1. Basic Principle
An altimeter measures air pressure, decreasing with increasing altitude ( barometric formula), and from the surrounding's pressure calculated and displayed the corresponding altitude.

2. Principle Pressure Decreases in increase of Altitude
For every 30’ difference the static pressure decreases by 1 Hecto Pascal upto 5000’
Static pressure at a point is the weight of surrounding air moleculesexerting a force on that point
A Capsule in the Altimeter pressure decreases when the aircraft increases its altitude
A knob provided on the front to allow a pilot to set the current sea-level pressure in the Kollsman window

3. Altimeter

4. Internal Construction4

5. In Detail
It is an aneroid barometer calibrated to indicate altitude instead of barometric pressure.
Actual Barometric pressure at that location subtracted from the current sea-level pressure and
Expressed this difference as an altitude
The pressure Window is called Kollsmans Window. 5

6. ALTIMETER
Altitude indicated = ( 1,000 feet/in Hg ) x (Kollsman -- Actual Barometric pressure 6

7. Transition Level and Transition Altitude
TA-Transition altitude (TA) is the altitude, when climbing, at which Pilot reset his altimeter to the standard pressure setting of 29.92" or 1013 hpa
TL-Transition level (TL) is the flight level (FL) at which, when descending, Pilot reset his altimiter to QNH, or the local altimeter setting.

8. Transition Altitude-define
The transition altitude (TA) is the altitude above sea level at which aircraft change from the use of altitude to the use of flight levels Lowest flight level FL available for use above the transition altitude
Transition Altitudes are given for all airports & indicated in the instrument approach charts

9. QNH & QFE QFE: Height above Air Port
Airport Elevation as Datum Point
For a parked aircraft, QFE =0.For an airport of altitude of 2000’.
QNH:Height above Sea Level-QNH 2000’ : Pressure above the SEA LEVEL & cause the altimeter to read altitude above mean sea level-Unit : millibars 9

10. QFE & QNH-define
QNH defined as, "barometric pressure adjusted to sea level.“and is a Barometric Setting to read the Altitude above the MSL
QFE-defined as the pressure adjusted to Aerodrome level

11. Altimeter setting methods
By setting QFE on the altimeter
By setting QNH on the altimeter
By Transition level setting 11

12. General ALTIMETER SETTING PROCEDURE in India
Altimeter setting procedure using FIS of Chennai, Bombay, Delhi and Calcutta
Transition Altitudes are given for all airports & indicated in the instrument approach charts
ATC provide QNH and Met reports in determining altitude clearances

13. 1.QFE Setting α 1/altitude
On the ground, the altimeter will show 0 (zero)
Atmospheric pressure measured at the airport; higher the airport elevation, the lower the QFE.
Aircraft HEIGHT- expressed in feet in AGL above ground level
QFE value of an airport is set on the altimeter, the pressure altimeter will show the HEIGHT above that airport. 13

14. 1. QFE setting on the altimeter14

15. 2. QNH setting on the altimeter
A QNH altimeter setting be included in the descent clearance
First cleared to an altitude below the transition level in approach clearances or clearances to enter the traffic circuit, and in taxi clearances for departing aircraft, 15

16. 2.By setting QNH given by ATC on the altimeter16

17. 2.QNH setting on the altimeter
, the QNH between airfields across the country vary significantly when the weather is unstable.
Cruising at M.080 with a high performance aircraft, would be changing the altimeter setting to the QNH of each airfield when crossing head every few minutes, from London to Kuala Lumpur
Not practical for cruising at high altitudes and speeds! 17

18. 3.Use of TL & TA settings 18

19. 3.Use Transition level TL and Transition Altitude TA
Standard Altimeter setting 1013,25 hPa or In Hg (Red part on the drawing)When cruising at or above Transition Level (TRL),
QNH selected during descent through Transition Level (TL),
Cruising at or below Transition Altitude (TA), use QNH (Blue part on the drawing).
When climbing through the Transition Altitude (TA), the Standard Altimeter is set to 1013,25 hPa or In Hg 19

20. Methods in ALTIMETER SETTING
By Vertical position of the aircraft-height above or below TL
By Determining transition level
By Minimum cruising level for IFR flights
By Provision of altimeter setting information 20

21. 1.Vertical position of aircraft
Vertical position of aircraft expressed in terms of altitudes at or below the transition altitude and in terms of flight levels at or above the transition level.
Vertical position expressed in terms of flight levels when climbing and in terms of altitudes when descending. 21

22. 2. Determination of Transition Level
appropriate ATS unit shall establish the transition level
Transition level available for use above the transition altitude established for the aerodrome(s) concerned 22

23. 3.Provision of altimeter setting information
The flight crew shall be provided with the transition level in due time prior to reaching it during descent.
This may be accomplished by voice communications, ATIS broadcast or data link.
transition level shall be included in approach clearances or requested by the pilot. 23