When calculating the maximum permissible mass for takeoff, several critical limits must be considered. Each of these limits ensures the safety and performance of the aircraft under various conditions.
- Field Limit Mass
- Climb Limit Mass
- Obstacle Limit Mass
- Tyre Speed Limit Mass
- Brake Energy Limitations
- Runway Strength Limitations
- Maximum Structural Mass
Field Limit Mass
Field Limit Mass is the maximum mass at which an aircraft can safely take off from a specific airfield, meeting all necessary field length requirements. This limit is determined by evaluating the Takeoff Run Available (TORA), Takeoff Distance Available (TODA), and Accelerate-Stop Distance Available (ASDA). Most of the times Balanced Field Length is used for Field Limit Mass calculation.
Climb Limit Mass
Climb Limit Mass, also known as the Weight Altitude Temperature (WAT) or Mass Altitude Temperature (MAT) limit, is the maximum weight at which an aircraft can achieve a specified minimum climb performance. Climb performance is defined in terms of climb gradient. Refer undermentioned link to understand climb limit mass considerations in detail.
Climb Limit Mass – Climb Gradient Requirements
Obstacle Limit Mass
Obstacle Limit Mass is the maximum weight at which an aircraft can safely clear all obstacles in its flight path during takeoff, maintaining a required vertical or horizontal margin. Refer undermentioned link to understand climb limit mass considerations in detail.
Obstacle Limit Mass Calculations
Tyre Speed Limit Mass
As the aircraft’s tyres rotate during takeoff and landing, the friction between the tyres and the runway generates heat. Excessive heat can cause the tyres to disintegrate and may also expand the air inside the tyres, potentially leading to over-pressurization and a tyre blowout. Although fusible plugs are installed to mitigate this risk, there is still a maximum ground speed and mass that the tyres can safely handle.
The maximum ground speed the tyres will experience occurs at the lift-off speed (Vlof). Tyre speed limits are therefore designed to be greater than or equal to the highest Vlof that the aircraft will encounter. For most medium-range jets, the maximum tyre speed limit is set at 195 knots.
Brake Energy Limitations (Vmbe)
When brakes are applied during an aircraft’s takeoff or landing, the kinetic energy of the aircraft is converted into heat energy through the brakes. The faster the aircraft is moving, the higher its kinetic energy, resulting in more heat being generated when the brakes are applied. The brakes have a finite capacity to absorb and dissipate this heat. If this capacity is exceeded, the brakes can overheat, potentially leading to brake failure or damage. Vmbe is the maximum speed at which the aircraft can be safely stopped using the brakes without exceeding their energy capacity. It is essential that Vmbe is not less than V1 to ensure that the aircraft can be safely stopped if the takeoff is aborted at V1.
If Vmbe is less than V1, the aircraft’s mass must be reduced until Vmbe equals V1. The takeoff mass at which this condition is met is known as the Brake Energy Limit Mass.
Runway Strength Limitation (ACN/PCN)
The bearing strength of an airport pavement is quantified using the Pavement Classification Number (PCN), which is compared to the Aircraft Classification Number (ACN) to determine if a given aircraft can safely operate on that pavement. The PCN represents the bearing strength of the pavement and includes a safety factor to ensure its durability and integrity under various loads. The ACN is a value that represents the load imposed by an aircraft on the pavement, calculated based on the aircraft’s weight, configuration, and the type of pavement.
An aircraft can operate on a pavement if its ACN is less than or equal to the PCN, ensuring that the pavement can support the aircraft without sustaining damage. Due to the safety factor included in the PCN, a 10% increase of the ACN over the PCN is generally acceptable for pavements in good condition. Occasionally, use by aircraft with an ACN up to 50% greater than the PCN may be permitted, but this requires close monitoring to detect any potential damage to the pavement and the aircraft.
Maximum Structural Mass
The maximum structural mass of an aircraft is the highest weight at which the aircraft can safely operate. This value is obtained after considering Maximum Takeoff Weight (MTOW), Maximum Landing Weight (MLW), and Maximum Zero Fuel Weight (MZFW).
