This is the subject of a debate
between my roommate Poncz and I,
How safe is it to land a commercial plane with
full wing gas tanks? I argue that there is a risk of the wings failing,
pointing to a story of a friend whose plane from Italy was turned around due
to the events of September 11th, and the pilot jettisoning the fuel from the
wings over the ocean. Poncz argues that the wings of an aircraft are
incredibly strong and that they can withstand such rigorous forces.
What's your take on this?
Regards,
Chris Vale
Georgia Tech
Hi Chris,
Thank you for the question.
Before I begin with my answer, I would like to give you some basic
information about the airplane landing weight limitation.
Since this site is specially dedicated to the Boeing 777, I will
discuss the answers based on this aircraft.
Generally, in a commercial jet
airplane, there is usually a fairly big difference between the maximum take
off weight and the maximum landing weight.
For instance, on a Boeing 777-200IGW (Increased Gross Weight), the
maximum take off weight is about 286,800 kg and the maximum landing weight
limitation is around 208,600 kg, giving a difference of 78,200 kilograms.
When you mentioned “full wing
gas tanks”, I take it to mean that the aircraft is at or around its maximum
take off weight when the wing tanks are full.
First of all, I must say that
Poncz is right. An aircraft can
safely land with “full wing gas tanks” or with wing tanks full. (By the
way, a jet engine uses kerosene rather than “gas” or gasolene).
Remember, when an airplane is designed and tested, it has to satisfy
the FAA (Federal Aviation Agency) Regulations in every aspects before they can
certify it suitable for carriage of commercial passengers.
Even with the wing tanks full,
the airplane may still be below the maximum take off weight as most airplanes
have a center fuel tank inside the body too.
When you often hear about
airplanes having to dump or jettison fuel before returning for an unscheduled
landing, most of the times it is because the pilots were taking the
precautions of minimizing the risks of further emergencies.
A heavy aircraft lands at a
higher speed and hence the chances of a brake fire are increased.
You also require a longer runway and if you have to abort the landing,
the safety margin is reduced where the surrounding terrain are fairly hilly.
So, if given a choice, a pilot
would like to jettison any excess fuel prior to executing a normal landing.
However, if the airplane is at its maximum take off weight, and it
suffered a serious emergency like a fire that cannot be extinguished, he can
still make a safe landing at this weight as the aircraft landing gears have
been designed to absorb the entire landing load without any serious problems.
All Boeing airplanes have been
designed with adequate strength margins during overweight landing when normal
operating procedures are used.
In fact, some Airlines have
encouraged landing overweight for financial reason.
Imagine the cost in fuel saving which can be as much as 78,200 kg in a
Boeing 777 but there is a penalty to pay in terms of safety and reduction in
the life cycles of the related components.
Furthermore, most airplane
manuals recommend a special inspection any time an airplane is landed
overweight regardless of how smooth the landing is. The inspection consist of
looking for obvious structural damages such as wrinkled skins, popped
fasteners or bent components in areas which are readily accessible.
If signs of overstress are
found, then a more thorough and detailed inspection will be required.
So, to answer your question
again, it is safe to land with “full wing gas tanks”.
The risk of wing failing does not arise as the airplane have been
designed to take a typical sink rate for a normal touchdown or even a
“hard” landing rate. The
landing loads are based on the worst possible landing attitude resulting in
high loading on individual gears.
Hope I have answered your
question satisfactorily.