Jump to content
  • Welcome to Recreational Flying!
    A compelling community experience for all aviators
    Intuitive, Social, Engaging...Registration is FREE.
    Register Log in
  • 05. Altitude and altimeters

    • The instrument for indicating the aircraft's altitude, the altimeter, measures static air pressure and is calibrated in accordance with the ICAO international standard atmospheric pressure and temperature model. For conformity, and thus safety, there is a need to clearly define the concepts of altitude and the related pressure setting codes. Air density has a significant effect on aircraft take-off and landing performance, and air density at the airfield must be ascertained by the pilot from the ambient air pressure and air temperature.

    5.1 Altitude sensing instruments
    The sensitive altimeter
    The altimeter is the cockpit instrument that indicates the aircraft's altitude. The instrument is a refined aneroid barometer with a dial indicating height above mean sea level rather than atmospheric pressure. The main component of such an instrument is a small, flexible, corrugated metal capsule from which the air has been partially evacuated — fitted with a metal closure or diaphragm. There is a spring within the capsule that applies a constant force to the bottom of the diaphragm, while atmospheric static pressure applies a counter force to the top, so that the diaphragm moves as atmospheric pressure changes. The movement of the pressure-sensing capsule is transferred and magnified — via a mechanical linkage or piezo-quartz component — to a dial pointer or pointers, or a digital display, which indicate the altitude reading. The static pressure is drawn from the aircraft's static vent/s, which may induce slight position errors due to aerodynamic effects around a vent. There may be two static vents in different locations on the airframe and the pilot may be provided with the ability to select either or both.

    The level in the atmosphere at which any particular pressure occurs is also dependent on temperature — as we saw in the 'Airspeed and the properties of air' module — but the altimeter does not sense the air temperature. Consequently, all altimeters are calibrated in accordance with the International Standard Atmosphere [ISA] model, which utilises a standard temperature lapse rate with height of 6.5 °C per km (2 °C per 1000 feet). The atmosphere in any region rarely corresponds to the ISA model, so aneroid altimeters do not indicate totally accurate height. This is not that important, as true altitude can be calculated, in the rare circumstance that it is needed for terrain clearance purposes by an aircraft operating under the visual flight rules. There is no problem with air traffic management, in that all aircraft in the same region, with properly set (and functioning) altimeters, will be out by the same amount.

    altimeter1.jpgIt is, of course, desirable to set the current local surface pressure into the altimeter by setting that reference pressure into a baro-setting scale or 'sub-scale' (known since the 1930s as the 'Kollsman* window'), which in turn resets the position of the height-indicating pointers against the dial. Or, if the aircraft is on the ground, the same result is achieved by turning the baro-setting knob until the altimeter indicates the known airfield elevation. The sensitive altimeter in the image indicates an altitude of about 1410 feet with the baro-scale setting at 29.9 inches of mercury [in.Hg] — equivalent to 1012.5 hPa. If the altitude was 11400 feet, the pointer with the inverted triangle on the end would be past the figure 1 on the image, indicating +10 000 feet.

    *Paul Kollsman invented his 'sensitive altimeter' in 1929 which was a far superior instrument to those existing at the time but it didn't gain widespread use until 'instrument flying' became common later in the 1930s.

    In Australia, all barometric pressures are reported in hectopascals (equivalent to millibars); and in the USA in units of inches of mercury (one in.Hg = 33.865 hPa so 29.92 in.Hg = 1013.25 hPa). The baro-scale setting range provided in modern altimeters may be from 800 to 1050 hPa.


    Electronic altimeter
    Electronic flight instrument systems [EFIS] use solid-state electronic componentry plus software to display the usual flight instrument readings on a liquid crystal, or similar, screen. In such systems, the atmospheric static pressure is fed to a pressure transducer, which senses and convert pressures to voltages. See the screen display of the Dynon D10A  light aircraft EFIS. Note that the EFIS has an outside air temperature probe and the software can calculate density altitude (see section 'Altitude and Q-code definitions') when needed.

    Electronic altimeters are also available as single instruments or possibly combined with an ASI function.


