Errors in Altimeter

Wg Cdr Rajagopal

Errors in Altimeter

Index

Position and Instrument Errors in Altimeter

  • Position or Pressure error is caused due to location of pitot head
  • Instrument Error is caused due to mechanical imperfections
  • Position and Instrument error are corrected on ground
  • Residual errors are listed on a correction card

Manoeuvre and Time lag Errors in Altimeter

  • Manoeuvre Induced Error causes temporary fluctuations in attitude indications
  • Time Lag Error is caused due to sluggish response of capsule to pressure
  • Servo assisted altimeter avoids time lag error

Barometric Error

  • Barometric error is caused due to change in MSL pressure

Effect of Incorrect QNH Setting

  • If incorrect QNH set in sub-scale, indicated altitude would be in error from actual altitude
  • If the sea level pressure reduces, the set datum would be lower than MSL
  • Altimeter would over read
  • If the sea level pressure increase, the set datum would be higher than MSL
  • Altimeter would under read

Flight From High to Low Pressure Area

  • During a flight from high pressure to low pressure area, the indicated altitude would be higher than true altitude
  • True altitude is lower than that indicated by the altimeter
  • Altimeter over reads, reducing terrain clearance which is potentially dangerous

Flight From Low to High Pressure Area

  • During a flight from low pressure to high pressure area, the indicated altitude would be lower than true altitude
  • True Altitude is higher that indicated by the altimeter
  • Altimeter under reads, increasing terrain clearance
  • Not so dangerous from terrain clearance point of view

Temperature Error – Cold Air

  • Consider flight between two places with same Mean sea level pressure
  • Colder air has higher lapse rate
  • At a particular altitude, the observed pressure is lower
  • Indicated altitude would be higher than True altitude
  • Altimeter over read in a flight from high to lower temperature region

Temperature Error – Warm Air

  • Consider flight between two places with same Mean sea level pressure
  • Warmer air has lower lapse rate
  • At a particular altitude, the observed pressure is higher
  • Indicated altitude would be lower than True altitude
  • Altimeter under reads in a flight from low to higher temperature region

Regional QNH

  • Regional QNH
  • Lowest forecast QNH in an Altimeter setting region
  • Issued every one hour for use while flying close to ground
  • Ensures minimum terrain clearance

Temperature Variation from ISA Temperature

  • Effect of temperature variation from ISA
  • For 1 deg colder from ISA – altimeter over reads by 4 ft / 1000 ft
  • For 1 deg warmer from ISA – altimeter under reads by 4 ft / 1000 ft

Effect of Pressure and Temperature

  • For both Temperature and Pressure
  • High to Low – Altimeter Over reads
  • Low to High – Altimeter Under reads

Alticor

  • Alticor = indicated altitude – true altitude
  • Add 30 ft for 1 hPa higher than MSL pressure
  • Add 1 % of indicated altitude for ISA +3 conditions
  • D value = True altitude – Indicated altitude

Blockage of Static Vent

  • Static vent blockage would make the altimeter indicate the altitude at which the blockage occurred
  • Under read during climb
  • Over read in descent

Leak in Static Vent

  • Leakage in static line outside pressurized compartment
  • No problem, altimeter would indicate correctly
  • Leak in static line inside pressurized compartment
  • Altimeter would indicate cabin altitude instead of actual altitude

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