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Pilots might find themselves needing to deliberately burn off fuel before landing, and, though the practice may seem out of kilter with the industry’s push towards more efficient flight, it is actually a routine procedure. In fact, intentionally burning off fuel can be critical to ensuring a safe flight.
One might wonder why this is, and the answer is realistically very simple: Weight. That is, any model of aircraft, landing anywhere around the world, is bound by its weight. Therefore, ensuring an aircraft is not too heavy before touching down is a vital factor pilots must consider, and can more often than not make for an explanation as to why excess fuel has been burned off on purpose beforehand.
The Importance Of Weight And Balance
As mentioned, when and why pilots intentionally burn off fuel before landing almost always comes down to weight. Considerations around weight are, naturally, vital at all stages of a flight and go hand in hand with the balance of an aircraft. Both are obvious but key factors behind event-free flying, whether that be for a pilot in a single-propeller aircraft, like a Cessna Skyhawk, or a huge passenger jetliner, such as an Airbus A380.
Weight specifically has to be capped based on the limitations of a particular aircraft model in order to protect its structural integrity and ensure its performance. As the Federal Aviation Administration (FAA) writes in its Pilot’s Handbook of Aeronautical Knowledge: “Excessive weight reduces the flight performance in almost every respect.” Simply put, an overweight aircraft will need to be faster and have more room upon taking off; will suffer from reduced range, cruising speed, and maneuverability; and will land faster and harder, in turn needing more runway to roll. Such reasons are just to name a few, mind, but offer a glimpse into why weight is so important to flight crew.
Balance is then intrinsically linked to weight, of course, and determines stability. Incorrectly loading an aircraft can leave a pilot having to adjust trim or even holding constant pressure on controls to account for a displaced center of gravity, for instance. Needless to say, neither scenario is good. Back to the point at hand, and getting the numbers right on weight is key. This is arguably among its most important upon coming into land.
Maximum Landing Weight
Every model of aircraft has what is known as a maximum landing weight (MLW). Alongside maximum takeoff weight (MTOW) and maximum zero fuel weight (MZFW), this is one of three key aircraft weight classifications that flight crew are required to know about their hardware. As one might have guessed, the former dictates how heavy an aircraft is authorized to be in order to carry out a normal landing.
Generally, aircraft have a higher MTOW than their MLW, and this difference is often significant. The reason is due to fuel burned over the duration of a flight, which usually is the only reason an aircraft’s weight will change while it is in the air — at least, this is the case on passenger services. MLW itself is then determined by the strength of an aircraft’s landing gear, wing structure, and fuselage.
|
Aircraft |
MTOW |
MLW |
|---|---|---|
|
Airbus A319 |
141,000 lbs – 172,000 lbs (64 t – 78 t) |
134,000 lbs – 141,000 lbs (61 t – 64 t) |
|
Airbus A320 |
154,000 lbs – 174,000 lbs (70 t – 79 t) |
141,000 lbs – 150,000 lbs (64 t – 68 t) |
|
Airbus A321 |
176,000 lbs – 214,000 lbs (80 t – 97 t) |
157,000 lbs – 174,000 lbs (71 t – 79 t) |
|
Airbus A350-900 |
463,000 lbs – 624,000 lbs (210 t – 283 t) |
452,000 lbs – 456,000 lbs (205 t – 207 t) |
|
Airbus A350-1000 |
595,000 lbs – 710,000 lbs (270 t – 322 t) |
514,000 lbs – 520,000 lbs (233 t – 236 t) |
|
Airbus A380 |
1,058,000 lbs – 1,268,000 lbs (480 t – 575 t) |
851,000 lbs – 871,000 lbs (386 t – 395 t) |
*Figures via Airbus
Manufacturers, like Airbus and 
Boeing, lay out the maximum weight parameters for each of their aircraft depending on such structural factors. Albeit simplified from pages and pages of documents, the table above provides examples from Airbus of the rough MTOW against MLW of a range of aircraft in the manufacturer’s portfolio. Note, the difference between the two figures tends to be far greater on larger jets, thanks to their more significant fuel requirements.
