Part 6. Altimetry

2. Part 6. Altimetry

3. TOPICS 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

4. 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 -

5. PRESSURE, HUMIDITY AND TEMPERATURE PRESSURE 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.

6. PRESSURE, HUMIDITY AND TEMPERATURE HUMIDITY 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.

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

8. 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.

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

10. 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.

11. INTERNATIONAL STANDARD ATMOSPHERE The ICAO defined values are - Mean Sea Level Temperature - + 15°C MSL pressure - 1013.25 hPa/mb (29.92 ins) MSL density - 1225 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 (104 987ft)

12. 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.

13. MET02/16 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.

14. 0 0 9 1 9 1 8 2 8 2 1020 1020 7 3 7 3 6 4 6 4 5 5 1020hPa 1000hPa HEIGHT AND PRESSURE A B

15. 980 hPa level Flight path 980 hPa level 600ft 300ft True 990 hPa SURFACE 1000hPa 600ft Indicated Sub scale setting level 1000 hPa HEIGHT AND PRESSURE

16. 697hPA WARMER THAN ISA 697hPA 10 000ft ISA 697hPA COLDER THAN ISA 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.

17. 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 1013.2 hPa because their vertical separation from each other is more important than their separation from the ground.

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

19. 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.”

20. 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.”

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

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

23. 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.

24. FLIGHT LEVEL “A level of constant atmospheric pressure above a datum of 1013.2hPa and separated from other levels by specific pressure intervals.”

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

26. 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.”

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

28. 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.

29. 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) Transition Level Transition Layer 1000ft Transition Altitude 2000ft Mean Sea Level 995hPa 540ft 1013hPa 1000 + 2000 + 540 = 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

30. 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) Transition Level Transition Layer 1000ft Transition Altitude 3000ft Mean Sea Level 1008hPa 150ft 1013hPa 1000 + 3000 + 150 = 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

31. 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) A B 5500ft 4700ft 1004hPa Mean Sea Level Z 1013hPa Calculate distance Z  1013 - 1004 = 9 9 x 30 = 270 Therefore Z = 270ft Aircraft A is at 5500 - 270 = 5230ft above a pressure datum of 1004hPa Therefore the vertical separation between the aircraft is 5230 - 4700 = 530ft MET03/20

32. 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) A B 5500ft 4700ft 1013hPa Z 1024hPa Mean Sea Level Calculate distance Z  1024 - 1013 =11 11 x 30 = 330 Therefore Z = 330ft Aircraft A is at 5500 + 330 = 5830ft above a pressure datum of 1024hPa Therefore the vertical separation between the aircraft is 5830 - 4700 = 1130ft

33. Questions…