Altimeter

Overview

An altimeter is a vital aviation instrument that measures the vertical distance between an aircraft and a reference level, typically mean sea level (MSL) or the ground directly below. It is a foundational part of an aircraft’s avionics, critical for navigation, safe terrain clearance, airspace management, and maintaining separation from other aircraft. Altimeters display altitude in feet or meters, and the process of measuring altitude is called altimetry.

The word “altimeter” combines “altitude” (height) and “meter” (measuring device). Both under Instrument Flight Rules (IFR) and Visual Flight Rules (VFR), pilots rely on the altimeter for situational awareness and compliance with airspace and obstacle clearance requirements.

Barometric (pressure) altimeters are the standard in aviation, but modern aircraft may use additional types, such as radar or GPS-based altimeters, each providing unique benefits for specific flight phases or operations. Altimeter readings are also transmitted to air traffic control (ATC) via the aircraft’s transponder, supporting traffic surveillance and collision avoidance.

For more, see SKYbrary: Altimeter and Wikipedia: Altimeter .

Principle of Operation

Atmospheric Pressure and Altitude

Barometric altimeters function on the principle that atmospheric pressure decreases predictably with altitude. The International Standard Atmosphere (ISA) defines a baseline: at sea level, standard pressure is 1013.25 hPa (29.92 inHg) and temperature is 15°C. For every 1,000 feet increase in altitude, pressure drops by about 1 inHg, though this rate is not exactly linear at higher altitudes.

Key Concepts:

• Pressure at sea level: 1013.25 hPa (29.92 inHg)
• Pressure decreases with altitude: ~1 inHg per 1,000 feet (approximate near sea level)
• ISA: Used for calibration and reference

The altimeter samples static air pressure through the aircraft’s static port. Lower pressure at higher altitudes causes the altimeter’s internal aneroid capsule(s) to expand, moving the mechanical pointer or updating the digital display.

Barometric (Pressure) Altimeter

The barometric altimeter is the most common type in aviation. It uses a sealed, flexible aneroid capsule that expands as outside air pressure drops (i.e., as the aircraft climbs). This movement drives the instrument’s pointer or digital readout.

Key Components:

• Aneroid Capsule: Sensitive to pressure changes, typically made from beryllium-copper alloy.
• Static Port: Provides undisturbed outside air pressure to the instrument.
• Kollsman Window: Adjustment knob for setting the reference pressure (QNH, QFE, QNE).
• Mechanical Linkages/Digital Processing: Translate movement into altitude indication.

Operation:

1. Static port channels outside air pressure into the altimeter.
2. Aneroid capsule expands/contracts with pressure changes.
3. Movement is displayed as altitude.
4. Kollsman window allows setting the appropriate reference pressure for the phase of flight.

Modern digital cockpits use air data computers to process this information for display, autopilot, and other avionics.

See SKYbrary: Altimeter and Wikipedia: Altimeter .

Types of Altimeters

Aviation uses several altimeter types, each suited to specific operations.

1. Barometric (Pressure) Altimeter

• Principle: Measures altitude above a reference by atmospheric pressure.
• Use: All certified aircraft, from light planes to airliners.
• Features: Mechanical or digital, with Kollsman window for pressure setting.
• Strengths: Reliable, does not require external power (mechanical).
• Limitations: Sensitive to atmospheric pressure and temperature errors.

2. Radar (Radio) Altimeter

• Principle: Sends radio waves downward; measures time to ground and back.
• Use: Low-level flight, approaches, automatic landing, terrain awareness.
• Features: Accurate height above ground level (AGL), typically up to 2,500 feet.
• Strengths: Not affected by weather or pressure; direct terrain reference.
• Limitations: Only works at low altitudes; less accurate over water/irregular terrain.

3. Laser Altimeter

• Principle: Emits laser pulses; measures reflection time to calculate AGL.
• Use: Terrain mapping, obstacle detection, UAVs, research.
• Features: High-precision, rapid data for mapping.
• Strengths: Not dependent on pressure; very accurate.
• Limitations: Performance reduced in fog, rain, or over reflective surfaces.

