# Part 6. Altimetry. Part 6. Altimetry TOPICS Pressure, Humidity & Temperature ISA and the Aircraft Altimeter 4 Pressure, Humidity & Temperature 4 ISA.

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Part 6. Altimetry

TOPICS Pressure, Humidity & Temperature ISA and the Aircraft Altimeter
4 Pressure, Humidity & Temperature 4 ISA and the Aircraft Altimeter 4 Height, Pressure, and the Aircraft Altimeter 4 Temperature and the Aircraft Altimeter 4 Altimeter Settings and Terminology

Pressure decreases with increasing height.
PRESSURE, HUMIDITY AND TEMPERATURE The study of pressure variation within the atmosphere is called ALTIMETRY. Pressure decreases with increasing height. Not only does the pressure decrease at altitude, but the density of the atmosphere does too. However, there are a number of other factors that affect density -

PRESSURE, HUMIDITY AND TEMPERATURE
The greater the pressure, the greater the density. This is because, as you increase the pressure of a gas, the molecules are squashed together within the gas and it’s weight for a given volume must also increase.

PRESSURE, HUMIDITY AND TEMPERATURE
Water vapour is less dense than dry air because the molecules are further apart. However, it combines readily with dry air so the higher the water vapour content of the air the lower the overall density.

PRESSURE, HUMIDITY AND TEMPERATURE
The lower the temperature the greater the density because the atoms take up less space as temperature is reduced.

PRESSURE, HUMIDITY AND TEMPERATURE
A column of cold air will weigh more than an identical column of warm air and the pressure at the bottom of the cold column will be higher. Similarly, if the pressure at the bottom of the warm column was the same as the cold column they would have to weigh the same and the warm column must be taller to achieve this. That being the case, the pressures would also be the same at the top of each column.

London and Bath have different atmospheric conditions
although the surface pressures are the same at 1000 hPa. Cold Air Warm London Bath 700mb (hPa) 1000mb (hPa) 9 500ft 10 000ft

INTERNATIONAL STANDARD ATMOSPHERE
This standard atmosphere, which has been internationally agreed, is a set of average values which are utilised for the calibration of aircraft altimeters, the cockpit instrument that indicates height by sampling the static (undisturbed air) pressure. Thus in theory, all aircraft altimeters should react in exactly the same manner to any change in air conditions.

INTERNATIONAL STANDARD ATMOSPHERE
The ICAO defined values are - Mean Sea Level Temperature °C MSL pressure hPa/mb (29.92 ins) MSL density gm cu m Lapse rate temp decreasing at 1.98°C/1000ft up to 11kms (36 090ft) - remaining at -56.5°C thereafter up to 20kms (65 617ft) - increasing at 0.3°C/1000ft thereafter up to 32kms ( ft)

AIRCRAFT ALTIMETER The principle of the aircraft altimeter is exactly
the same as that of the aneroid barometer. An evacuated capsule reacts to changes in air pressure and these changes are transmitted to a pointer on a dial that is suitably calibrated in feet or metres.

Altimeters are fitted with a digital subscale, that is
HEIGHT AND PRESSURE Altimeters are fitted with a digital subscale, that is set by a rotating knob to indicate the pressure datum above which the altimeter is operating. This is necessary because air pressure does not remain constant at any place and varies from hour to hour.

A B HEIGHT AND PRESSURE 1020hPa 1000hPa 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6
1020hPa 1 2 3 4 5 6 7 8 9 1020 1000hPa A B

HEIGHT AND PRESSURE 980 hPa level Flight path 600ft 300ft True 990 hPa
Sub scale setting level 1000 hPa 1000hPa 990 hPa 300ft True 600ft 980 hPa level 600ft Indicated SURFACE Flight path

TEMPERATURE VARIATION
MET02/20 TEMPERATURE VARIATION Cold air is denser than warm air. Consider three columns of air with identical pressures at MSL, if the temperatures of the columns are different then the height at which the pressure has fallen to a specified level will also be different. COLDER THAN ISA 10 000ft ISA WARMER THAN ISA 697hPA

ALTIMETER SETTINGS AND TERMINOLOGY
The altimeter subscale setting depends upon the phase of flight that the aircraft is undergoing. When operating at, or near an airfield the subscale setting may be set on either the pressure at the official aerodrome elevation, which is known as QFE, or at MSL which is known as QNH. When flying at higher levels all aircraft set the ISA pressure of hPa because their vertical separation from each other is more important than their separation from the ground.

