Download Temperature, Pressure and Density

Section 03 - Altimetry
Lesson 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
temperature of 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
 When
Error
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.
Variation of Density Altitude with
Temperature
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 overread 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 underread 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.
Typical Altimeters
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.
QFF v QNH
ABOVE
MSL
BELOW
MSL
Warmer
than ISA
QFF < QNH QFF > QNH
Cooler
than ISA
QFF > QNH QFF < QNH
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
in ft
-1000
0
5000
10000
15000
18000
24000
30000
34000
39000
Flight
Level
-10
0
50
100
150
180
240
300
340
390
Height Temp
in m °C
-305 +17
0
+15
1524 +05
3048 -05
4572 -15
5486 -21
7315 -33
9144 -44
10363 -52
11900 -56.5
Pressure
hPa
1050
1013
843(850)
697(700)
572
506(500)
393(400)
301(300)
250
197(200)
Density
kg/m3
1.26
1.23
1.06
0.90
0.77
0.70
0.57
0.458
0.394
0.317
Standard Pressure
Levels
Pressure (mb)
1000
700
500
400
300
250
200
150
Approx. Height
Mean sea level
10,000 ft.
18,000 ft.
24,000 ft.
30,000 ft.
34,000 ft.
39,000 ft.
45,000 ft.
An aircraft cruising at FL80, flying
from low to high pressure maintains
a constant true altitude. Why?
FL100
WARM
A. 1012 hPa
COLD
B. 1015 hPa