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Why aeroplanes dump excess fuel

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Excess fuel is dumped from a nozzle protruding from the left wing of a Royal Australian Air Force AP-3C Orion aircraft before landing. AFP

Q: I saw a photo in one of the newspapers of an Australian Air Force jet ‘dumping excess fuel’ before landing after a mission in search of the lost Malaysian plane. What was happening?

The planes involved in the search operation for the missing Malaysian flight MH370 took off from Perth, Australia, with about nine hours worth of fuel.

Recently, when the search for the missing airliner was abandoned early due to poor weather, an Australian AP 3C Orion aircraft was photographed dumping fuel prior to landing.

All planes are designed with certain design weight limits. For most flights, it’s expected a plane will be at its heaviest at takeoff, burn fuel during the flight and land significantly lighter.

This means that for all airliners the maximum design takeoff weight is always more than the landing weight. For modern jets that can fly halfway around the world, the difference between these two weights can be quite significant.

If an emergency occurs that makes it untenable to continue with the flight shortly after takeoff. When this happens, a plane can be several tonnes above it’s maximum certified landing weight. A pilot then usually has three options each with various advantages and disadvantages.

The first is that the plane can dump the fuel until the weight returns to within limits. This apart from taking time which may make the situation worse, also wastes tonnes of very expensive fuel.

Despite what might be expected, the fuel vaporises almost immediately and with sufficient altitude never reaches the ground.

Pilots can also elect to enter a holding pattern in a high drag configuration, for example, with the landing gear extended, to burn the fuel. This is essentially the same as fuel dumping with the added disadvantage of wasting even more time. However, it might be the only option for planes without a fuel jettison system.

Lastly, the captain can elect to land overweight. This is the preferred choice for the airlines as it saves both fuel and time.

However, since the plane is heavier the approach speeds will be faster and it will take a longer distance to stop on the runway. The safety margins are also slightly reduced.

This does not mean that if you hear of an immediate return to the origin airport you should start writing a will. It so happens that the regulations determining the installation of fuel jettison systems are determined by an aircraft’s climb performance after a go around at its takeoff weight.

It has nothing to do with the structural ability of the aircraft to withstand such a landing.

During certification, planes land on one wheel at the maximum takeoff weight with a descent rate of up to six feet per second, as opposed to a normal landing at only two feet per second on all the main wheels.

There have been no accidents on record during either overweight landings or fuel dumping making both very safe procedures. In fact, in service history reveals that the plane rarely gets damaged.

After such a landing, expect a brief inspection defined by the manufacturer no matter how smooth the touch down, before resuming on your trip.

Dr Ondieki is a pilot with an international airline.