It is well known for all aviators that the
maximum gross take-off weight, for most aircrafts, 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 highly likely that its overweight
will prevent its landing as early as possible, therefore the pilot in command
will have the following 3 options at this point:
1.
Fly around at low altitude and high power
to burn fuel.
2.
Land overweight.
3.
Dump fuel (if able).
Flying around at low altitude and high engine power,
could be the best choice to burn the extra fuel if there is enough time before
emergency landing. As an aircraft climbs from take-off to its cruise altitude
the air pressure decreases. Interestingly, the “standard” air pressure at sea
level is 14.7 psi. The exact pressure varies somewhat with the weather
patterns. In other words, the aircraft consumes more fuel when flying at low
altitude. Accordingly, the fuel consumption decreases with increasing aircraft
altitude as the air pressure decreases, and vice versa.
On
the other hand, overweight landing is severely dangerous in every situation on
every aircraft, the major concerns for overweight landing are:
§
As the landing weight increases, the
approach and touchdown speeds increase, and the aircraft require more distance
to decelerate. In this case the landing field length may become an issue if the
selected runway is smaller than the requirements.
§ Loads
on the landing gear according to the higher rates of the touchdown in case of
overweight landings. Although, the wheel, brake and tire design are based on
the landing stop conditions at maximum take-off weight, but if these limits
exceeded, the airplane will need maintenance for sure.
In some cases, the pilot can
dump fuel in order to make an emergency landing. This is done to avoid the
excessive weight of the aircraft while landing at places where suitable
conditions might not exist. However, this option may not be available for some
airplanes including airplanes that are not equipped to dump fuel or are
not approved to dump fuel at their current location and altitude. For these
airplanes, only option 1 and 2 are available.
Sometimes, this same concept of fuel dumping is used
in what is called “a dump-and-burn” when fuel is ignited intentionally using
the plane’s afterburner to create a spectacular flame for air shows or as a
finale to fireworks.
Anyway, when there is a need to proceed with fuel dumping this should be coordinated with air traffic control, and precautions are taken to keep other aircraft out of the dumping areas. Moreover, fuel dumping usually takes place at a high enough altitude so that the fuel will evaporate before reaching the ground. The speed with which fuel can be dumped varies according to Aircraft type, but most of aircrafts can reach its standard maximum landing weight within few minutes.
Thankfully, most of the
fuel will evaporate before it even hits the ground when it has been done in suitable
altitude. This is more likely on warmer days and when an aircraft is high up in
the atmosphere -- ideally above 5,000 feet (1,524 meters), if the jettison is
done in lower altitude it may cause a water pollution or another environmental
risks for plants, animals and humans. Fortunately, fuel jettison isn't very
common and is done in emergency-type situations only, and it is normally done after
coordination with the air traffic controller over an area of little population
or over water when the aircraft altitude is suitable to avoid pollution and
other bad environmental effects.
Well planned flights that have well managed fuel calculation will let airlines minimize fuel consumption and avoid such fuel waste in emergency time, experienced flight planning companies like IFPLS will help you to save your fuel and money and provide you with pre-flight emergency plan if needed.
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