    Altitude encoding devices
    Altitude encoding devices continually supply pressure altitude data (in Gillham 'Gray' code format) to aircraft transponders and/or GNSS receivers – 'baro-aiding'. There are two types;  encoding altimeters and blind encoders; the latter are stand-alone digital devices with no display (hence 'blind') probably with a pressure transducer  connected to the aircraft's static pressure system.  Standard pressure (1013.25) is  factory pre-set as the scale basis in all altitude encoding devices so both types send pressure altitude not altimeter-indicated altitude. This pressure setting within the device cannot be altered by pilots, such devices being primarily an air traffic management aid.

    A user's manual for the Australian Microair EC2002 low power encoder may be downloaded from the  Microair website.

    5.2 Altitude and Q-code definitions
    Altitude - the third positional dimension
    An aircraft's 3-dimensionsal position may be very accurately defined by its latitude, longitude and altitude; and the latter is normally the most safety-critical dimension. Contour lines and spot points on WACs and VNCs provide an indication of terrain elevation, i.e. height above the reference datum, which is the Australian Height Datum (AHD). The aircraft's altimeter reading provides the aircraft's vertical position and thus an indication of the current height above the terrain indicated on the chart — height above ground level (agl) or airfield level (afl) and the terrain clearance — may be determined. However, in aviation, that altitude reading and the altitude term itself, have many connotations; particularly important is the concept of density altitude.

    Altimeter indicated altitude: is the approximate height of the aircraft above the AHD or above mean sea-level [amsl], calculated in accordance with the ISA but using a local or area QNH as the pressure setting rather than the ISA Standard Pressure of 1013.2 hPa. In Australia the AHD represents mean sea level.

    However an aircraft maintaining a constant altitude — with 1013 hPa or a local/area QNH set in the baro-setting window — is following an isobaric  or contour surface whose height above the AHD will vary according to atmospheric temperature and pressure conditions.

    • In the Australian summer temperatures the 'thickness' of the atmosphere is greater than the ISA standard and consequently the rate of pressure decrease with height is less than ISA and the altimeter indicated altitude will be lower than the true altitude.
    • If the atmosphere is colder than the ISA the thickness of the atmosphere is less than the ISA standard and consequently the rate of pressure decrease with height is greater than ISA and the altimeter indicated altitude will be higher than the true altitude.
    • Also note that;  
      • if you fly from an area of higher pressure to lower and do not obtain and reset the new area/local  QNH the altimeter will be  over-reading (the aircraft is lower than indicated)
      • conversely flying from an area of lower pressure to higher the altimeter will be  under-reading (the aircraft is higher than indicated)
      • but if you fly from an area that is warmer to a cooler one, the altimeter  will be over-reading
      • and conversely, flying from an area that is cooler to one that is warmer the altimeter will be  under-reading and the aircraft is higher than indicated.

    So the adage "From high to low, look out below" is incomplete and the adage "From high to low, hot to cold, look out below" doesn't really apply in  Australia where the continental  low pressure systems (rather than those emanating from the Southern Ocean) are not 'cold core lows' but  'surface heat lows' and troughs. See 'Height contours and thickness charts'.

    These altimeter indicated altitude variations should not be a concern to pilots of aircraft flying under the day visual flight rules and maintaining visual meteorological conditions, particularly so if en route area/local QNH baro-setting information is acquired and properly applied. What should be a particular concern is density altitude rather than true altitude.

    Calibrated altitude: is the altimeter indicated altitude, corrected for internal instrument error and static vent position error by means of reference data for that aircraft installation.

    Pressure altitude: is the altimeter reading when the baro-setting scale is set to 1013.2 hPa; usually termed pressure height in reference to an airfield reading. It is the ISA standard pressure setting.  Standard pressure is also the standard factory setting for altitude encoding devices. All aircraft cruising in the Standard Pressure Region —  above a transition altitude that (in Australia) commences at 10 000 feet  —  use the standard pressure setting, and the subsequent altimeter reading is normally referred to as flight level [FL].

    True altitude: the calibrated/indicated altitude corrected for the outside air temperature conditions. However, there are still problems in the determination of the true height above the AHD, as demonstrated in the following paragraphs. True altitude as calculated in flight from an altimeter reading is of little value to recreational aircraft operating in VMC.

