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You climb with a climb speed schedule 300/0.78M. What do you expect in the crossover altitude 29 200 ft (OAT = ISA)?
  • A
    The rate of climb increases since the constant IAS-climb is replaced by the constant Mach-climb.
  • B
    No noticeable effect since the true airspeed at 300 kt IAS and 0.78 Mach are the same (at ISA temperature TAS = 460 kt).
  • C
    During the acceleration to the Mach number 0.78 the rate of climb is approximately zero.
  • D
    The rate of climb decreases since climb performance at a constant Mach number is grossly reduced as compared to constant IAS.

Refer to figures.

The Crossover Altitude is the altitude where a specific Indicated Airspeed (IAS) becomes a specific Mach number (M). It is the altitude where a specified IAS and Mach number represent the same TAS.

Above the Crossover Altitude, the aircraft climbs maintaining a constant Mach number. At the Crossover Altitude, there is one value of TAS which corresponds to the given Mach number - in this case 460 kt TAS.
During the initial climb (below the Crossover Altitude), when maintaining a constant IAS, we are "chasing" the targeted TAS (in this case, 460 kt) by gradually pitching down, as the IAS decreases with an increase in altitude. Once this value is reached, further increase in TAS with altitude must be stopped by pitching up slightly, leading to a short increase in Rate of Climb. However, afterwards, the ROC will decrease again due to a lower TAS and reduced excess of Thrust.

Below and above the Crossover Altitude, Rate of Climb decreases. Bottom line is, the question only refers to the crossover altitude itself.

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