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I have heard ATC ask an emergency aircraft if they will be overweight when landing.
Why do they ask? I am not asking about the consequences of an overweight landing or what procedures are required following one, but rather why ATC would ask for this in advance of the landing.

E.G.

  • Does ATC take it into account when offering a runway?
  • Does passing it on to emergency services somehow allow them to be better prepared than if they were informed after the fact?
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    $\begingroup$ Higher possibility of a brake fire or runway overrun is one of the reasons. $\endgroup$ – Ron Beyer Aug 13 '17 at 17:49
  • $\begingroup$ Besides, landing a highly loaded aircraft is not advisable, since –apart from the reasons stated above– a tire (or more) may easily burst if the high-speed contact is hard, and that will be a serious extra complication... $\endgroup$ – xxavier Aug 13 '17 at 21:11
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    $\begingroup$ They want to know because of the consequences of an overweight landing, but you said you didn't want to know about that. $\endgroup$ – OrangeDog Aug 14 '17 at 10:36
  • $\begingroup$ The brakes are designed for a certain amount of energy and it doesn't matter much how long time it takes for that amount of energy to be dumped into the brakes (time is too short for any significant amount of heat to transfer to air). So, unless you give it a ultra long runway, e.g. 10 minutes of deceleration time, the brake will overheat no matter what. The best ATC can do is probably to assign a clear runway so when the plane loses control after the brake overheats and blows a tire or two it won't hit anything on the ground. $\endgroup$ – user3528438 Aug 14 '17 at 23:09
  • $\begingroup$ @OrangeDog sorry I wasn't sufficiently clear - A separate question asked what are the consequences which I didn't want to duplicate. My question was was one of timing - why ATC would want to know ahead of the landing as opposed to checking after the fact.. Answers in the other question didn't seem to indicate that knowing ahead of time would matter. $\endgroup$ – byte wrangler Aug 15 '17 at 2:48
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If an aircraft encounters a serious problem quite soon after departure that forces it to land immediately, the aircraft may be above its certified maximum landing weight. This is because there is still a lot of fuel in the tanks, which adds a lot of weight.

As @RonBeyer mentioned in a comment, landing overweight can have a number of serious consequences. When landing a heavy aircraft, stopping it on the runway already requires a lot of energy (or rather, a lot of energy needs to be converted into something else). If the aircraft is heavier than it is designed to be on landing, this may cause excessive stress on the brakes. In extreme cases, this can make the brakes catch fire. Now, open flames under an aircraft that is almost fully loaded with fuel is the recipe for disaster.

In some cases, even though the crew brakes hard on landing, the aircraft may be so heavy that stopping on the runway is impossible. A runway excursion will then follow (a situation where the aircraft leaves the runway onto the grass or similar). This is not neccesarily dangerous, but it certainly can be, depending on the surrounding terrain:

enter image description here

(image from http://avherald.com/h?article=46f3d531. This aircraft was not overweight, but I still think the situation shown in the picture is relevant to this question.)

Obviously, in an emergency ATC, and in turn the fire and rescue crew at the airport, need to be prepared for such events. If ATC expects an aircraft will be landing overweight, getting the crew to confirm this will allow ATC to inform the fire crew that they can expect brake fires or a potential runway excursion. It can also enable the rescue leader to position the fire vehicles at the best possible position, which would be further down the runway than normal, since the landing roll will be longer. So basically, it's a way to get information to the rescue crew so that they can prepare for what might be coming.

Depending on the type of emergency, ATC will also ask the crew about other details, in some cases also with the purpose of passing this information on to the rescue crew, since the rescue crew will normally not be in direct contact with the cockpit as long as the plane is in the air. See also this related question: Why does ATC ask emergency aircraft about fuel on board?

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  • $\begingroup$ And even if everything goes well, the brakes will be hotter than normal, which means the aircraft may need to wait a bit for them to cool down before it can taxi to the stand. Maybe even on the runway. At the very least, the ground controller will need to find a parking space for them near the runway, where they can let the brakes cool down. Worst case, they need to be towed from the runway. $\endgroup$ – Jörg W Mittag Aug 14 '17 at 8:03
  • $\begingroup$ So why doesn't ATC ask "are you going to crash-land, and if so how?" or something :P Or is the idea to offload that part of the planning from the pilots? If so, there must be other questions about what kind of ground support might be needed. $\endgroup$ – Peter Cordes Aug 14 '17 at 16:48
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    $\begingroup$ @PeterCordes "Are you overweight" is a much simpler question, with a much simpler answer, than "Are you going to crash land and if so, how?" In an emergency, the pilot(s) doesn't want to deal with anything that's not necessary, so simple questions are preferred when information is needed to coordinate activities on the ground. ATC has a whole bunch of people who can take that simple information and formulate a rescue plan without burdening the pilot(s). $\endgroup$ – mmathis Aug 14 '17 at 22:05
  • $\begingroup$ AFACT, emergency vehicles are always positioned near the approach end of the runway and chase the aircraft. This is to minimize the risk that the aircraft collides with the vehicles (in case it has lateral runway excursion). $\endgroup$ – Jan Hudec Aug 15 '17 at 7:03
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To add to the other answer, a great example of needing to burn off fuel before attempting an emergency landing was JetBlue 292. The nose gear would not retract after takeoff and, after flying by the tower to get a visual, they determined the nose gear was stuck in a twisted way. The flight had to circle for 2 hours to burn off fuel before landing, since it was obvious the landing gear would be damaged upon doing so.

enter image description here

The pilots flew the aircraft, which can carry up to 46,860 pounds (21,255 kg) of aviation fuel, in a figure eight pattern between Bob Hope Airport in Burbank and LAX for more than two hours in order to burn fuel and lower the risk of fire upon landing. This also served to lighten the plane, reducing potential stress on the landing gear and dramatically lowering landing speed as well. The Airbus A320 does not have the mechanical facility to dump fuel, despite various news agencies reporting that the aircraft was doing so over the ocean.

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    $\begingroup$ "The Airbus A320 does not have the mechanical facility to dump fuel, despite various news agencies reporting that the aircraft was doing so over the ocean." - excellent example of how a lot of aviation related stuff is misreported in mass media. $\endgroup$ – Burhan Khalid Aug 14 '17 at 4:58
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    $\begingroup$ Wow, I can't imagine having to announce to the cabin, "We're going to have an emergency landing in about 2 hours. Just sit tight, nothing to be worried about. (There will be plenty to worry about when we actually try to land)" $\endgroup$ – Cort Ammon Aug 14 '17 at 16:18
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    $\begingroup$ @CortAmmon It was even worse. They had satellite TV so they could watch TV news commentators talk about the plane they were stuck on. And you know how they love worst-case scenarios... $\endgroup$ – Machavity Aug 14 '17 at 16:40
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    $\begingroup$ @PeterCordes I don't see any advantage to that. LAX is a major airport; which would make finding alternate routing for the affected passgeners much easier than whatever small location they ended up at. If something did go horribly wrong on landing, again LAX is presumably better equiped to handle it and has access to major trauma centers to treat the injured. The same is true for access to repair facilities. A middle of nowhere airport would be worse in almost every possible way. $\endgroup$ – Dan is Fiddling by Firelight Aug 14 '17 at 18:36
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    $\begingroup$ @PeterCordes If you can't find a duplicate, it would make a good question $\endgroup$ – Machavity Aug 14 '17 at 23:09

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