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2.3.1. Altimeter Setting Correction

In case of ISA temperature conditions, and a standard altimetric setting, the aircraft true altitude can be obtained from the indicated altitude provided the local QNH is known.

True altitude = Indicated altitude + 28 x (QNH [hPa] - 1013)

2.3.2. Temperature Correction

Flying at a given indicated altitude, the true altitude increases with the temperature (Figure A7). The relationship between true altitude and indicated altitude can be approximated as follows:

ISA T

TA = IA T

TA = True altitude

IA = Indicated altitude

T = Actual temperature (in Kelvin)

TISA = Standard temperature (in Kelvin)

An example is provided in Appendix 1 of this manual.

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Figure A7: Temperature effect on True Altitude, for a constant Indicated Altitude

Conclusion:

If the temperature is higher, you fly higher.

If the temperature is lower, you fly lower.

Temperature correction is important, when flying a departure or arrival procedure in very low temperature conditions. For that purpose, the following table (Table A3) is proposed in the FCOM:

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Table A3: True Altitude Correction versus Temperature

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3. OPERATING SPEEDS

Different speed types are used to operate an aircraft. Some of them enable the crew to manage the flight while maintaining some margins from critical areas, whereas others are mainly used for navigational and performance optimization purposes. This is why the following sections propose a review of the different speed types that are used in aeronautics.

3.1. Calibrated Air Speed (CAS)

The Calibrated Air Speed (CAS) is obtained from the difference between the total pressure (Pt) and the static pressure (Ps). This difference is called dynamic pressure (q). As the dynamic pressure cannot be measured directly, it is obtained thanks to two probes (Figure A8).

Figure A8: Pitot Tube and Static Probes

To obtain the total pressure Pt, airflow is stopped by means of a forwardfacing tube, called the pitot tube (Figure A9), which measures the impact pressure.

This pressure measurement accounts for the ambient pressure (static aspect) at the given flight altitude plus the aircraft motion (dynamic aspect).

The static pressure Ps is measured by means of a series of symmetrical static probes perpendicular to the airflow. This measurement represents the ambient pressure at the given flight altitude (static aspect).

CAS = f (Pt-Ps) = f (q)

Flying at a constant CAS during a climb phase enables the aerodynamic effect to remain the same as at sea level and, consequently, to eliminate speed variations.

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Figure A9: CAS Determination Process

3.2. Indicated Air Speed (IAS)

The Indicated Air Speed (IAS) is the speed indicated by the airspeed indicator.

Whatever the flight conditions, if the pressure measurement were accurate, then the IAS should ideally be equal to the CAS. Nevertheless, depending on the aircraft angle of attack, the flaps configuration, the ground proximity (ground effect or not), the wind direction and other influent parameters, some measurement errors are introduced, mainly on the static pressure. This leads to a small difference between the CAS and the IAS values. This difference is called instrumental correction or antenna error (Ki).

IAS = CAS + Ki

3.3. True Air Speed (TAS)

An aircraft in flight moves in an air mass, which is itself in motion compared to the earth. The True Air Speed (TAS) represents the aircraft speed in a moving reference system linked to this air mass, or simply the aircraft speed in the airflow. It can be obtained from the CAS, using the air density (ρ) and a compressibility correction (K).

T AS = (ρo/ρ ) K CAS

3.4. Ground Speed (GS)

The ground speed (GS) represents the aircraft speed in a fixed ground reference system. It is equal to the TAS corrected for the wind component (Figure A10).

Ground Speed = True Air Speed + Wind Component

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Figure A10: Ground Speed and Drift Angle

3.5. Mach Number

The Mach Number is a comparison between the TAS and the speed of sound.

a

M= TAS

With TAS = True Air Speed

a = The speed of sound at the flight altitude

The speed of sound in knots is:

a(kt) = 39 SAT(K)

With SAT = Static Air Temperature (ambient temperature)in Kelvin

The speed of sound is solely dependent on temperature. Consequently, the Mach number can be expressed as follows:

M =

TAS (kt)

39 273 + SAT(° C)

Flying at a given Mach number in the troposphere: When the pressure altitude increases, the SAT decreases and thus the True Air Speed (TAS). Or :