Standard Atmosphere Questions and Answers

Answer: B

In cruise, altitude is usually measured using static pressure.

The barometric altimeter reads the static pressure of the surrounding air and converts it to altitude through the standard atmosphere. All aircraft carry one, and flight levels in cruise are pressure altitudes.

Dynamic pressure is what the airspeed indicator measures, not altitude. Radar altimeters measure height above the ground but are used mainly at low height, on approach and landing, not in normal cruise. Laser range finders are not the standard instrument for aircraft altitude.

Answer: A

Indicated airspeed is used during take-off.

Indicated airspeed reflects the dynamic pressure on the aircraft, which is what sets the aerodynamic forces. So the speeds that matter for take-off, such as the rotation and lift-off speeds and the stall speed, are given in indicated airspeed and stay the same whatever the airfield's altitude or temperature.

Navigation needs true airspeed and ground speed, not indicated airspeed. Engine RPM is set from engine instruments, and the elevator angle is a control input, not something read from an airspeed.

Answer: D

In the isothermal layer, density decreases exponentially with altitude.

Combine hydrostatic balance with the ideal gas law at constant temperature \(T\):

\[ \begin{aligned} dp &= -\rho g\,dh, \quad p = \rho R T \\ \frac{d\rho}{\rho} &= -\frac{g}{RT}\,dh \\ \rho &= \rho_{1}e^{-g(h - h_{1})/RT} \end{aligned} \]

In the ISA this layer runs from 11 km to 20 km at 216.65 K. Density does not stay constant, since the pressure keeps falling; it cannot increase with height; and a linear decrease belongs to neither pressure nor density in this layer. Even in the gradient layer below, the decrease follows a power law, not a straight line.

Answer: C

The FALSE statement is The true airspeed is 250 km/h.

With no instrument or position errors, calibrated airspeed equals indicated airspeed, 250 km/h. Neglecting compressibility, equivalent airspeed equals calibrated airspeed, also 250 km/h. True airspeed is related to equivalent airspeed through the density ratio:

\[ V_{true} = \frac{V_{e}}{\sqrt{\rho/\rho_{0}}} \]

At 5000 m the air density is well below sea-level density (about 60% of it), so the true airspeed is larger than 250 km/h, roughly 320 km/h. That makes 'true airspeed is greater than 250 km/h' true and 'true airspeed is 250 km/h' false.