Groundschool – Theory of Flight
Altitude and altimeters
Revision 41 — page content was last changed November 2, 2008. The page has been edited by RA-Aus member Dave Gardiner www.redlettuce.com.au.
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Groundschool – Theory of Flight |
Altitude and altimetersRevision 41 — page content was last changed November 2, 2008. The page has been edited by RA-Aus member Dave Gardiner www.redlettuce.com.au. |
The instrument for indicating the aircraft's altitude, the altimeter, measures static air pressure and is calibrated in accordance with an international standard atmospheric pressure and temperature model. Air density has a significant effect on aircraft take-off performance, and equivalent air density at the airfield can be ascertained from current pressure and air temperature.
3.1 The sensitive altimeterThe sensitive altimeter is the cockpit instrument that indicates the aircraft's altitude. The instrument is a refined aneroid barometer with a dial indicating height above a pre-set 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, which may induce slight position errors due to aerodynamic effects around the vent.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. The atmosphere in any region rarely corresponds to the ISA, 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. 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 pressure-setting 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 pressure-setting scale until the altimeter indicates the known airfield elevation. The altimeter in the image indicates an altitude of 1400 feet with the baro-scale set at 29.9 inches of mercury [in/Hg] — equivalent to 1013 hPa. If the altitude was 11 400 feet, the pointer with the inverted triangle on the end would be past the figure 1 on the image, indicating +10 000 feet.
In Australia, all barometric pressures are reported in hectopascals [equivalent to millibars]; and in the USA in units of inches of mercury [in/Hg =33.86 hPa]. The sub-scale setting range provided in modern altimeters is from 850 to 1050 hPa. Electronic altimeterElectronic 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 encodingIn some flight conditions, an aircraft must operate a transponder for traffic separation purposes. The transponder obtains altitude data from a special altitude encoding altimeter or from a blind encoder; the latter being an electronic device that obtains current atmospheric pressure from the static pressure line and the reference pressure used is preset at 1013.2 hPa. That same reference pressure is used for the altitude encoding function of the altimeter, thus the transponder broadcasts pressure altitude only. |
3.2 Altitude and Q-code definitionsAltitudeAltimeter indicated altitude: the approximate height of the aircraft above mean sea-level [amsl], calculated in accordance with the ISA.Calibrated altitude: the indicated altitude, corrected for internal instrument error and static vent position error. 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: seasonal charts showing regional values to be added to airfield elevation to give declared density altitude were published in section 20.7 of the Civil Aviation Orders. For example the summer chart shows regional values of 2000 feet on the eastern coast and 3600 feet in south-west Queensland. These regional values are to be used only if there are no other means of calculating current density altitude. 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'. Pressure altitude: the altimeter reading when the pressure-setting scale is set to 1013.2 hPa. It is the ISA Standard Pressure setting, sometimes termed pressure height. Standard pressure is also the standard factory setting for altitude encoding devices. All aircraft cruising in the Standard Pressure Region — above a transition layer 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]. However an aircraft maintaining a constant altitude using 1013.2 hPa, or any other fixed setting for that matter, is following an isobaric surface whose height amsl will vary according to atmospheric conditions. An aircraft maintaining FL145 (i.e. 14 500 feet), and flying towards a lower pressure area, will actually be descending at a rate approximating 40 feet per one hPa decrease in surface level pressure. True altitude: the calibrated altitude corrected for atmospheric temperature conditions. But as the correction will assume standard pressure and temperature lapse rates between the surface and the aircraft level, it will not be an accurate reflection of the aircraft's height above mean sea-level. If you maintain a particular altitude, you will be following an isobaric surface and not maintaining a constant height. The only way to measure height accurately is by triangulation — and that can only be done by a GPS receiver in the aircraft. However, there are still problems in determining the vertical datum. See geoid-ellipsoid separation. Q-codesNote: 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. 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 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) 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'. 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 27 (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 × 27 = 1431 + 500 = 1931 feet (the reading at touchdown). |
3.3 Physiological effects of altitudeThe 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.) 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; and a poor memory recall. Unfortunately, the afflicted person is usually unaware of the symptoms occurring. For more information read the article 'Hypoxia' from Flight Safety Australia magazine. The Australian Civil Aviation Order Part 20.4 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." |
3.4 High density altitude: effect on take-off/landing performanceHigh 'density altitude' conditions at an Australian airfield, particularly in summer, can provide severely hazardous conditions for any aircraft where the difference between power required and power available is small. This concerns most general aviation and all ultralight aircraft engaged in take-off or landing at that airfield.What we are really doing when calculating density altitude is estimating the density of the air. 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). So the weight of the charge delivered to the cylinders, in a normally aspirated engine, will be only 80% of the standard sea-level density. 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. As well as that, the lower air density (½r in the lift equation) directly reduces the thrust performance of the propeller by 10% in which case the thrust performance will be 90% of 80%, or about 72% of the rated sea-level performance. 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 higher; 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. Thus, the ground roll speed prior to reaching IAS/CAS for rotation must be higher than that at sea-level, and 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. And that is before taking into account the effect of reduced engine/propeller performance on take-off distance. Remember that V² in the lift equation refers to TAS not CAS. (From the previous module, to convert CAS to TAS multiply the density altitude, in 1000s of feet, by a factor of about 1.5 to get the percentage increase to apply; i.e. at 6000 feet density altitude, TAS will be about 9.0% higher than CAS.) 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:
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.
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. For example: Armidale, New South Wales, airport (elevation 3550 feet) on a warm day, temperature 30 °C. Pressure altitude with 1013.2 standard pressure setting reads 3400 feet. Or, conversely with the altimeter set so that altitude reads 3550, the pressure-setting sub-scale displays 895 hPa (i.e. QFE). In the ISA table 895 hPa equates with a pressure altitude of 3400 feet.
Alternatively, the density of dry air at altitude can be calculated using the equation: r = P / (2.87 T), where:
e.g. In the Armidale example, 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 more accurate calculation of density altitude than the prior method. In ISA, the density of dry air at msl is 1.225 kg/m³, thus at 1.029 kg/m³ the density reduction is 0.196 kg/m³ (or 16%). Maximum possible lift at Armidale in the above conditions is 16% less than that possible under standard msl conditions. The efficiency of a fixed-pitch propeller in converting engine power to thrust would also be reduced by a similar amount because of the reduced density. Similarly, the TAS at Vlof 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. As a small, normally aspirated engine may only produce 80% power (or less) on take-off at a density altitude of 5600 feet, and the power converted to thrust is also reduced considerably, then the take-off thrust available in the Armidale example might be less than 60% of the rated msl thrust. The same conditions apply when landing; the TAS at Vref will be 10% 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 less than 50% of the rated msl thrust, which would probably preclude a go-around. Also, it must be borne in mind that the air is not dry; rather, the absolute humidity may be very high. This does not have a significant effect on lift, but does adversely affect the engine performance a little. In a carburation or injection system fuel is metered on the volume of gas being inducted whether it is air or water vapour. With water vapour in the gas, there is less air present. This enriches the mixture slightly as the fuel is metered for the total volume of gas. Water vapour slows burning, which slightly affects power. In addition, under high density altitude conditions, the mixture may be excessively over-rich. The recommendation for normally aspirated Lycoming engines is that the mixture should be leaned to maximum rpm before take-off, but only if the density altitude is 5000 feet or greater.
[ The next section in the airmanship and safety sequence is section 9.1 Consequences of exceeding MTOW ] |
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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. |
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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. It is not known whether this was QFE or QFF. 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 |
Supplementary documents
| 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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Copyright © 2000–2008 John Brandon [contact information]
