I promise I don’t start a series, but as my previous post was already about dumping things, this post was quite natural as the next step. It is also motivated by a very recent case.

Each plane has some limitations in terms of weight for various phases of flight, and in this case what is of interest is the maximum take-off weight, and the maximum landing weight.

The maximum take-off weight is obviously limited by some structural constraints and by the power available. The maximum landing weight is also limited by strength of the aicraft fuselage, and of the dampers, and for long-range aicrafts, the maximum take-off weight can be higher than the maximum landing weight. Keep cool, I did not say that a plane landing with a weight higher than its maximum landing weight would explode or be destroyed, but it would need a serious inspection before going back in operation.

Under normal circumstances, a plane taking-off with a weight over its maximal landing weight will land within limits, as most of the fuel will be burnt in flight, making the plane weight below the maximum landing weight.

Nevertheless, if such a plane has to land earlier than expected for any reason, one option to make it light enough is to dump part of the fuel within the atmosphere before landing. All planes facing such possibility have to be equipped with proper jettisoning devices.

Now, what about our ecological friends ? Given the speed and altitude at which fuel will be “sprayed”, all of it will remain in the atmosphere, as the droplets are so small that they are maintained in the air by local airflows.

But let’s do some calculations, based on the particular case I mentionned in the introduction. This particular plane dumped 60 tons of fuel between Geneva and Dijon. This looks to be a lot. To make the computation easier, let’s assume that fuel has a density of 0.7, which is not that wrong. This means that this plane did dump about 86 cubic meters of fuel. Still seems a lot ?

This dumping took place between Geneva and Dijon, so over a distance of 75 nautical miles, equivalent to 139 kilometers, so 0.6 cubic meter per kilometer. This is roughly a bathtub every kilometer.

Given the altitude of the plane, one can imagine that when reaching ground (but remind this is not the case !), the fuel would spread over a 1km area, so the 0.6 cubic meter would be “spread” in a square of 1km x 1km, which is 1′000′000 square meters. Assuming it is spread evenly, this would result in a fuel layer of 0.0006 millimeters, or 0.6 microns !

Compare that to what is emitted daily by cars, trucks, and also envisage how bad an problematic landing could be with 60 tons more on board, and you will realise that this fuel dumpings are not so critical things.

All details of the case I used as example, and links to additional resources can be found here.