An altimeter is a barometric instrument fitted in the cockpit to measure the vertical height of an aircraft above mean sea level. It is an aneroid barometer that uses a stack of sealed aneroid wafers within the pitot-static system to sense changes in atmospheric pressure and translate them into altitude. By calibrating these pressure readings against standard atmospheric models, the altimeter displays the aircraft's height above a fixed level, enabling pilots to maintain safe vertical separation from terrain and other traffic during all phases of flight.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is an altimeter in aircraft?
An altimeter or altitude meter is an instrument used to measure the altitude of an object above a fixed level, and it compares the pressure of outside static air to the standard pressure of 29.92" Hg of air at sea level. This barometer is supplied with nonlinear calibration so as to indicate altitude.
An altimeter is a pressure instrument used in the cockpit to measure vertical height above particular pressure levels and is one of the components that make up an aircraft's pitot static system. Altimeter is a barometer calibrated to a reference level like sea level or the airport, with an adjustable barometric scale visible in the Kollsman window. Pressure decreases as altitude increases and the instrument senses this change and translates it into altitude. Thus, the altimeter is calibrated to standard sea level conditions, and the standard altimeter setting is 29.92 inches Mercury (1013.25 hPa) when flying above the transition level.
What are the types of altimeters?
The types of altimeters are listed below.
- Barometric Altimeter: A barometric altimeter consists of a barometric capsule linked to a pointer by a suitable mechanical or electronic system. The pointer moves across the dial in response to changes in barometric pressure. The dial is calibrated in feet or in metres. There are different types of barometric altimeters - three-pointer, drum-pointer, counter-pointer, and counter drum-pointer.
- Radio Altimeter: Early radio altimeters determined altitude by measuring the time between transmission of a radio signal from the aircraft and reception of the reflected signal. Modern systems measure the change between the transmitted and reflected signal.
A Three-pointer altimeter uses three pointers to indicate tens, hundreds, and thousands of feet whereas a counter drum-pointer altimeter replaces the thousand-foot pointer with a rotating drum geared to the mechanism that drives the pointer. A sensitive altimeter is a barometric altimeter equipped with a Kollsman window whose range is from 948 to 1050 millibars (28.00 to 31.00 inches of mercury); the setting is set based on local AWOS/ASOS/ATIS reading. A non-sensitive altimeter lacks this fine adjustment and is used where precision is less pivotal. Laser altimeter uses electromagnetic waves to measure altitude and is compared to phase radio-altimeters for ground-effect vehicles. Lidar altimeter, a downward-facing laser altimeter, is used to help navigate the helicopter Ingenuity on Mars.
What is the difference between a sensitive and a non-sensitive altimeter?
A sensitive altimeter is more sensitive than a regular altimeter, now often called non-sensitive. All altimeters are sensitive to some extent, but sensitive altimeters provide a more accurate measurement of pressure and are essentially a refined aneroid barometer. The sensitive altimeter employs a minimum of two aneroid capsules to provide more power to drive the mechanical linkage.
Sensitive altimeters have a 100-foot scale marked in thousands of feet, with 20-foot tick marks as their most precise display. Non-sensitive altimeters have a 1000-foot scale, with 200-foot tick marks as their most precise display. Sensitive altimeters usually require test and calibration equipment with repeatable accuracies of 0.005 inHg (0.127 mmHg) or better, while the instrument used for final calibration of nonsensitive altimeters or sensitive altimeters certificated for use below 35,000 feet (10,668 meters) has a repeatable accuracy of at least 0.01 inch (0.254 mm).
The Kollsman window is a small window used for calibration by altering the pressure setting. Sensitive altimeters have an adjustable barometric scale. Altimeters differ in detail depending on the accuracy of the instrument and the altitude band covered.
What is the function of an altimeter of an aircraft?
An altimeter measures the height of an aircraft above a fixed level. Altimeter provides indicated altitude, true altitude, pressure altitude, density altitude and GPS altitude. A barometric altimeter provides an output to the transponder system to enable transmission of flight level or altitude to air traffic control. An altimeter takes ambient air pressure from a static port and provides an output to the transponder system. A radio altimeter determines aircraft height above terrain and supplies height data to safety systems, automation systems, terrain awareness warning systems, and Traffic Collision Avoidance Systems (TCAS).
How does an aircraft altimeter work?
The aircraft altimeter is an aneroid barometer. It uses aneroid capsules sealed inside the instrument case which expand as pressure decreases, and contract as pressure increases. When aircraft climbs, pressure decreases as altitude increases, so the wafers expand and the bellows inside the case expand. When the aircraft descends, pressure inside the case increases and the bellows contract. Expansion and contraction of the wafers move gears and linkages that drive a pointer across a dial marked in feet. The drum is geared to the mechanism and is marked in thousands of feet; each number on the drum represents 100', and each smaller mark represents 20'. The pointer makes one circle for every 1,000' of altitude change. The static port senses pressure in the outside air and feeds it to the altimeter case, so the instrument measures height above particular pressure levels. A Kollsman window shows the pressure setting scale from 28.00" to 31.00" inches of mercury. This calibration follows the standard pressure lapse rate of 1" Hg per 1,000'. Thus, altimeter calibrates pressure to altitude using nonlinear calibration, and the striped segment visible below 10,000' is masked above that level to avoid misreading.
