I have just seen that a BA flight from London to Miami took a 2 hour detour to use up fuel before heading to its destination. It is not the first time I have heard of this.
Why do planes need to use up/dump the fuel before landing? Surely it’s better to be safe and have more than enough fuel for the entire journey therefore not use up a full tank just before landing?
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To reduce weight for landing typically. For long flights especially they take off heavier than they can safely land in weight.
In emergencies they’ll dump fuel to ensure their weight and the fuel aren’t a factor in any outcome.
general theory:
– you want as little weight as possible in the plane when you land, because it’s less mass to brake down the speed of.
– the planes ability to land with a certain amount of fuel may never have been explicitly tested by the manufacturer, to the point where the plane is actually not insured if it lands with that amount of fuel
– bringing down an excess amount of highly flammable liquid during the phase of the flight that is most dangerous is…not really a bright idea, if you boil it down
– weight distribution is unpredictable if you carry a small-ish amount of liquid in a tank that can hold a lot more capacity, and you want the landing to be as predictable as possible.
Also, no. They don’t use up the tank entirely just before landing, just down to a minimum threshold (in minutes) so that they don’t land with more than they absolutely need.
Remember momentum from high school physics? That’s why. A heavier plane means it takes more time to stop with the same braking force as a lighter plane. This can be a problem for larger planes which are built to *just* be able to stop in time when at the usual weight for landing (plus a reasonable window of safety). Planes will often take on extra fuel for safety as well in case the destination has them holding for a while. In certain flight conditions the plane may actually burn less fuel and then the pilot is left with a dilemma of what to do with the extra fuel. This is especially a problem for older and shorter runways which have trouble accommodating large planes in conditions that extend the time it takes them to stop.
What flight was it that did the detour?
If you look up British Airways 207, the flight all have a flight time of 9 hours to 9 hours 40 minutes along reasonable paths, the difference is likely because of theater.
There is two exceptions, the one on Monday was diverted to Orlando and it too just over 10 hours, the change is after it reached Miami.
The last one is today
https://flightaware.com/live/flight/BAW207/history/20230622/0935Z/EGLL/EGLL where the flight diverted back to London a bit after it passed over Ireland. It has a lot if circular flying. The reason is that it did not fly to the destination and the fuel was not used up as usual. Airplanes’ max takeoff weight is larger than the max landing weight.
Normally it is not a problem because the extra weight is the fuel that is used during the flight. The amount of fuel you take a bit simplified what is needed to reach the destination and to that a buffer for a diversion to an alternative landing and a bit of extra time in the air. It will be less then max landing weight at the destination.
You need to dump or waste fuel if the length of the trip significantly decreases. So the plane was fuel because it flew London to London not London to Miami. If it had flown to Miami that would not have been needed.
A little bit of further detail to expand on the answers that are already here… The plane’s computer keeps track of how much weight there is on board, including the plane body itself, passengers, luggage/cargo, and fuel. It uses that weight and the length of the runway it will be landing on (plus some other factors) to determine if it will have enough of a margin of error to safely stop the plane without going off the end of the runway or setting the brakes on fire, this is called the Maximum Landing Weight (MLW).
The plane must be under this weight when landing, otherwise it is considered an overweight landing which comes with a hefty dose of bureaucracy, required inspection, etc etc. Pilots don’t want to do that.
I find it a bit puzzling that such a long flight would have to ‘dump fuel’ (commercial airlines, to the best of my knowledge, do not have fuel-dump capabilities). The pilots would be well-aware of the fuel situation long before they start descent. Excessive fuel could be burned by making the descent and approach less effective, i.e. a “non-idle descent”. It is not hard for a big heavy plane to use huge amounts of fuel in a short time, even without altering the flight plan. Worst case, they’d ask to be put in a hold in the STAR near the destination airport at low altitude where the air is thicker, thus burning even more fuel to keep the speed.
When a plane lands, it bumps into the ground on its undercarriage quite heavily, and then has to be brought to a stop by the brakes (and reverse thrust) in the length of runway available. Each of these can only deal with an aircraft up to a certain weight. If the aircraft is heavier than that, it can not land safely.
When a plane takes off, its engines have to accelerate it down the runway up to a speed where the wings generate enough lift for it to become airborne, within the length of runway available. There is a maximum weight that a plane can take off with.
Maximum takeoff weight is generally larger than maximum landing weight. For a long distance flight, the fuel needed for the flight will be a lot, so generally the plane will be loaded with the fuel it needs for the flight, the passengers it needs, and any leftover allowance on takeoff weight will be filled with airfreight. For a long flight, it will be below the maximum for landing once it has used up its fuel.
If a plane that had planned for a long flight gets into trouble and needs to land somewhere soon after takeoff, if it hasn’t used up enough fuel, it may be above its maximum safe landing weight. If the problem isn’t super urgent, then just flying around for a bit to burn off fuel until the maximum safe landing weight is reached can be a sensible strategy.
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