    GPS altitude: the global positioning system uses the WGS84 ellipsoid as its basis for GPS altitude, whereas the AHD (a 'geoid') is the basis for elevations on Australian navigation charts.  The  difference in elevation of a particular point on the Earth's surface — when measured against both the ellipsoid and a national geoid — can be quite considerable, as much as ±250 feet ; this is known as the geoid-ellipsoid separation. In Australia the degree of geoid-ellipsoid separation is quite unusual, in the south-west corner  the AHD geoid is  about 102 feet below the WGS84 ellipsoid while at the north-east corner it is 237 feet above it, so the value of the geoid-ellipsoid separation at all locations must be available to derive true altitude. See 'Geoid-ellipsoid separation and GPS altitude'.

    Density altitude: a calculation used to determine possible  aircraft performance — see section 'High density altitude'  below.   This is the pressure altitude adjusted for variation from standard temperature, or  the height in ISA having a density corresponding to the location density, then called density height.

    Declared density altitude: see  'Method 3: use the CASA declared density altitude charts' below.

    Pivotal altitude: is not associated with altimeter setting; it is a term used by the proponents of 'ground reference' manoeuvres such as 'eights on pylons'. It is a particular height above ground at which, from the pilot's viewpoint, the extended lateral axis line of an aircraft doing a 360° level turn (in nil wind conditions) would appear to be fixed to one ground point, and the aircraft's wingtip thus pivoting on that point. The pivotal altitude in nil wind conditions is easily calculated by squaring the TAS in knots and dividing by 11.3. So an aircraft circling at 80 knots would have a pivotal altitude around 550 feet, no matter what the bank angle.

    When an aircraft is turning at a height greater than the pivotal altitude, the wingtip appears to move backwards over the landscape. When an aircraft is turning at a height less than pivotal altitude (i.e. usually close to the ground) the wingtip appears to move forward over the landscape. For more information see 'pivotal altitude and reversal height'.


    Note: the letters in the Q-code nomenclature have no literal significance; these are remnants of an extensive notation system from the days of wireless-telegraphy and particularly used in marine and aerial navigation/communication. There were some 200 three-letter Q-codes, each representing a sentence, a phrase or a question. For instance, QRM "I am being interfered with"!. Some 30 Q-codes are still used by amateur radio/morse code enthusiasts and the four below, plus QDM (the magnetic bearing to a station), still survive in aviation. For a full listing of Q-codes google 'all Q codes'. The following four codes relate to altimeter settings.

    QFE: the barometric pressure at the station location or aerodrome elevation datum point. If QFE is set on the altimeter pressure-setting scale while parked at an airfield, the instrument should read close to zero altitude — if the local pressure is close to the ISA standard for that elevation. However, the use of QFE is deprecated and anyway, if the airfield elevation is higher than perhaps 3000 feet, older/cheaper altimeters may not be provided with sufficient sub-scale range to set QFE.

    QFF:  the mean sea-level [msl] pressure derived from the barometric pressure at the station location. This is derived by calculating the weight of an imaginary air column extending from the location to sea-level — assuming the temperature and relative humidity at the location are the long-term monthly mean, the temperature lapse rate is ISA, and the relative humidity lapse rate is zero. This is the method used by the Australian Bureau of Meteorology; QFF calculations differ among meteorological organisations. QFF is the location value plotted on surface synoptic charts and is closer to reality than QNH, though it is only indirectly used in aviation.

    QNH: the msl pressure derived from the barometric pressure at the station location by calculating the weight of an imaginary air column extending from the location to sea-level — assuming the temperature at the location is the ISA temperature for that elevation, the temperature lapse rate is ISA and the air is dry throughout the column.

    The Australian aviation regulations state that when an 'accurate' QNH is set on the pressure-setting scale at an airfield, the VFR altimeter indication should read within 100 feet of the published airfield elevation, or 110 feet if elevation exceeds 3300 feet; otherwise the altimeter should be considered unserviceable. However, due to the inherent inaccuracy possible in QNH, this may not be so. The difference between QFF and QNH when calculated on a hot day at a high airfield in Australia can be as much as 4 hPa, equivalent to about 120 feet. The advantage to aviation in using the less realistic QNH is that all aircraft altimeters in the area will be out by about the same amount, and thus maintain height interval separation.

    The local QNH at an airfield is normally derived from an actual pressure reading. But the area QNH used outside the airfield zone is a forecast value, valid for three hours, and may vary by up to 5 hPa from any local QNH in the same area.  Either local QNH or area QNH may be set on the altimeter pressure-setting scale of all aircraft cruising in the Altimeter Setting Region,  which (in Australia) extends from the surface to the Transition Altitude of 10 000 feet. The cruising levels within the Altimeter Setting Region are prefixed by 'A'; e.g. A065 = 6500 feet amsl.