An aircraft altimeter is an aneroid barometer that measures pressure altitude. Inside the instrument, a sealed capsule expands or contracts as the external atmospheric force changes which drives a needle across a three-handed dial. By equating the sensed pressure to a standard datum, the device represents compression elevation rather than true height above the land. Its precise measuring offers the upright height to my piloting, giving data for situational cognizance and safe separation from terrain and other aircraft.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
How does a pilot use an altimeter?
A pilot uses an altimeter to maintain assigned altitude, obtain accurate Mean Sea Level altitude, and assure terrain clearance. Before takeoff the altimeter must be set to the altimeter setting reported by AWOS, ASOS, or ATIS. During the ground altimeter check the pilot sets the reported airfield altimeter setting on the altimeter scale, then reads the indication to confirm it shows airport elevation. In flight, 14 CFR section 91.121 requires the pilot to set the altimeter to the setting of a station within 100 miles (160.93 kilometers) of the aircraft. The Kollsman window allows the pilot to set the altimeter to the current local pressure. Turning the Kollsman knob changes the barometric scale visible in the window and simultaneously moves the altitude pointers. When flying from high-pressure to low-pressure systems the pilot must adjust the altimeter and look out below, because a 1" Hg change changes the pointer indication by 1000 ft (304.8 m).
At the transition altitude pilots change from the local altimeter setting to 29.92, and above that level the altimeter must be set to 29.92. If the setting is more than one hour old and no current station is available, ATC requires the pilot to set 31.00 prior to reaching a mandatory crossing altitude. In helicopters the same barometric principle applies: the pilot sets local pressure in the Kollsman window and uses the indication to hover at assigned heights and avoid obstacles. In jets, the crew set the current local pressure and monitor the radar altimeter for discrepancies and report any anomaly to ATC. A glass cockpit includes an altimeter and the information is presented on electronic tapes rather than traditional drums and pointers, yet the pilot must still set the same local altimeter setting to obtain accurate MSL altitude.
When reading an altimeter, I learned the delicate motions of the three pointers. The device required my continuous attention and I had to cross-check it against other signs. My main focus was on keeping the aircraft at stable height. The device thus became an important location indicator.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
Why can altimeter readings differ?
Altimeter readings can differ because the instrument senses only the weight of the air beneath it, and that weight is never constant. If barometric pressure changes, the altimeter reading will differ; a high-pressure system will read lower and a low-pressure system will read higher. Atmospheric pressure changes hour to hour, so the altimeter reading at the same airport on a different day will be different.
Temperature affects altimeter readings. Colder than standard temperature will overstate altitude, so the instrument reads higher than actual whereas warmer than standard temperature will understate altitude, so it reads lower than actual. Humidity levels also affect altimeter readings - when air is more humid than standard, the altimeter reads higher.
Wind affects altimeter readings because wind causes changes in air pressure. Westward wind decreases air pressure, causing the altimeter to read higher. After a rapid descent, the altimeter reading is higher than actual because static pressure inside the case lags behind the outside air.
What is the structure of an altimeter?
A standard altimeter contains a stack of sealed aneroid wafers with an internal pressure of 29.92" Hg. Inside the airtight 3 1/8" case these wafers expand and contract in response to the static pressure delivered through a tube attached to the aircraft's static ports. The movement of the wafers is transferred by gears to three pointers that sweep the main dial, which is calibrated in feet and numbered 0 through 9. A striped segment below 10,000 ft (3,048 m) and a drum marked in thousands of feet provide additional read-out information, while the Kollsman window shows the pilot the barometric scale set by the rotating knob. The entire dial-and-pointer assembly is therefore driven mechanically from the sealed aneroid stack, translating pressure changes into a continuous altitude indication for the pilot.
What causes an altimeter breakdown?
An altimeter breakdown begins with air pressure impacts on the aneroid wafers. Inside the casing, static air enters through a tube attached to the static ports, and higher static pressure presses down on the wafers, making them collapse. When the aircraft goes up, pressure inside the case decreases, the bellows expand, and the pointer rises. Any blockage or leak in the Pitot-static system therefore causes altimetry system error.
The very air the altimeter samples may mislead it. Nonstandard temperature affects altimeter: warmer air is less dense, so warmer than standard temperature causes the altimeter to underread and to understate altitude whereas colder than standard temperature causes the altimeter to overread and to overstate altitude. A Venturi effect, produced when air accelerates around the airframe, causes a drop in pressure at the peak. This pressure drop is proportional to wind speed and forces the instrument to over-read.
Altimeter error can be caused by human error, by failing to follow procedures, and by lack of crew resource management skills. Distraction by other cockpit tasks is a frequently reported cause of altimeter-missetting incidents. Extreme barometric pressure is only one of the causes cited for the altimeter-missetting incidents, yet failure to input correct setting remains the dominant trigger: if the crew does not set the current setting when flying from high pressure into low pressure, the aircraft is closer to the surface than the altimeter indicates, and the altimeter will over-read. A large-scale altitude change causes altimeter lag, so a momentary snapshot looks false even when the mechanism is intact. Inaccurate altitude readings lead to controlled flight into terrain or near-mid-air collisions.