ALTIMETER SETTINGS AND TERMINOLOGY
flight level height altitude QFE 1013.2 elevation QNH MSL 1013.2

QFE “Atmospheric pressure at official aerodrome level. When set on the
subscale of a pressure altimeter it will read zero when the aircraft is on the ground at the station.”

QNH “Atmospheric pressure at mean sea level.
When set on the subscale of a pressure altimeter it will read aerodrome elevation when the aircraft is on the ground at the station.”

HEIGHT “The vertical distance of a level, point
or object considered as a point measured from a specified datum.”

ALTITUDE “The vertical distance of a level, point
or object considered as a point measured from mean sea level.”

ELEVATION “The vertical distance of a point or level,
on or affixed to the surface of the earth, measured from mean sea level.” NOTE - Aerodrome elevation is the elevation of the highest point on the landing area. A separate threshold elevation is published if it is 7ft or more BELOW aerodrome elevation and for precision approach runways.

FLIGHT LEVEL “A level of constant atmospheric
pressure above a datum of hPa and separated from other levels by specific pressure intervals.”

CHANGES OF REFERENCE Flight Levels Transition Level Transition Layer
Altitude Level 1013.2 MSL QFE QNH Transition Level Transition Altitude

TRANSITION ALTITUDE “The altitude at or below which the
vertical position of an aircraft is controlled by reference to altitudes. The transition altitude is located at a fixed level and published in aeronautical information publications.”

TRANSITION LEVEL “The lowest flight level available for
use above the transition altitude.”

TRANSITION LAYER “The airspace between the transition
altitude and the transition level.” NOTE - The actual depth of the transition layer varies as the pressure at MSL changes and vertical separation of 1000ft does not always exist between the transition altitude and the transition level.

The next available flight level above 3540ft is FL40 - The TL is FL40
EXAMPLE 1 If the Transition Altitude is 2000ft, the QNH is 995 hPa and the Transition Layer at least 1000ft in depth, what is the Transition Level ? (Assume 1hPa = 30ft) 540ft 2000ft 1000ft Transition Layer Transition Level Transition Altitude Mean Sea Level 995hPa 1013hPa = 3540 Therefore the top of the TL is 3540ft above a pressure datum of 1013hPa The next available flight level above 3540ft is FL40 - The TL is FL40

The next available flight level above 4150ft is FL45 - The TL is FL45
EXAMPLE 2 If the Transition Altitude is 3000ft, the QNH is 1008 hPa and the Transition Layer at least 1000ft in depth, what is the Transition Level ? (Assume 1hPa = 30ft) 150ft 3000ft 1000ft Transition Layer Transition Level Transition Altitude Mean Sea Level 1008hPa 1013hPa = 4150 Therefore the top of the TL is 4150ft above a pressure datum of 1013hPa The next available flight level above 4150ft is FL45 - The TL is FL45

If aircraft A is flying at FL55 and aircraft B is operating at 4700ft
EXAMPLE 1 If aircraft A is flying at FL55 and aircraft B is operating at 4700ft on the QNH of 1004hPa, what is their vertical separation ? (Assume 1hPa = 30ft) MET03/20 Z 4700ft Mean Sea Level 1004hPa 1013hPa A B 5500ft Calculate distance Z  = x 30 = 270 Therefore Z = 270ft Aircraft A is at = 5230ft above a pressure datum of 1004hPa Therefore the vertical separation between the aircraft is = 530ft

If aircraft A is flying at FL55 and aircraft B is operating at 4700ft
EXAMPLE 2 If aircraft A is flying at FL55 and aircraft B is operating at 4700ft on the QNH of 1024hPa, what is their vertical separation ? (Assume 1hPa = 30ft) Z 4700ft Mean Sea Level 1024hPa 1013hPa A B 5500ft Calculate distance Z  = x 30 = 330 Therefore Z = 330ft Aircraft A is at = 5830ft above a pressure datum of 1024hPa Therefore the vertical separation between the aircraft is = 1130ft

Questions…

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