    When there is no official Local QNH available at an airfield and the site elevation is known, the local QNH can be derived by setting the sub-scale (when the aircraft is on the ground at the location) so that the altimeter indicates the known airfield elevation. The use of local QNH is important when conducting operations at an airfield, as the circuit and approach pattern is based on determining height above ground level [agl].

    Note that it is not mandatory for VFR aircraft to use the area QNH whilst enroute. You may substitute the current local QNH of any aerodrome within 100 nm of the aircraft or the local QNH at the departure airfield. See 'Acquiring weather and QNH information  in-flight'.

    The purpose of the Transition Layer is to maintain a separation zone between the aircraft using QNH and those using the standard pressure setting. Cruising within the Transition Layer is not permitted.  If Area QNH was 1030 hPa, there would be about 500 feet difference displayed between setting that value and setting standard pressure. The Transition Layer extends from the Transition Altitude to the  Transition Level  which, in Australia, is usually at FL110 but it may extend to FL125  —  depending on Area QNH. More detail is available in 'Aeronautical Information Publication (AIP) Australia' section ENR 1.7; downloadable from Airservices Australia.

    QNE:  common usage accepts QNE as  the ISA Standard Pressure  setting of 1013.2 hPa. However another definition of QNE is the 'altitude displayed on the altimeter at touchdown with 1013 set on the altimeter sub-scale' i.e. 'pressure height'. It is also referred to as  the 'landing altimeter setting'.

    Within the latter meaning, the term is only likely to be used when an extremely low QNH is outside an aircraft's altimeter sub-scale range, and the pilot requests aerodrome QNE from air traffic services. In Australia, such extreme atmospheric conditions are only likely to occur near the core of a tropical depression/cyclone and as QNE is not listed in the ICAO "Procedures for Air Navigation Services", air traffic services would not provide QNE on request.

    However, QNE can be calculated by deducting the QNH from 1013, multiplying the result by 28 (the appropriate pressure lapse rate per hPa) and adding the airfield elevation.

    For example: QNH 960 hPa, airfield elevation 500 feet, pressure setting 1013.
    QNE = 1013 –960 = 53 × 28 = 1484 + 500 = 1984 feet (the reading at touchdown).


    5.3 High density altitude: effect on take-off/landing performance
    High 'density altitude' conditions at an Australian airfield can provide a severely hazardous environment for any aircraft where the difference between power required and power available is small. This concerns most general aviation and all sport and recreational aircraft engaged in take-off or landing at that airfield. It is the density of the air that provides engine power, propeller performance and lift.

    What we are really doing when calculating density altitude is estimating the density of the air. In ISA conditions, at a  density altitude of 6000 feet amsl,  the air density will be about 1.0 kg/m³, about 20% less than sea-level standard density. The maximum lift possible to be generated will be reduced by 10% (lift = CL × ½rV² × S ) and the ground roll speed related to IAS/CAS prior to take-off will be greater; i.e. during take-off at msl in ISA sea-level conditions TAS = IAS/CAS, but in high density altitude conditions  TAS is greater than IAS/CAS. Remember that V² in the lift equation refers to TAS not CAS. So, at 6000 feet density altitude the TAS at lift-off would be about 10% higher (see rule of thumb) than msl conditions thus the aircraft has to accelerate to a 10% higher ground roll speed before reaching lift-off IAS, and that is before taking into account the effect of the engine and propeller performance reductions on the aircraft's ground roll acceleration performance and its climb-out capability.

    The weight of the charge delivered to the cylinders, in a normally aspirated engine, will be only 80% of the standard sea-level value. Thus, only 80% of the engine's rated power can be supplied at the propeller shaft for take-off and climb-out, or for a go-around. The lower air density ( ½r in the ½rV² term of the lift equation) directly reduces the thrust performance of the fixed-pitch propeller by 10% in which case the thrust performance will be 90% of 80%, or about 72% of the rated sea-level performance. So both the time and the distance needed to acquire take-off lift — and to clear obstacles at the end of the strip — must be increased, the aircraft's rate of climb, and thus angle of climb, will be less than it is near sea level.

