If you're about to start your first job as an airline pilot, maximum landing weight probably isn't something you've dealt with. Managing MLW is critically important and here's how it works.
Why Does A Maximum Landing Weight Exist?
Until now, you might've been flying in a plane where maximum takeoff weight (MTOW) matches maximum landing weight (MLW). If you've flown the Cessna 172, you know that both the maximum takeoff and landing weights are 2,400 lbs. On the Boeing 777-300ER, the maximum takeoff weight is 775,000 lbs. The maximum landing weight is 554,000 lbs... A difference of 221,000 lbs! Even with a total burn of roughly 16,500 pounds per hour (both engines), it would take over 13 hours to burn off the difference between MTOW and MLW. Factor in bad fuel planning or an early diversion, and you have a serious problem to deal with.
The higher the landing weight, the heavier the landing gear and the supporting structures must be. Large aircraft are designed to burn huge amounts of fuel over vast distances, not do pattern work like piston aircraft. According to Boeing, transport aircraft certification criteria require that landing gear design be based on a sink rate of 10 feet per second at the maximum designed landing weight AND a sink rate of 6 feet per second at the maximum designed takeoff weight. Keep in mind, typical sink rates at touchdown are about 2 to 3 feet per second, and even "hard" landings rarely exceed 6 feet per second.
A Tight Balance: Fuel, Passengers, Cargo, Alternates, Distance, and More
If you're anything like me you've mindlessly "topped off" a light airplane or two on a cross-country. In the airline world, this doesn't exist and airlines carefully plan fuel loading to perfectly meet legal requirements, economic goals, and aircraft limitations. Simply put, it costs money to carry heavy jet fuel in the wings, so airlines err on the side of cost-savings by balancing minimum fuel requirements with safety.
It can feel like threading the needle to balance maximum takeoff weight and maximum landing weight with a full load of passengers and IFR alternate requirements. It's not uncommon for pilots to see that their flight has been planned by dispatch to land AT the maximum landing weight of the airplane. Sometimes, payload (passengers and cargo) must be taken off before departure to meet the fuel requirements of a flight while honoring weight limitations.
Over-Fueling Example: Lansing to Chicago
When I was flying for a regional airline, I experienced a flight that perfectly illustrates how limiting max landing weight can be. We were planned to land around MLW and our tanks were filled to maximum capacity due to enroute deviations for thunderstorms, holding, and alternate requirements. Even for a short route from Lansing to Chicago, filling up the tanks was required for our route. Unfortunately, the flight was canceled when ATC delays finally exceeded our duty period limits.
The next day was much nicer with clear skies. The same airplane sat at the gate waiting for us to take a full load of passengers to Chicago. It was still filled to the top with jet fuel and unfortunately, airport operations was unable to de-fuel the airplane. The new required fuel load was about half of the onboarded fuel. In order to land below maximum landing weight in Chicago, either 25 passengers would not be able to board due to a weight restriction, or the route could be planned a few hundred miles longer... Just to burn off the excess weight.
Dispatch chose to plan a long 2.5 hour long flight to Chicago for what normally would take 45 minutes. We carefully monitored the fuel progress page, and immediately requested an expedited turn to Chicago once we knew we'd land under MLW. This is the power of maximum landing weight in an extremely unusual situation.
Most Restrictive: Maintaining Legal Fuel Requirements
Maximum landing weight may become your most limiting factor when you're tight on maintaining legal fuel requirements. If you need a destination alternate, or two, you're more likely to land close to MLW if the plane is even close to fully loaded with passengers.
Unfortunately, this required fuel usually can't be reduced and this may cause the flight to be weight restricted. In these cases, fuel takes priority over payload, so people or cargo are removed to "make room" for the excess weight of fuel.
Less Restrictive: Burning Excess Fuel To Carry More Payload
Extra fuel always sounds great, right?... Not so fast. Too much fuel can prevent you from carrying a maximum payload, meaning lost income. Dispatch and the flight crew will work in partnership to determine how much extra everyone is comfortable with, and if over that number whether they will de-fuel or purposefully burn more fuel enroute to ensure a landing below MLW.
In this case, the flight may be planned at a lower altitude in order to burn excess fuel while maximizing payload. It's not what we want for the environment, but it works.
Predicted Overweight Landing - What Can Pilots Do Enroute?
If you're enroute and notice that you will be significantly overweight on arrival, there are a few things you can do. The easiest choice is requesting a lower altitude to increase the fuel burn. You can also request vectors, holding, or increase speed to increase fuel burn.
Some large aircraft have the ability to jettison fuel, but this capability is seldomly used.
Last Resort... Add Drag or Configure for Landing
You've started your final descent into busy airspace. Holding and vectors aren't a reasonable option and you're concerned you may land overweight. As you begin to slow below aircraft limitations, you can start increasing drag by configuring for landing with flaps, speed brakes, and landing gear. All of these choices incresae drag, requiring higher engine power and significantly increasing fuel burn.
It's not the most comfortable option for passengers due to noise and vibration, but it gets the job done!
What else do you want to learn as you become a professional pilot? Tell us in the comments below!
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