3TOPICS Pressure, Humidity & Temperature ISA and the Aircraft Altimeter 4Pressure, Humidity & Temperature4ISA and the Aircraft Altimeter4Height, Pressure, and the Aircraft Altimeter4Temperature and the Aircraft Altimeter4Altimeter Settings and Terminology
4Pressure decreases with increasing height. PRESSURE, HUMIDITY AND TEMPERATUREThe study of pressure variation within theatmosphere is calledALTIMETRY.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 -
5PRESSURE, HUMIDITY AND TEMPERATURE 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 volumemust also increase.
6PRESSURE, HUMIDITY AND TEMPERATURE 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 airthe lower the overall density.
7PRESSURE, HUMIDITY AND TEMPERATURE The lower the temperature the greater the densitybecause the atoms take up less space astemperature is reduced.
8PRESSURE, 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 columnthey would have to weigh the sameand the warm columnmust be taller to achieve this.That being the case, the pressures would alsobe the same at the top of each column.
9London and Bath have different atmospheric conditions although the surface pressures are the same at1000 hPa.ColdAirWarmLondonBath700mb (hPa)1000mb (hPa)9 500ft10 000ft
10INTERNATIONAL STANDARD ATMOSPHERE This standard atmosphere, which has beeninternationally agreed, is a set of average valueswhich are utilised for thecalibration of aircraft altimeters,the cockpit instrument that indicatesheightby sampling thestatic (undisturbed air) pressure.Thus in theory, all aircraft altimeters should reactin exactly the same manner to any changein air conditions.
11INTERNATIONAL STANDARD ATMOSPHERE The ICAO defined values are -Mean Sea Level Temperature °CMSL pressure hPa/mb (29.92 ins)MSL density gm cu mLapse 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 ( ft)
12AIRCRAFT ALTIMETER The principle of the aircraft altimeter is exactly the same as that of the aneroid barometer.An evacuated capsule reacts to changes in airpressure and these changes are transmittedto a pointer on a dial that is suitablycalibrated in feet or metres.
13Altimeters are fitted with a digital subscale, that is HEIGHT AND PRESSUREAltimeters are fitted with a digital subscale, that isset 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 andvaries from hour to hour.
16TEMPERATURE VARIATION MET02/20TEMPERATURE VARIATIONCold air is denser than warm air. Consider threecolumns of air with identical pressures at MSL, if thetemperatures of the columns are different thenthe height at which the pressure has fallen to aspecified level will also be different.COLDER THAN ISA10 000ft ISAWARMER THAN ISA697hPA
17ALTIMETER SETTINGS AND TERMINOLOGY 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 pressureat the official aerodrome elevation, which isknown as QFE, or at MSL which is known asQNH.When flying at higher levels all aircraft set the ISApressure of hPa because their verticalseparation from each other is more importantthan their separation from the ground.
18ALTIMETER SETTINGS AND TERMINOLOGY flightlevelheightaltitudeQFE1013.2elevationQNHMSL1013.2
19QFE “Atmospheric pressure at official aerodrome level. When set on the subscale of a pressure altimeter it willread zero when the aircraft is onthe ground at the station.”
20QNH “Atmospheric pressure at mean sea level. When set on the subscale of a pressurealtimeter it will read aerodromeelevation when the aircraft ison the ground at thestation.”
21HEIGHT “The vertical distance of a level, point or object considered as a pointmeasured from a specifieddatum.”
22ALTITUDE “The vertical distance of a level, point or object considered as a pointmeasured from mean sealevel.”
23ELEVATION “The vertical distance of a point or level, on or affixed to the surface of theearth, measured from meansea level.”NOTE - Aerodrome elevation is theelevation of the highest point on thelanding area. A separate thresholdelevation is published if it is 7ft ormore BELOW aerodrome elevationand for precision approach runways.
24FLIGHT LEVEL “A level of constant atmospheric pressure above a datum of hPaand separated from other levels byspecific pressure intervals.”
26TRANSITION ALTITUDE “The altitude at or below which the vertical position of an aircraft is controlledby reference to altitudes. Thetransition altitude is located at afixed level and published inaeronautical informationpublications.”
27TRANSITION LEVEL “The lowest flight level available for use above the transition altitude.”
28TRANSITION LAYER “The airspace between the transition altitude and the transition level.”NOTE - The actual depth of the transitionlayer varies as the pressure at MSLchanges and vertical separation of1000ft does not always existbetween the transition altitude and thetransition level.
29The next available flight level above 3540ft is FL40 - The TL is FL40 EXAMPLE 1If the Transition Altitude is 2000ft, the QNH is 995 hPaand the Transition Layer at least 1000ft in depth,what is the Transition Level ?(Assume 1hPa = 30ft)540ft2000ft1000ftTransition LayerTransition LevelTransition AltitudeMean Sea Level995hPa1013hPa= 3540Therefore the top of the TL is 3540ft above a pressure datum of 1013hPaThe next available flight level above 3540ft is FL40 - The TL is FL40
30The next available flight level above 4150ft is FL45 - The TL is FL45 EXAMPLE 2If the Transition Altitude is 3000ft, the QNH is 1008 hPaand the Transition Layer at least 1000ft in depth,what is the Transition Level ?(Assume 1hPa = 30ft)150ft3000ft1000ftTransition LayerTransition LevelTransition AltitudeMean Sea Level1008hPa1013hPa= 4150Therefore the top of the TL is 4150ft above a pressure datum of 1013hPaThe next available flight level above 4150ft is FL45 - The TL is FL45
31If aircraft A is flying at FL55 and aircraft B is operating at 4700ft EXAMPLE 1If aircraft A is flying at FL55 and aircraft B is operating at 4700fton the QNH of 1004hPa, what is their vertical separation ?(Assume 1hPa = 30ft)MET03/20Z4700ftMean Sea Level1004hPa1013hPaAB5500ftCalculate distance Z = x 30 = 270Therefore Z = 270ftAircraft A is at = 5230ft above a pressure datum of 1004hPaTherefore the vertical separation between the aircraft is = 530ft
32If aircraft A is flying at FL55 and aircraft B is operating at 4700ft EXAMPLE 2If aircraft A is flying at FL55 and aircraft B is operating at 4700fton the QNH of 1024hPa, what is their vertical separation ?(Assume 1hPa = 30ft)Z4700ftMean Sea Level1024hPa1013hPaAB5500ftCalculate distance Z = x 30 = 330Therefore Z = 330ftAircraft A is at = 5830ft above a pressure datum of 1024hPaTherefore the vertical separation between the aircraft is= 1130ft