    There are many conditions that exist, or might exist, at high density altitude which, though they may be individually slight, all affect the airframe and engine performance adversely. For instance, attempting take-off with a combination of some of the following conditions may cause some difficulty; attempting take-off when most conditions exist may well be disastrous:

    • at an elevated airfield
    • with moderate to high surface temperature
    • on a short, soft strip with unslashed,  wet grass
    • at maximum weight
    • incorrect flap setting
    • light and variable winds
    • departing into rising terrain and a sinking air environment.

    The same conditions apply when landing; the TAS at Vref will be  higher and the consequent ground roll will be longer. The thrust available for a go-around, in the event of an aborted approach, might be very much less than the rated msl thrust, which would probably preclude any late go-around.

    In addition, under high density altitude conditions, the mixture may be excessively over-rich. The recommendation for normally aspirated  engines with cockpit mixture control is  that the mixture should be leaned to maximum rpm before taxying, take-off   or landing  if the density altitude is 5000 feet or greater.

    Density altitude at a particular location can vary considerably from day to day, and also according to time of day. For instance, the table below shows a mid-afternoon and an early morning reading at Alice Springs, in central Australia, on different days. The airfield elevation is 1900 feet.

    QFE Temperature Air density Pressure altitude Density altitude
    941 hPa 43 °C 1.037 2020 feet 5600 feet 957 hPa –2 °C 1.230 1580 feet –100 feet

    The isotherms plus  colour in-fills on the following Australian Bureau of Meteorology map indicate the mid-afternoon surface screen temperatures on a late-spring day. Note that, except for the mountain area near the south-east coast, the surface temperatures greatly exceed the 15 °C ISA standard.




    5.4 Calculating the dry air density altitude
    The density of dry air (r) varies according to ambient air pressure and ambient air temperature, this is reflected in the equation density = pressure divided by 2.87 times the temperature(K). Pressure (or pressure altitude) is readily obtained from the altimeter, and temperature can be obtained from various sources.


    Method 1: use the temperature differential
    Density altitude is roughly 120 feet greater than pressure altitude for each 1 °C that the temperature exceeds ISA for that level, and 120 feet less for each 1 °C that the outside air temperature is less than ISA. In the ISA table sea-level temperature is 15 °C and the ISA temperature lapse rate is 2 °C per 1000 feet.

    For example: Armidale, New South Wales, airport (elevation 3550 feet) on a warm summer day, temperature 30 °C. Altimeter, with 1013.2 standard sea-level pressure sub-scale setting, reads 3400 feet pressure height/altitude.

    • So,
    • ISA standard temperature for an elevation of 3550 feet = [15 –(3.55 x 2)] = 8 °C.
    • The Armidale  temperature then exceeds standard by 22 °C, thus adjustment to be added= 22 × 120 = 2640 feet
    • Pressure altitude = 3400 feet
    • Then the approximate density altitude = 2640 + 3400 = 6040 feet.


    Method 2: calculate using the air density equation
    The density of dry air at altitude can be calculated using the equation:
    r = P / (2.87 T), where:

    • r = rho — the density of dry air  [kg/m³]
    • P = the pressure [hPa]
    • 2.87 = the gas constant for dry air
    • T = the air temperature in kelvin units [K].


    Using the Armidale example, with the altimeter set so that altitude shows the elevation of 3550 feet, the pressure-setting sub-scale will display  895 hPa (i.e. QFE). The temperature is 303 K  (30 °C + 273) thus density = 895 / (2.87 × 303) = 1.029 kg/m³. The height in ISA having a corresponding density is about 5850 feet. This gives a slightly more accurate calculation of density altitude than method 1.


    Method 3: use the CASA declared density altitude charts
    The ICAO International Standard Atmosphere model, used for flight instrument calibration, is based on average climatic conditions at 40° to 45° N latitudes and as such does not reflect conditions over much of Australia in all seasons, with the discrepancy peaking in summer. The Civil Aviation Safety Authority recognises this and publishes seasonal 'declared density altitude' charts with isopleths delineating regional values to be added to airfield elevation to give declared density altitude.

    The three seasonal charts (summer, winter and autumn/spring) are published as appendices to Civil Aviation Order 20.7.0. For example the summer chart shows regional values of 2000 feet in some south-east and 3600 feet in central areas. These regional values are to be used only if there are no other means of calculating current density altitude at the departure and destination airfields.

