Pressure Altimeter

Wing Commander Rajagopal

Measurement of Aircraft Altitude using Pressure Altimeter

A pressure altimeter measures altitude by sensing atmospheric pressure and converting it into a height reading above sea level. Pilots set the local barometric pressure (QNH) to obtain accurate altitude information during flight.

Introduction to Pressure Altimeter

  • The Pressure Altimeter is the primary instrument used to measure aircraft altitude.
  • It is essentially an aneroid barometer that measures atmospheric pressure.
  • The altimeter senses changes in static air pressure and displays them as altitude.

Principle of Pressure Altimeter

  • Static (atmospheric) pressure decreases with increasing altitude at a non-uniform rate.
  • This occurs because the weight of the air column above the aircraft decreases with height.
  • Static pressure is also affected by variations in temperature and atmospheric pressure.
  • These variations must be understood and corrected to obtain accurate altitude measurements.

Simple, Sensitive and Servo Assisted Altimeters

Construction of a Simple Altimeter

  • A simple altimeter consists of a sealed instrument case containing an expandable aneroid capsule.
  • Static pressure from the aircraft’s static source is supplied to the sealed instrument case.
  • The aneroid capsule contains a fixed internal pressure.
  • As altitude increases, external static pressure decreases, causing the capsule to expand.
  • The capsule movement is transmitted through a linkage to indicate altitude.
  • The instrument displays the altitude at which the sensed pressure would exist under International Standard Atmosphere (ISA) conditions.

Manual Setting Knob in a Simple Altimeter

  • A manual setting knob is used to adjust the datum altitude by rotating the pointer.
  • On the ground, the pointer may be set to:
    • Zero – to indicate height above the airfield (Above Ground Level).
    • Airfield Elevation – to indicate altitude above Mean Sea Level (MSL).

Construction of a Sensitive Altimeter

  • A Sensitive Altimeter is an improved version of the pressure altimeter.
  • It uses multiple aneroid capsules to increase sensitivity.
  • Jewelled bearings reduce mechanical friction.
  • A temperature-compensating mechanism improves accuracy.
  • Mechanical vibrators help overcome friction within the instrument.
  • A variable sub-scale allows selection of the desired pressure datum.
  • Multiple pointers indicate hundreds, thousands, and tens of thousands of feet for improved readability.

Sub-Scale of a Sensitive Altimeter

  • The sub-scale setting is independent of altitude changes and surface pressure variations.
  • It is used only to select the desired pressure reference (QNH, QFE, or Standard Pressure).
  • The normal adjustment range is approximately 800 to 1050 hPa.

Construction of a Servo-Assisted Altimeter

  • At higher altitudes, pressure changes become progressively smaller, reducing capsule movement.
  • A Servo-Assisted Altimeter uses an electromagnetic system known as the E-I Bar (Electromagnetic Indicator Bar).
  • The E-I Bar amplifies very small capsule movements to improve instrument performance.

Advantages of a Servo-Assisted Altimeter

  • The E-I Bar generates an electrical error signal for even very small capsule movements.
  • Reduces or eliminates lag error.
  • Improves measurement accuracy.
  • Provides a faster instrument response.
  • Allows digital altitude display and integration with electronic flight systems.

Calibration of Altimeter

  • The altimeter is calibrated according to the International Standard Atmosphere (ISA).
  • It indicates true altitude only when ISA atmospheric conditions exist.
  • Pressure Altitude is the altitude in the ISA at which the measured atmospheric pressure exists.
  • For example, if the altimeter senses a pressure of 1013.25 hPa, it will indicate an altitude of 0 ft under ISA conditions.