INTRODUCTION Getting to Grips with Aircraft Performance

10

• Operations: The Flight Crew Operating Manual (FCOM) can be viewed as the AOM (aircraft-related portion of the Operations Manual), which contains all the necessary limitations, procedures and performance data for aircraft operation.

The following table (Table 1) illustrates the large aircraft regulatory basis:

All aircraft of the Airbus family are JAR 25 and/or FAR 25 certified. On the other hand, compliance with the operating rules remains under the airline’s responsibility.

This brochure is designed to address three different aspects of aircraft performance:

• The physical aspect : This brochure provides reminders on flight mechanics, aerodynamics, altimetry, influence of external parameters on aircraft performance, flight optimization concepts…

• The regulatory aspect : Description of the main JAR and FAR certification and operating rules, leading to the establishment of limitations. For a clear understanding, regulatory articles are quoted to assist in clarifying a given subject. In such cases, the text is written in italics and the article references are clearly indicated to the reader.

• The operational aspect : Description of operational methods, aircraft computer logics, operational procedures, pilot’s actions…

11

A. GENERAL

1. THE INTERNATIONAL STANDARD ATMOSPHERE (ISA)

1.1. Standard Atmosphere Modeling

The atmosphere is a gaseous envelope surrounding the earth. Its characteristics are different throughout the world. For this reason, it is necessary to adopt an average set of conditions called the International Standard Atmosphere (ISA).

1.1.1. Temperature Modeling

The following diagram (Figure A1) illustrates the temperature variations in the standard atmosphere:

Figure A1: ISA temperature

The international reference is based on a sea-level temperature of 15°C at a pressure of 1013.25 hPa1. The standard density of the air at sea level is 1.225 kg/m3.

1 1013.25 hPa is equal to 29.92 in Hg, ‘hPa’ meaning hectoPascal and ‘in Hg’ inches of mercury.

12

Temperature decreases with altitude at a constant rate of -6.5°C/1000m or -1.98°C/1000ft up to the tropopause. The standard tropopause altitude is 11,000 m or 36,089 feet.

From the tropopause upward, the temperature remains at a constant value of -56.5°C.

Therefore, the air which is considered as a perfect gas in the ISA model presents the following characteristics:

• At Mean Sea Level (MSL):

ISA temperature = T0 = +15°C = 288.15 K

• Above MSL and below the tropopause (36,089 feet):

ISA temperature (ºC) = T0 - 1.98 x [Alt(feet)/1000]

For a quick determination of the standard temperature at a given altitude, the following approximate formula can be used:

ISA temperature (ºC) = 15 - 2 x [Alt(feet)/1000]

• Above the tropopause (36,089 feet):

ISA temperature = -56.5ºC = 216.65 K

This ISA model is used as a reference to compare real atmospheric conditions and the corresponding engine/aircraft performance. The atmospheric conditions will therefore be expressed as ISA +/- ΔISA at a given flight level.

Example:

Let’s consider a flight in the following conditions:

Altitude = 33,000 feet

Actual Temperature = -41ºC

The standard temperature at 33,000 feet is : ISA = 15 - 2 x 33 = -51ºC, whereas the actual temperature is -41ºC, i.e. 10ºC above the standard.

Conclusion: The flight is operated in ISA+10 conditions

13

1.1.2. Pressure Modeling

To calculate the standard pressure P at a given altitude, the following assumptions are made:

• Temperature is standard, versus altitude.

• Air is a perfect gas.

The altitude obtained from the measurement of the pressure is called pressure altitude (PA), and a standard (ISA) table can be set up (table A1).

Figure A2: Pressure Altitude function of Pressure

Table A1: Example of Tabulated Pressure Altitude Values

PA

PA = f(P)

GENERAL Getting to Grips with Aircraft Performance

14

Assuming a volume of air in static equilibrium, the aerostatic equation gives:

dP = - ρgdh

With ρ = air density at an altitude h

g= gravity acceleration (9.80665 m/s2)

dh = height of the volume unit

dP = pressure variation on dh

The perfect gas equation gives:

With R = universal gas constant (287.053 J/kg/K)

Consequently:

• At Mean Sea Level (MSL):