    Armidale is located at 30° S and 151° E between the 2800 and 3000 feet isopleths of the summer chart, so adding 2900 to the airfield elevation of 3550 feet gives a declared density altitude of 6450 feet.


    density_altitude_chart_armidale.jpgMethod 4: use a density altitude computational chart

    First determine current pressure altitude with 1013 hPa standard pressure setting on the altimeter sub-scale, for example 3400 feet. Also determine outside air temperature, for example 30 °C. Draw a horizontal line from the 3400 feet position to the 30 °C vertical line. Determine the density altitude scale at which the line terminates, for this example density altitude = 6000 feet.

    Click the image for a larger scale  printable  computational chart.

    Method 5: use an E6-B type circular scale computer
    The plastic circular slide rule flight planning computers have a density altitude facility that just entails placing the pressure altitude opposite temperature and reading off density altitude from a third scale. Because the scales are close to the centre of the instrument they are small and difficult to set accurately, but the Jeppesen Model CR-3 computer indicates about 6000 feet density altitude for the Armidale example.

    It can be seen that the five methods listed provide much the same result — about 6000 feet density altitude — so use the method which is most convenient; but do the estimate and then calculate the effects on aircraft performance at estimated weight while also including the effects of runway conditions and the wind velocity.


    Method 6: use the information in the Pilot's Operating Handbook
    Charts in the Pilot's Operating Handbook or Flight Manual should provide density altitude plus aircraft performance and maximum weight figures from the input of pressure altitude and temperature.

    Method 7: use one of the density altitude calculators available on the internet
    Google "density altitude calculator". It's probably advisable to do a number of trial runs before choosing a particular DA calculator.

    Know the normal take-off distance required
    Before you can start to estimate the take-off distance required under high density altitude conditions, you must know the take-off distance required under standard ISA mean sea-level conditions.

    CAO 101-28, an airworthiness certification requirements for commercially supplied amateur-built kit ultralights, states in part:
    "The take-off distance shall be established and shall be the distance required to reach a screen height of 50 feet from a standing start, with ... short, dry grass surface ... the aeroplane reaching the screen height at a take-off safety speed not less than 1.2 Vs1 ... take-off charts ... shall schedule distances established in accordance with the provisions of this paragraph, factored by 1.15."

    Sea-level ISA and nil wind conditions are implied.

    CAO 101.55 has much the same wording, but specifies 1.3 Vs1 as the take-off safety speed. FAR Part 23 is similar.

    CAO 101-28 also requires that the landing distance stated will be that to come to a full stop from a screen height of 50 feet at the threshold, with the screen being crossed at 1.3 Vso and the same conditions as specified for the take-off distance. Refer to Vref.

    If buying an aircraft or kit, you should require that the standard take-off and landing distance chart information for the airframe/engine/propeller combination be supplied. Statements such as "Take-off ground roll 10 m to 40 m" have no value. You must insist, particularly with imported aircraft, that the distances should be stated clearly in one form only and for nil wind conditions "Take-off distance to clear 50 feet (15 m) screen" or "Landing distance over 50 feet (15 m) screen". You have to know without doubt, having done the necessary calculations,  that you can clear obstacles at the end of the unslashed paddock on a hot, bumpy day without risk to you or your passenger, and that if it is necessary to abort a landing, the aircraft will have the ability to go-around safely.

    5.5 Calculating and adding the effect of humidity on density altitude
    The ISA is based on dry air and though air density (mass per unit volume) is chiefly dependent on temperature and pressure, humidity — the presence of water vapour — does decrease the density of the air a little and so has a small effect on lift. Humidity also has a small adverse affect on engine performance as combustion performance is dependent on the oxygen intake during each engine cycle, and that amount of oxygen is dependent on the air density.

    The molecular mass ratio of water vapour to dry air is 0.62:1 and the water vapour molecule occupies about the same space as any oxygen or nitrogen molecule it displaces, so air density decreases as the relative humidity of the air increases, and this should be considered when calculating density altitude.

    *Note: see atmospheric moisture for more information.

