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Groundschool – Theory of Flight
Altitude and altimeters
Revision 50 — page content was last expanded 27 November 2013
|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.|
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.
It 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. 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. 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.
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.
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'.
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).
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.
The next module in this Flight Theory Guide discusses lift generation, aerofoils and wings.
Groundschool – Flight Theory Guide modules
| Flight theory contents | 1. Basic forces | 1a. Manoeuvring forces | 2. Airspeed & air properties |
| [3. Altitude & altimeters] | 4. Aerofoils & wings | 5. Engine & propeller performance |
| 6. Tailplane surfaces | 7. Stability | 8. Control | 9. Weight & balance |
| 10. Weight shift control | 11. Take-off considerations | 12. Circuit & landing |
| 13. Flight at excessive speed | 14. Safety: control loss in turns |
| Operations at non-controlled airfields | Safety during take-off & landing |
|The next section within the Aviation Meteorology ground school covers cloud, fog and precipitation|
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