Ask Captain Lim

Hello Capt Lim,

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.