    Effect on density altitude. The table below gives the density in grams per cubic metre of the water vapour at the saturation point (i.e. relative humidity (RH) = 100%), for air temperatures between zero and 45 °C. As can be seen, at 35 °C and 100% RH the water vapour density is 40 grams/m³ which is 3.5% of the dry air mass at that temperature. However as the mass ratio of water vapour to dry air is 0.62:1, the 40 grams of vapour in the moist air would displace 65 grams of dry air (13g of oxygen). The effect on air density is a net reduction of 25 grams or 2% and, below 5000 feet elevation, equivalent to a density altitude increase of about 750 feet. At 25 °C and 100% RH the water vapour density is 25 grams/m³ which would displace 40 grams of dry air (8g of oxygen) and the effect on air density is a net reduction of 15 grams or 1.2%; equivalent to a density altitude increase of about 500 feet. It can be seen that high humidity has an additional detrimental effect on aircraft take-off and landing performance under high density altitude conditions.


    Water vapour saturation partial pressure, density at sea level
    and effect on density altitude (values rounded)
    Dry air
    Net density
    Net density
    0 6 5 1290 3 0.2% +100 ft
    10 °C 12 10 1250 6 0.5% +200 ft
    15 °C 17 15 1225 9 0.8% +250 ft
    20 °C 23 20 1200 12 1.0% +350 ft
    25 °C 30 25 1180 15 1.3% +450 ft
    30 °C 42 30 1160 19 1.6% +550 ft
    35 °C 56 40 1150 25 2.2% 750 ft
    40 °C 73 50 1130 31 2.7% +900 ft
    45 °C 97 65 1110 40 3.6% +1200 ft


    The Australian Bureau of Meteorology publishes maps of the average monthly relative humidity observations and these might be used as a basis for estimation if you are unable to find the current RH at the location. If the dry-bulb and wet-bulb temperatures are known the following table will provide a reasonable estimate of the current relative humidity. Wet-bulb temperatures are always lower than dry-bulb temperatures.

    Calculation of relative humidity, for dry-bulb temperatures from 20°C to 45°C, knowing the difference between the dry-bulb and wet-bulb temperatures
    Difference -1° -2° -3° -4° -5° -6° -7° -8° -9° -10°
    Relative humidity 95% 90% 85% 80% 75% 70% 65% 60% 55% 50%

    A handy rule of thumb to allow for the effect of humidity — after completing the dry air density altitude calculation:

    • for air temperatures below 30 °C add 50 feet to the dry air result for each 10% that the local relative humidity exceeds 10% e.g. air temperature 25 °C, RH 60%, altitude to be added is 5×50=250 feet
    • for air temperatures 30 °C and above, add 100 feet to the calculated density altitude for each 10% RH that RH exceeds 20%; e.g. air temperature 35 °C, RH 65%, altitude to be added is 5×100=500 feet.


    The rule is based on the following calculations providing the increase in density altitude for air temperatures from 15 °C to 40 °C and relative humidities between 10% and 100%:

      100% 90% 80% 70% 60% 50% 40% 30% 20% 10%
    15 °C +260 ft +230 ft +210 ft +180 ft +160 ft +130 ft +100 ft - - -
    20 °C +350 ft +320 ft +280 ft +250 ft +210 ft +180 ft +140 ft +100 ft - -
    25 °C +450 ft +400 ft +350 ft +300 ft +260 ft +220 ft +180 ft +130 ft - -
    30 °C +560 ft +500 ft +450 ft +390 ft +340 ft +280 ft +220 ft +170 ft +110 ft -
    35 °C +740 ft +670 ft +600 ft +530 ft +450 ft +380 ft +300 ft +220 ft +150 ft -
    40 °C +900 ft +810 ft +720 ft +630 ft +540 ft +450 ft +360 ft +270 ft +180 ft -

    For more information on take-off and climb performance in high density altitude conditions, see take-off considerations.


    5.6 Physiological effects of altitude
    The tissues and organs of the human body need a constant and adequate supply of oxygen to function at maximum efficiency; insufficient oxygen in those tissue and organs is called hypoxia. There are many causes for the condition, but the one of most interest to sports and recreational aviators is the hypobaric form of hypoxia  caused by continuing flight at an altitude where the partial pressure of the atmospheric oxygen is less than that required for proper functioning of the brain. The body utilises the oxygen partial pressure to pass it through the membrane of the lung alveoli into the bloodstream.

