Errors in Altimeter

Wing Commander Rajagopal

Errors and Remedial Actions for Pressure Altimeter

The main altimeter errors are instrument error, position error, and pressure/temperature error, which can cause incorrect altitude readings. These errors occur due to instrument imperfections, airflow effects, and changes in atmospheric conditions.

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 errors are corrected on the 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 the sluggish response of the capsule to pressure changes.
  • Servo-assisted altimeter avoids time lag error.

Barometric Error

  • Barometric error is caused due to changes in Mean Sea Level (MSL) pressure.

Effect of Variation of Pressure and Temperature on Altimeter

Effect of Incorrect QNH Setting

  • If an incorrect QNH is set in the sub-scale, the indicated altitude will differ from the actual altitude.
  • If the sea level pressure decreases, the set datum becomes lower than MSL.
  • The altimeter will over-read.
  • If the sea level pressure increases, the set datum becomes higher than MSL.
  • The altimeter will under-read.

Flight from High to Low Pressure Area

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

Flight from Low to High Pressure Area

  • During a flight from a low-pressure area to a high-pressure area, the indicated altitude will be lower than the true altitude.
  • True altitude is higher than that indicated by the altimeter.
  • The altimeter under-reads, increasing terrain clearance.
  • This is generally less dangerous from a terrain clearance point of view.

Temperature Error – Cold Air

  • Consider a flight between two places with the same Mean Sea Level pressure.
  • Colder air has a higher lapse rate.
  • At a particular altitude, the observed pressure is lower.
  • The indicated altitude will be higher than the true altitude.
  • The altimeter over-reads during a flight from a warmer region to a colder region.

Temperature Error – Warm Air

  • Consider a flight between two places with the same Mean Sea Level pressure.
  • Warmer air has a lower lapse rate.
  • At a particular altitude, the observed pressure is higher.
  • The indicated altitude will be lower than the true altitude.
  • The altimeter under-reads during a flight from a colder region to a warmer region.

Regional QNH

  • Regional QNH is the lowest forecast QNH within an Altimeter Setting Region.
  • Issued every hour for use while flying close to the ground.
  • Ensures minimum terrain clearance.

Temperature Variation from ISA Temperature

  • Effect of temperature variation from ISA:
  • For every 1°C colder than ISA, the altimeter over-reads by 4 ft per 1,000 ft.
  • For every 1°C warmer than ISA, the altimeter under-reads by 4 ft per 1,000 ft.

Effect of Pressure and Temperature

  • For both pressure and temperature:
  • High to Low → Altimeter Over-reads
  • Low to High → Altimeter Under-reads

Alticor

  • Alticor = Indicated Altitude − True Altitude.
  • Add 30 ft for every 1 hPa higher than Mean Sea Level pressure.
  • Add 1% of the indicated altitude for ISA +3°C conditions.
  • D-value = True Altitude − Indicated Altitude.

Effect of Blockage and Leaks on Altimeters

Blockage of Static Vent

  • A blocked static vent causes the altimeter to indicate the altitude at which the blockage occurred.
  • It will under-read during climb.
  • It will over-read during descent.

Leak in Static Vent

  • Leakage in the static line outside the pressurised compartment causes no significant problem; the altimeter indicates correctly.
  • A leak in the static line inside the pressurised compartment causes the altimeter to indicate cabin altitude instead of actual altitude.

Error Summary

Remember:

  • High to Low Pressure → Altimeter Over-reads
  • Low to High Pressure → Altimeter Under-reads
  • Warm to Cold → Altimeter Over-reads
  • Cold to Warm → Altimeter Under-reads