Why do planes dump fuel?

Why do planes dump fuel?

Why do planes dump fuel
Why do planes dump fuel? - Wikimedia

Fuel cost represents one of the biggest expenses for the aerospace and airline industries. On an average fuel costs account for 29% of all operating expenses. So, why do planes dump fuel? Before answering this let me tell you that this is not done frequently and is done only in emergency situations, like mechanical failure or a medical emergency.

In technical terms, fuel dumping is known as fuel jettison

Aircrafts have two types of weight limits: 

The maximum structural landing weight is always less than the maximum takeoff weight. Now one might think that taking off at a heavier weight would be more difficult as opposed to having to land at a lighter weight. However, this is not the case, as landing puts more stress on the plane.

Why do planes dump fuel?

Fuel Jettison Duct
Fuel Jettison Duct - Wikimedia
One of the factors increasing the weight of an airplane is the fuel stored in it. Now when an airplane takes off, its weight is under the limit of the maximum takeoff weight. The amount of fuel in the plane is such that most of it will be consumed as it reaches the destination thereby decreasing the weight of the aircraft and bring it under the maximum landing weight. 

But in the unfortunate situation of an emergency in which the plane has to land suddenly it is not possible to do so without dumping the fuel . This is because the weight of the airplane is higher than its maximum landing weight and hence it is far too heavy for a safe touchdown. Thus fuel is dumped so that the weight of the plane becomes safe for landing.

In certain situations, dumping of fuel will prove to be cheaper than not dumping it. If an airplane is made to land at a weight higher than its maximum landing weight then it can cause structural damages primarily in landing gear and also to the airframe structure. Sometimes planes can suffer severe structural damages that can take months to repair or worse yet, be irreparable. The plane can even break apart on landing. Thus fuel dumping is not as wasteful as it seems.

Fuel dumping is a simple procedure. Fuel is stored in the hollow wings of the plane. The fuel is jettisoned from nozzles which are located in the wings. The pilot performs a three step process to engage the plumbing and start dumping fuel. 

Plane fuel tanks
Plane Fuel tanks - Wikimedia
Sometimes there occurs an emergency in which there is no time to dump the fuel. In such cases, the pilot has to take the risk of overweight landing. Jets flying with US airlines in the 1950s and early 1960s tended to have fuel dump systems. However, most of the planes developed today are designed by considering the maximum overweight landing. 

For example, Boeing 757 has no fuel dump capability as its maximum landing weight is the same as its maximum takeoff weight. Larger, wider body planes like the Boeing 777 and 747 can dump fuel.

What happens to the dumped fuel?

Since the fuel is dumped in the sky, do we need to hide under roofs when a plane goes over us to protect ourselves from it? Well, the answer to this is NO. Fuel dumping is usually restricted by altitude, that is, fuel cannot be dumped below a particular altitude generally 4500 feet (1371.6 meters). Dumped fuel flows behind the plane like a contrail

Most of the fuel evaporates into the atmosphere before reaching the ground. The evaporation of fuel depends on several factors like altitude, air temperature, and dumping pressure .Technical advancements lead to the development of systems which are installed in planes which help to vaporize the fuel stream to a large extent so as to aid evaporation. 

F111 burning dumped fuel
F111 burning dumped fuel - Wikimedia
Air shows sometimes include dump-and-burn which is a type of fuel dump in which the dumped fuel is burnt by using plane’s afterburner. The dumped fuel burns and produces a tail of fire. However, Dump-and-Burn is banned in the USA as fuel costs a lot of money and to dump it would be a criminal waste.

How long is the fuel to be dumped for?

So how does the pilot decide how long the fuel dump is to be performed until the plane reaches appropriate landing weight. This can be explained by an example
Boeing 777
Boeing 777 - Aircraft Nerds
For Boeing 777

Maximum takeoff weight = 247,200 kg
Maximum landing weight = 201,840 kg
Weight to lose = 45,360 kg or approx 46,000 (46 tons)

The fuel dumping rate for Boeing 777 is 2 tons/min

Fuel burn rate = 6 tons/hr
                       = 0.1 tons/min

Thus fuel dumping rate + Fuel burn rate = 2.1 tons/min

To lose 46 tons the time required will be 46/2.1 = 21.9 min

So the pilot will need to perform fuel dumping for approx 22 min to reach the appropriate landing weight.
Generally landing with less fuel is better than to land with more fuel. Fuel makes up a very large proportion of its total weight and it’s inefficient and uneconomic to fly around with unneeded fuel. The point of dumping fuel is to decrease the weight of the aircraft to the point where it is safe to land. Large planes with a full load are too heavy to land safely, however, if it is a serious emergency like a fire or something similar, it has to be done.  

So to answer the question, dumping fuel is not routine and though fuel dumping leads to wastage of loads of gallons, it needs to be done in emergency situations to protect the plane from any harm and also the people traveling in it.

Thanks for reading!

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