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Section 03 - Altimetry Lesson 08

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- The Altimeter
- Altimeter Errors
- Altimeter Definitions
- Altimeter Settings

- Close connection between pressure and height
- It is primarily an instrument merely for measuring pressure.
- Similar to an aneroid barometer

- The relationship between pressure and height is constantly changing and depends on the surface pressure and the mean temperature of the air column up to the height concerned.
- The relationship between pressure and height is governed by the pressure-height equation.
- The height indicated on an altimeter is directly dependent on the surface pressure and the mean temperatureof the air column from the surface up to the height of the altimeter.

- Note: It is not necessary to memorize this equation for the exams
- Calculation of Height/Thickness between two pressure levels
- H2 - H1 = 221.1 T (logP1 - logP2)

- ‘T’ in the formula is the MEAN temperature of the layer.
- Thus the thickness of a layer is directly dependent on this mean temperature ‘T’.
- The true altitude of an aircraft will depend on the mean temperature of the air layer below the aircraft.

- Altimeters are calibrated according to the ISA.
- Altimeters will only indicate correctly when:
- the air column corresponds to ISA
- ELR 1.98°C/1000 ft.

- The surface pressure is standard;
- 1013.25 mb.

- the air column corresponds to ISA

- This results in two main errors:
- Barometric Error
- When the pressure at sea level varies from ISA causing variations aloft.

- Temperature Error
- When the air column is colder or warmer than ISA.

- Barometric Error

- Air behaves according to the gas laws.
- If the air is warmed the density will decrease.
- If it is cooled the density will increase.
- This will affect what is called the “Density Altitude.”

- Density Altitude is that height in the ISA which will correspond to the prevailing (ambient) density.
- The rule of thumb to calculate this is:
- A change of 1°C will result in 120 ft change in density altitude.
- If air column is ISA the Density Altitude =ISA Altitude.

- Height indicated on an altimeter is:
- directly dependent on the height of the air column above the altimeter.

- If the temperature of the air column increases there will be a greater height of air above the altimeter and it will sense a greater pressure and register a lower altitude. (It will under read.)
- If the temperature of the air column is decreased there will be less air above the altimeter and this will be sensed as a decrease in pressure and the altimeter will indicate a greater altitude. ( It will over read.)

- If the surface pressure or mean temperature of the air layer below the aircraft is decreasing then true altitude will decrease and the altimeter will over-read on arrival at destination.
- If the surface pressure or mean temperature of the air layer below the aircraft is increasing then true altitude will increase and the altimeter will under-read on arrival at destination.

- In most cases density is unknown
- Temperature is easily measured so it is better to speak of “Temperature Error,” rather than density error.
- Rule of thumb: 1% of height for 2.5°C (4% per 10°C), or
- for every 1°C of ISA deviation the altimeter is in error by 4ft per 1000ft of altitude

- 10°C deviation = 4%
- 4% of 3000ft = 4x30=120ft.

- with 1013.25 mb set on its subscale then the Pressure altitude: Is the height of a given level in the ISA above the 1013.25mb pressure datum.
- If an aircraft is flying vertical position is expressed as a Flight Level.
- e.g. FL050; FL180.

- The setting of 1013.25mb is referred to as ‘standard altimeter(or pressure)setting.’ (SPS)
- Flight levels (FL) are pressure altitudes expressed in units of 100 feet; e.g. FL340 is a pressure altitude of 34,000 ft.

- Density Altitude: Is that altitude in the ISA at which the prevailing air density is to be found.
- If the air is warmer than ISA it will be less dense and the density altitude will be higher in the ISA than the pressure altitude.
- If the air is cooler than ISA then the density altitude will be less than the pressure altitude.

- True Altitude: is the exact vertical distance above Mean Sea Level (MSL).
- This will differ from indicated altitude if the temperature varies from ISA conditions and the sub-scale setting is different from the value of mean sea level pressure directly below the aircraft.

- QFE
- QFF
- QNH
- Regional QNH
- Standard Pressure Setting
- QNE

- Is a pressure setting which when set on the sub-scale of an altimeter will cause it to read zero at the airfield elevation.
- An airfield may have a touchdown QFE if there is a difference in height of 2m (7ft) or more between aerodrome reference and runway threshold.

- Is the local station barometric pressure adjusted to mean sea level assuming an isothermal column of air at station temperature
- It is expressed to one decimal point.

- Is a pressure setting which when set on an altimeter sub-scale will cause it to read the airfield elevation above mean sea level.
- ISA conditions are assumed for the air column
- It is always expressed in integer values.

- Is the lowest forecast QNH for a period of one hour for a designated region

- is defined as 1013.25 mb.
- When flying with standard pressure set on the subscale the vertical position of the aircraft is referred to as it’s ‘flight level.’

- Is the height indicted on an altimeter with 1013 mb set on the subscale.
- It is the same as pressure altitude.

- All altimeters are calibrated according to the International Standard Atmosphere.
- Any variation from this introduces an error mainly due to temperature deviation from ISA.
- Provided all aircraft use the same sub-scale setting, separation can be maintained.

HeightFlightHeightTempPressureDensity

in ftLevelin m°ChPakg/m3

-1000-10-305+1710501.26

00 0+1510131.23

5000501524+05843(850)1.06

100001003048-05697(700)0.90

150001504572-155720.77

180001805486-21506(500)0.70

240002407315-33393(400)0.57

300003009144-44301(300)0.458

3400034010363-522500.394

3900039011900-56.5197(200)0.317

FL100

COLD

WARM

A. 1012 hPa

B. 1015 hPa

1013 hPa