4. Sonic Altimeter

• Principle: Sends ultrasonic sound waves downward; times echo for AGL.
• Use: UAVs, hover control in helicopters, research.
• Features: Lightweight, short-range.
• Strengths: Inexpensive, effective at low heights.
• Limitations: Sensitive to wind, noise, and surface irregularities.

5. GPS-Based (Satellite) Altimeter

• Principle: Uses GPS satellites to calculate geometric altitude.
• Use: Modern avionics, navigation, electronic flight bags.
• Features: Global coverage, not dependent on weather.
• Strengths: Not affected by pressure/temperature; useful for cross-checks.
• Limitations: Altitude is referenced to a geodetic ellipsoid, not mean sea level; not approved for primary ATC separation.

Comparison Table:

MSL: Mean Sea Level; AGL: Above Ground Level

References:

• SKYbrary: Altimeter
• Wikipedia: Altimeter

Altimeter Settings and Reference Pressures

Correct altimeter settings are critical for accurate altitude readings. The reference pressure selected (via the Kollsman window) determines whether the altimeter shows altitude above sea level, above a specific airfield, or pressure altitude for flight levels. The three main standards are QNH, QFE, and QNE.

QNH

• Definition: Altimeter set to local mean sea level pressure.
• Use: Most common; gives altitude above mean sea level (MSL).
• Example: On the ground, the altimeter reads the airport elevation when set correctly.

QFE

• Definition: Altimeter set to airfield pressure; reads zero on the ground at the airfield.
• Use: Used in some military or glider operations for local AGL indication.

QNE

• Definition: Standard pressure setting (1013.25 hPa/29.92 inHg); used for flight levels above transition altitude.
• Use: Ensures uniform reference for ATC separation at higher altitudes.

References:

• SKYbrary: Altimeter Pressure Settings

Terminology

• Altitude: Vertical distance above mean sea level (QNH).
• Height: Vertical distance above ground/reference (QFE, AGL).
• Flight Level (FL): Pressure altitude referenced to standard pressure (QNE).

Accuracy, Limitations, and Common Issues

Factors Affecting Altimeter Accuracy

1. Atmospheric Pressure Variation: Changing surface pressure (e.g., weather fronts) requires resetting the altimeter. Incorrect setting can lead to significant altitude errors.
2. Temperature Deviations: Altimeters assume standard temperature. Cold air compresses pressure levels, making the altimeter overread (aircraft lower than indicated). Corrections are required in cold climates (ICAO Cold Temperature Corrections ).
3. Mechanical/Instrument Errors: Blocked static ports (due to ice, insects, or debris) or instrument wear can cause erroneous readings. Regular checks and maintenance are mandated.
4. Position/Installation Errors: Poorly located static ports or airframe changes can cause errors, especially at high speeds or unusual attitudes.
5. User Error: Failure to set the correct pressure reference (QNH, QFE, QNE) is a frequent source of error.

Regulatory and Operational Considerations

• Redundancy: Most aircraft carry at least two independent altimeters for safety.
• ATC and Transponder Integration: Altitude information is transmitted to ATC via the aircraft’s transponder, supporting radar surveillance and collision avoidance systems.
• Standardization: ICAO and national regulators provide detailed requirements for altimeter accuracy, calibration, and use.

Importance in Aviation

The altimeter is indispensable for:

• Terrain and Obstacle Clearance: Preventing Controlled Flight Into Terrain (CFIT).
• Vertical Separation: Ensuring safe spacing between aircraft at different altitudes.
• Navigation: Maintaining assigned flight levels or altitudes.
• Approach and Landing: Providing accurate height information for safe landing.
• Legal Compliance: Meeting regulatory requirements for flight operations.

References

• SKYbrary: Altimeter
• Wikipedia: Altimeter
• ICAO: International Civil Aviation Organization

Summary

An altimeter is a fundamental aviation instrument, using barometric, radar, laser, sonic, or GPS principles to measure an aircraft’s altitude relative to sea level or the ground. Its accurate operation is vital for safety, navigation, and compliance with global aviation standards.

For comprehensive regulatory details and operational guidance, consult the International Civil Aviation Organization (ICAO) and SKYbrary .