    (The 'stagnant' forms of hypoxia — greyout and blackout —   caused by reduced blood flow to the eyes and brain at aircraft accelerations exceeding +3g to +4g is also, of course, of interest to aerobatic pilots.  For a pilot of average fitness, greyout (dimness of vision) will start between +3.5g and +4.5g, reaching  blackout (complete loss of vision)  between +4g and +5.5g and g-induced loss of consciousness [GLOC]  between +4.5g and +6g.) The application of perhaps –2g or –3g causes increased blood flow to the eyes, resulting in leakage from the blood vessels –redout. Prolonged application of high negative g may severely damage the optic nerves.

    Atmospheric oxygen partial pressure declines as altitude increases; see the atmospheric oxygen section in the Aviation Meteorology Guide. The table in that section shows the time a reasonably fit person will remain conscious at those altitudes without using supplemental oxygen. However, the effects of hypoxia commence at much lower altitudes, probably around 8000 feet for a fit person, less if unfit though much lower for a heavy smoker. These effects include a gradual deterioration in thinking, calculating and reacting; inability to make appropriate judgements; light headedness and a poor memory recall. Unfortunately, the afflicted person is usually quite unaware of the symptoms occurring and may enjoy a feeling of well-being even, perhaps, euphoria. For more information read the article 'Hypoxia' from Flight Safety Australia magazine.

    In Australia recreational aircraft may only be flown at or above 10 000 feet amsl if the pilot has applied to and received written permission for that flight from the Civil Aviation Safety Authority.  The aircraft must be equipped with an operating Mode A/C or S transponder. Also the Australian Civil Aviation Order Part 20.4 paragraph 6 which applies to all Australian aircraft, requires that:  "A flight crew member who is on flight deck duty in an unpressurised aircraft must be provided with, and continuously use, supplemental oxygen at all times during which the aircraft flies above 10 000 feet altitude." Note that   an aircraft may not cruise within the transition layer and that layer could extend to FL125.


    Things that are handy to know
    Altimeter rules of thumb
    • For each 10 °C that the outside air temperature is warmer than ISA standard, increase the indicated altitude by 4% to give true altitude. Conversely, for each 10 °C cooler, decrease indicated altitude by 4% — 10/273 approximates to 4%; refer to Charles' law.

    • When flying from higher to lower pressure conditions, without altering QNH, the altimeter will — if below 10 000 feet — overread (indicate higher than actual altitude) by about 30 feet for each one hPa pressure change.

    • When flying from lower to higher pressure conditions, without altering QNH, the altimeter will — if below 10 000 feet — underread (indicate lower than actual altitude) by about 30 feet for each one hPa pressure change.

    • If the altimeter sub-scale setting is less than QNH the altimeter will overread. Conversely, if the setting is greater than QNH, the altimeter will underread.

    • Air density decreases by about 1% for each:
    — 10 hPa fall in pressure, or
    — 300 feet increase in height, or
    — 3 °C increase in temperature, from the msl standard.

    Stuff you don't need to know
    • There is a semi-diurnal atmospheric tide, similar to the oceanic tide, which is most apparent in the lower latitudes. The tide peaks at 1000 hrs and 2200 hrs local solar time, with the minima at 0400 hrs and 1600 hrs. At Cairns, 17° S latitude, the daily minima and maxima are 2 hPa either side of the mean pressure; e.g. 0400 hrs — 1014 hPa; 1000 hrs — 1018 hPa; 1600 hrs — 1014 hPa; 2200 hrs — 1018 hPa. The runway elevation at Cairns is 10 feet amsl, so that if you left a parked aircraft at 1600 hrs with the altimeter reading 10 feet, six hours later it would be reading 110 feet below mean sea-level. When making their regular pressure reading reports, weather observation stations adjust the reported QFF according to a 'time of day' table.

    • There is also a semi-diurnal gravity variation at the Earth's solid surface, also peaking at 1000 hrs and 2200 hrs. A movement of 50 cm from the low to high earth tide has been ascertained in central Australia.

    •  Perhaps the highest surface pressure recorded is 1083.3 hPa at Agata, Siberia on 31 December 1968.  Agata is 850 feet amsl.



    User Feedback

    Recommended Comments

    There are no comments to display.

    Join the conversation

    You can post now and register later for your post to be seen If you have an account, sign in now to post with your account.

    Add a comment...

    ×   Pasted as rich text.   Paste as plain text instead

      Only 75 emoji are allowed.

    ×   Your link has been automatically embedded.   Display as a link instead

    ×   Your previous content has been restored.   Clear editor

    ×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...