight for landing.
On a routine flight, aircraft will be heavier due to the large amounts of fuel to get the plane from one point to another. This also takes into consideration any contingencies in flight where they might get delayed somehow or redirected due to bad weather, or some other unforeseen event.
These flights have already taken into account that by the time they’re ready to land, they’ve used up the fuel in transit, and are thus ready to land under their maximum structural landing weight.
If for some reason the plane arrives at the destination, and they’re over the maximum landing weight, they need to either dump fuel or somehow use up the excess fuel (usually by circling the airport a few times) just to get under the weight limit for landing.
Why do pilots have to dump airplane fuel?
An emergency might happen when an aircraft has to return to the airport shortly after takeoff or has to land short of its intended destination.
In cases like these, fuel dumping (or fuel jettison) is done to lose weight so it can land safely quickly.
Landing a plane when it is above the maximum allowable weight is very dangerous, as the landing systems will be taking a lot more stress than it should. With way too much weight, the plane can suffer structural damage or, worse, even break apart during landing.
When a fuel dump happens, the fuel is released at enormous pressure, projecting it into the air stream. Shooting out of the plane at a high flow rate of more than 200 miles per hour, the fuel disperses and breaks down into small droplets very quickly.
What happens to all the fuel dumped by the aircraft?
Simply put, the fuel doesn’t even reach the ground, as it had already vaporized somehow in the air.
When an aircraft attempts a fuel dump, this is normally done at a certain altitude. Air traffic controllers would typically assign the aircraft a bit of airspace in which the fuel can be dumped.
As much as possible, the fuel dump is done over an unpopulated area with a minimum altitude of about a mile above the ground. Most fuel dumps are carried out at an altitude of about two and a half to five miles, as this allows enough time for the fuel to evaporate.
Also, most modern planes capable of carrying out a fuel dump have systems to help vaporize the fuel stream quickly and efficiently.
Not all commercial passenger-carrying aircraft have the capability to dump fuel, however. And this includes two of the most common passenger jets in the world: the A320 and the B737.
The Airbus A340 and Boeing’s B707 and B727 typically have fuel dump systems, although not all might have them as these are custom options upon ordering.
Sources
- Simple Flying
- Wikimedia Commons
- Fuel dumping of an Airbus A340-600 above the Atlantic Ocean near Nova Scotia: photo by BobMil42.