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How Excess Fuel Is Burned
Though such parameters are an important tool for safety, aircraft are not always packed to hit their MTOW. Take a Boeing 777-300ER, with a MTOW of 775,000 lbs (351.5 t) and 554,000 lbs (251.3 t) MLW, as used in an example by flight training firm Boldmethod. A huge 221,000 lbs (100.2 t) difference in this case would require fuel to be burned at roughly 16,500 lbs (7.5 t) an hour for over 13 hours. All this goes to say, sometimes filling up completely is just not necessary. It can also pose the risk of a dilemma when the time comes to land. This exact case of an aircraft carrying too much fuel late on in a flight is not uncommon, though, so what happens then?
One of the main ways in which pilots will deliberately burn off fuel is by simply extending the time their aircraft is in the air. In practice, this usually means circling, or holding, near the destination airport until enough fuel has been burned to allow for a landing within the parameters of an aircraft’s MLW. Moving to lower altitudes is one way to speed up this process, given that engines burn fuel more quickly in denser air, but it can be a slow process and requires authorization in often busy airspace, so is not always practical.
In cases where holding is not possible, pilots do have a somewhat last resort to avoid an overweight landing. That is, increasing drag as early as possible during their descent into an airport, thereby increasing the engine power and fuel needed for the final stages of the flight. Deploying the landing gear, speed brakes, and flaps early can ensure this. That said, doing so makes for a loud and bumpy experience, so is far from ideal from a passenger’s perspective.
Fuel Might Also Sometimes Be Dumped
Some pilots do hold another trick up their sleeves: Fuel dumping. This is largely only an option for those of larger, long-range widebody aircraft, given the greater difference in MTOW and MLW compared to smaller, narrowbody jets.
For this, certain aircraft must have specialist equipment to allow excess fuel to be expelled. These appear as nozzles near an aircraft’s wingtips or trailing edges, which are often managed automatically in order to allow for a target weight to be more easily reached. Such equipment, when available, provides a way of rapidly cutting the weight of an aircraft in time for landing. However, this option comes with its risks and mainly makes for an emergency-only procedure.
Select aircraft capable of fuel dumping:
|
Airbus |
Boeing |
|
Airbus A330 |
Boeing 747 |
|
Airbus A340 |
Boeing 767 |
|
Airbus A350 |
Boeing 777 |
|
Airbus A380 |
Boeing 787 |
Regulations around fuel dumping are strict, given that both other aircraft and the ground below need to be considered, alongside ecological and health issues. Though the speed at which fuel is ejected from an aircraft usually means it will break up quickly and evaporate before even reaching the ground, fuel dumping below 5,000 to 6,000 feet (1,524 to 1,829 meters) may mean this does not happen as desired, per manufacturer BMT Aerospace.
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Overweight Landings Do Happen
Despite the options of burning or ditching excess fuel, sometimes these are not enough to ensure landing in a timely manner or are simply not available. Of course, sitting in a holding pattern to cut down on fuel takes time, and in an emergency may be out of the question, while the aircraft in question may also be incapable of dumping its fuel. As such, overweight landings do happen and can be necessary in certain conditions.
As aviation safety site SKYbrary puts it, “Generally, maximum landing weight is an operational limitation to be complied with. However, a deviation from this rule is possible in the interest of safety.”
Examples of situations that could warrant an overweight landing include “a malfunction rendering the aircraft unairworthy […] A situation where an expeditious landing would reduce the exposure to a hazard [or] a situation requiring immediate medical attention,” according to the site. While other exceptions might appear, why an overweight landing has been made can often be explained by a need for quick action in an emergency.
Risks Of Overweight Landing
Overweight landings come with their risks, though, namely to the aircraft itself, but also to passengers and crew onboard. For the former, an overweight landing might result in what is known as a “high load event”. Here, aircraft are subject to load conditions outside operational thresholds or limits, and such events on the ground are most commonly seen in hard or overweight landings.
Aircraft are designed to withstand the likes of hard landings over and above their MLW, but, reassuringly, do not need to most of the time. In this instance, the option on a pilot’s part to burn off excess fuel, or even dump it in extreme circumstances, allows for planes to safely make their way to the ground, without passengers perhaps even knowing the issue was there in the first place most of the time.
