part 6. altimetry topics isa and the aircraft altimeter height, pressure, and the aircraft altimeter...
TRANSCRIPT
Part 6. Altimetry
TOPICS
ISA and the Aircraft Altimeter
Height, Pressure, and the Aircraft Altimeter
Temperature and the Aircraft Altimeter
4
4
4
Altimeter Settings and Terminology4
Pressure, Humidity & Temperature4
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, butthe density of the atmosphere does too.
However, there are a number of other factors thataffect density -
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 thegas and it’s weight for a given volume
must also increase.
PRESSURE, HUMIDITY AND TEMPERATURE
HUMIDITY
Water vapour is less dense than dry air becausethe molecules are further apart.
However, it combines readily with dry air sothe higher the water vapour content of the air
the lower the overall density.
PRESSURE, HUMIDITY AND TEMPERATURE
TEMPERATURE
The lower the temperature the greater the densitybecause the atoms take up less space as
temperature is reduced.
PRESSURE, HUMIDITY AND TEMPERATURE
A column of cold air will weigh more than an identicalcolumn of warm air and the pressure at thebottom of the cold column will be higher.
Similarly, if the pressure at the bottom of the warmcolumn 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 alsobe the same at the top of each column.
PRESSURE, HUMIDITY AND TEMPERATURE
ColdAir
WarmAir
London Bath
700mb (hPa)
700mb (hPa)
1000mb (hPa)
9 50
0ft
10 0
00ft
London and Bath have different atmospheric conditionsalthough the surface pressures are the same at
1000 hPa.
This standard atmosphere, which has beeninternationally agreed, is a set of average values
which are utilised for thecalibration of aircraft altimeters,
the cockpit instrument that indicatesheight
by sampling thestatic (undisturbed air) pressure.
Thus in theory, all aircraft altimeters should reactin exactly the same manner to any change
in air conditions.
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/1000ftup to 11kms (36 090ft)
- remaining at -56.5°C thereafterup to 20kms (65 617ft)
- increasing at 0.3°C/1000ftthereafter up to 32kms (104 987ft)
INTERNATIONAL STANDARD ATMOSPHERE
The principle of the aircraft altimeter is exactlythe same as that of the aneroid barometer.
An evacuated capsule reacts to changes in airpressure and these changes are transmitted
to a pointer on a dial that is suitablycalibrated in feet or metres.
AIRCRAFT ALTIMETER
Altimeters are fitted with a digital subscale, that is set by a rotating knob to indicate the pressuredatum above which the altimeter is operating.
This is necessary because air pressure doesnot remain constant at any place and
varies from hour to hour.
HEIGHT AND PRESSURE
MET02/16
1
2
3
45
6
7
8
9
1020
0
1020hPa
1
2
3
45
6
7
8
9
1020
0
1000hPa
A B
HEIGHT AND PRESSURE
Sub scale setting level 1000 hPa
1000hPa990 hPa300ft True
600ft
980 hPa level
980 hPa level
600ft
Indi
cate
d
SURFACE
Flight path
HEIGHT AND PRESSURE
Cold air is denser than warm air. Consider threecolumns of air with identical pressures at MSL, if the
temperatures of the columns are different thenthe height at which the pressure has fallen to a
specified level will also be different.
TEMPERATURE VARIATION
COLDER THAN ISA10 000ft ISA
WARMER THAN ISA
697hPA
697hPA
697hPA
MET02/20
The altimeter subscale setting depends upon thephase of flight that the aircraft is undergoing.
When operating at, or near an airfield thesubscale 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.
ALTIMETER SETTINGS AND TERMINOLOGY
When flying at higher levels all aircraft set the ISApressure of 1013.2 hPa because their verticalseparation from each other is more important
than their separation from the ground.
1013.2
1013.2
QNH
QFE
MSL
altitude
flightlevel
elevation
height
ALTIMETER SETTINGS AND TERMINOLOGY
QFE
“Atmospheric pressure at officialaerodrome level. When set on the
subscale of a pressure altimeter it willread zero when the aircraft is on
the ground at the station.”
QNH
“Atmospheric pressure at mean sea level.When set on the subscale of a pressure
altimeter it will read aerodromeelevation when the aircraft is
on the ground at thestation.”
HEIGHT
“The vertical distance of a level, pointor object considered as a point
measured from a specifieddatum.”
ALTITUDE
“The vertical distance of a level, pointor object considered as a point
measured from mean sealevel.”
ELEVATION“The vertical distance of a point or level,
on or affixed to the surface of theearth, measured from mean
sea level.”NOTE - Aerodrome elevation is the
elevation of the highest point on thelanding area. A separate threshold elevation is published if it is 7ft or
more BELOW aerodrome elevationand for precision approach runways.
FLIGHT LEVEL
“A level of constant atmosphericpressure above a datum of 1013.2hPa
and separated from other levels byspecific pressure intervals.”
CHANGES OF REFERENCE
Flight Levels
Transition Layer
TransitionAltitude
TransitionLevel
1013.2
1013.2
MSL
QFE
QNH
Transition Level
Transition Altitude
TRANSITION ALTITUDE
“The altitude at or below which thevertical position of an aircraft is controlled
by reference to altitudes. Thetransition altitude is located at a
fixed level and published inaeronautical information
publications.”
TRANSITION LEVEL
“The lowest flight level available foruse above the transition altitude.”
TRANSITION LAYER
“The airspace between the transitionaltitude and the transition level.”
NOTE - The actual depth of the transitionlayer varies as the pressure at MSLchanges and vertical separation of
1000ft does not always exist between the transition altitude and the
transition level.
EXAMPLE 1If 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)
540ft
2000ft
1000ftTransition Layer
Transition Level
Transition Altitude
Mean Sea Level 995hPa
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
EXAMPLE 2If 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)
150ft
3000ft
1000ftTransition Layer
Transition Level
Transition Altitude
Mean Sea Level 1008hPa
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
EXAMPLE 1If 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)
Calculate distance Z 1013 - 1004 = 9 9 x 30 = 270Therefore 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
Z
4700ft
Mean Sea Level1004hPa
1013hPa
A
B
5500ft
MET03/20
EXAMPLE 2If 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)
Calculate distance Z 1024 - 1013 =11 11 x 30 = 330Therefore 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
Z
4700ft
Mean Sea Level1024hPa
1013hPa
A
B
5500ft
Questions…