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Section 03 - AltimetryLesson 08

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

The Altimeter

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

The Altimeter

- 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.

Pressure - Height Equation

- 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.

Altimeter Errors

- 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.

Altimeter Errors, cont’d

- 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.

Density Altitude

- 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

- 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.

Influence of Air Temperature on Altimeter Indications

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

Influence of Air Temperature on Altimeter Indications, cont’d

- 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.)

Effect of temperature and pressure variation

- 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.

Temperature Error

- 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

Temperature error calculation

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

Altimeter Definitions (1)

- 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.

Altimeter Definitions (1)

- 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.

Altimeter Definitions (2)

- 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.

Altimeter Definitions (3)

- 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.

Altimeter Pressure Settings

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

QFE

- 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.

QFF

- 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.

QNH

- 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.

Regional QNH

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

Standard Pressure Setting

- 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.’

QNE

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

Altimeter Calibration

- 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.

Table of ISA Values

Height Flight Height Temp Pressure Density

in ft Level in m °C hPa kg/m3

-1000 -10 -305 +17 1050 1.26

0 0 0 +15 1013 1.23

5000 50 1524 +05 843(850) 1.06

10000 100 3048 -05 697(700) 0.90

15000 150 4572 -15 572 0.77

18000 180 5486 -21 506(500) 0.70

24000 240 7315 -33 393(400) 0.57

30000 300 9144 -44 301(300) 0.458

34000 340 10363 -52 250 0.394

39000 390 11900 -56.5 197(200) 0.317

An aircraft cruising at FL80, flying from low to high pressure maintains a constant true altitude. Why?

FL100

COLD

WARM

A. 1012 hPa

B. 1015 hPa

1013 hPa

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