Maximum Take-Off Weight
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In aviation, the Maximum Take-Off Weight (or MTOW) is the maximum weight with which an aircraft is allowed to try to achieve flight. It is one of the three main "limiting design weights" with regards to aircraft performance, the others being the Maximum Zero-Fuel Weight and the Maximum Landing Weight. In some countries, especially those in Europe, Maximum Take-Off Weight is referred to as "Maximum Take-Off Mass."
[edit] Principle
The purpose of defining a safe MTOW is that the lift generated by the wings upon take-off must exceed the force exerted by gravity on the plane in order for the plane to lift off from the ground. Wings can only generate a limited amount of lift, and so the weight of the plane must be capped so that the lift will always be sufficient.
An aircraft's rated MTOW and other performance ratings are based on the International Civil Aviation Organization's International Standard Atmosphere (or ISA). These state that an aircraft must be able to continue take-off after it reaches the V1 speed, even if one engine has failed. MTOW calculations are all-inclusive, representing the sum of the weights of the airframe, fuel, cargo, and personnel.
Before a flight, the actual take-off weight of the plane is calculated. This is so the pilot knows how fast the plane must be travelling in order to generate enough lift to take off i.e. the V speeds, and to ensure the weight of the aircraft is within limits.
[edit] Limiting Factors
MTOW is affected by several factors, including the following:
- Airframe design - The physical mass of the plane and its aerodynamic characteristics.
- Type and power of engine - The amount of lift that a wing can generate is affected by the speed of the air moving over it. A more powerful engine will be able to generate more speed, or accelerate more quickly.
- The air pressure - A higher air pressure difference between upper and lower wing surface will allow the wing to generate more lift.
These factors (and more) determine an aircraft's Maximum Authorised Take-Off Weight. This however does not take into account the conditions at the time of take-off, which may include:
- Airfield altitude (distance above sea-level) - This affects air pressure.
- Air temperature - This affects air pressure.
- Length of Runway - A short runway means the aircraft has less time to accumulate speed (especially with only one engine).
- Condition of Runway - A runway covered in snow or in poor condition will have more friction and make it harder for the plane to gather speed. See the Munich Air Disaster
- Obstacles - For example, a plane must take off in time to gain enough height to clear a hill at the end of a runway.
If any of these factors is severe enough, a Regulated Maximum Take-Off Weight, or RTOW will be imposed.
[edit] Regulation
The same model of aircraft can have different MTOWs. An airline can choose to have its airliner certified for an increased weight at an additional cost. Some airlines who do not require a high MTOW choose to have a lower MTOW for that particular aircraft and can save money.
