Thursday 20 July 2023

Thrust limited or buffet limited at high altitude,

When is an aircraft thrust limited or buffet limited at high altitude, and how do we know?

When cruising at high altitude, the aircraft gets closer to the edge of its operating envelope, and as a result, it becomes more limited in how it can perform.

Generally speaking, at high altitude, the aircraft will either be buffet limited or thrust limited. Buffet is caused by the separalion of airflow across an aircraft's wings and manifests itself as a high frequency vibration or "rumble" that can be felt through the airframe.

There are generally two types of buffet: 
High-speed and low speed.

High-speed buffet is caused by supersonic airflow over parts of the wing. When the supersonic air slows to subsonic speeds, a shock wave is formed. This shock wave causes airflow separation, which leads to buffet.

Buffet margins become more limiting when the airplane is in a turn. This is because more lift is required during a turn than would be required in straight and level flight.

More lift = a higher angle of attack and therefore a faster flow of air on the top surface of the wing, which can result in reaching the high speed buffet earlier.

Since the load factor is directly proportional to bank angle, ie more bank angle = more g, load factor defines the maneuvering capability of the airplane.

The buffet limited maximum altitude in the Boeing 737, is the maximum altitude at which an aircraft can manoeuvre to 1.3g without it experiencing a specified level of buffet.

This translates to 40 degrees of bank in level flight.
The Thrust limited altitude is determined by the amount of thrust being produced by the engines, combined with the aircraft weight, speed and outside air temperature.

When the aircraft reaches an altitude where it would be unable to achieve a 100 feet per minute residual rate of climb (sometimes set at 300fpm in the FMC) it is said to be thrust

This is indicated to the pilot by either a "T" or "B" next to the maximum altitude displayed in the FMC.
B = buffet limited, T = thrust limited.

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