Altimetry

Wg Cdr Rajagopal

Altimetry

Index

Height Above Mean Sea Level

  • Mean sea level (MSL) is the reference sea level taking the average of tides
  • MSL is the datum for measuring altitude and elevation
  • Altitude is the vertical distance of a movable object from MSL
  • Elevation is the vertical distance of a point on surface of earth from MSL
  • Altitude and elevation are referred as Above mean sea level (AMSL)

Height Above Ground Level

  • Airfield Reference Point (ARP) is the reference point for an airfield
  • Height is the vertical distance from specified datum
  • ARP is the datum for measuring height above ground in an airfield
  • Height above an airfield is referred as Above ground level (AGL)

Pressure Altitude and Flight Levels

  • Standard Pressure Setting (SPS) is the pressure at MSL as per ISA
  • Value of standard pressure setting is 1013.25 hPa
  • Pressure altitude is the altimeter reading when altimeter subscale is set to SPS
  • Flight levels are levels of constant pressure
  • Vertical separation between each flight level corresponds to 500 ft
  • Flight level of 050 (FL 050) means a pressure altitude of 5000 feet

Use of Datum Setting for Pressure Altimeter

  • Height indicated on the altimeter, is the height above set datum pressure as per International Standard Atmospheric conditions
  • For example, when datum pressure is set to 1000 hPa on altimeter sub scale
  • Altimeter compares the prevailing pressure to the datum pressure setting
  • Converts the pressure difference to height as per ISA conditions
  • Points to this height to the pilot on a circular dial

QFE

  • QFE is the observed pressure reduced to ARP using ISA lapse rate
  • It is the atmospheric pressure on ground at an airfield
  • If QFE is set as datum
  • On ground, the altimeter reads nearly zero on the runway
  • During flight, altimeter indicates height above ground level (AGL)

QNH

  • QNH is the observed pressure reduced to MSL using ISA lapse rate
  • It is the same lapse rate by which altimeters are calibrated
  • When QNH is set on subscale
  • On Ground, altimeter will indicate elevation of airfield
  • During flight, the altimeter would read altitude
  • Altitude is the height above mean sea level

SPS

  • Standard Pressure Setting (SPS) is the pressure at MSL as per ISA
  • Value of standard pressure setting is 1013 hPa
  • Pressure altitude is the altimeter reading when altimeter subscale is set to SPS

Flight Level

  • Flight levels are levels of constant pressure
  • Vertical separation between each flight level corresponds to 500 ft
  • Flight level of 050 (FL 050) means a pressure altitude of 5000 feet

QNE – SPS

  • QNE is the the height indicated on airfield when standard pressure setting of 1013 is set on altimeter
  • Used at high altitude airfields where setting QFE is not possible
  • Out of range of altimeter sub-scale

Transition Altitude during Climb Phase

  • It is that altitude during climb out phase where an the altimeter
  • Has to be reset to SPS (1013 hPa)
  • Varies from one airfield to another
  • Does not vary from time to time

Altimeter indications During Climb at Transition Altitude

  • If the altimeter changes over from high QNH (1030) to SPS (1013)
  • Altimeter shows sudden decrease in altitude
  • If the altimeter changes over from low QNH (1000) to SPS (1013)
  • Altimeter shows sudden increase in altitude

Transition Level during Descent Phase

  • It is that altitude during descent phase where an the altimeter
  • Has to be reset to the local QNH
  • Varies from one airfield to another
  • Varies from day to day

Altimeter Indication at Transition Level during Descent

  • If the altimeter changes over from SPS (1013) to high QNH (1020)
  • Altimeter shows increase in altitude
  • If the altimeter changes over from SPS (1013) to low QNH (1000)
  • Altimeter shows decrease in altitude

Transition Layer

  • Transition layer is the airspace of vertical dimensions between
  • Transition Altitude & Transition level

Calculation of QNH From QFE

  • Airfields below 5000 feet
  • Add 1 hPa change for every 27 feet elevation
  • Airfields above 5000 feet
  • Add 1 hPa change for every 25 feet elevation

Density Altitude

  • Density altitude is that altitude in ISA where prevailing density is found
  • Density altitude is quite similar to Pressure altitude
  • Which is that altitude in ISA where prevailing pressure is found
  • In ISA conditions Pressure altitude and Density altitude would be same
  • Density Altitude is important for calculating Aircraft performance
  • Lower Density altitude good aircraft performance
  • Higher Density altitude bad aircraft performance

Air Density in High Hot and Humid Atmosphere

  • Air density would be lesser in these conditions
  • High altitude
  • Warmer than ISA
  • High Humidity
  • Lower air density means higher density altitude

Calculation of Density Altitude

  • Increased Density altitude results in poor aircraft performance
  • For example, Take-off run required would increase
  • Ensure performance parameters before take-off roll
  • Density Altitude can be found from calculation of ISA Deviation
  • Density altitude = Pressure altitude + (120 × ISA deviation)

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