Figure and drawings from the FAA Pilot’s Handbook of Aeronautical Knowledge
The altimeter is a crucial instrument during flight, as it provides pilots with correct altitude measurement above Earth’s surface and proper traffic separation. Legally, pilots need to have an operating altimeter to conduct flight activities. For such a fundamental instrument, it is important to understand how it works and the additional errors that it can be susceptible to. An aircraft that flies along a line of constant pressure may change altitude as the air temperature changes. Without being corrected for the temperature change, a pressure altimeter will continue to read the same elevation. If an aircraft is flown from an area of high pressure to an area of low pressure without adjustments to the altimeter, the altimeter will only display a constant altitude but the actual distance of the aircraft above the ground will be lower than the altimeter indications. Hence the saying “from hot to cold, look out below”.
On December 25, 2012 an Antonov 72 transport plane crashed because of altimeter problems that led to a controlled flight into terrain (CFIT). The plane was close to the Shymkent Airport (CIT) in Kazakhstan and all 20 passengers and seven crew were killed, according to Aviation Safety Net. While en route and during the descent the flight encountered poor weather with heavy snowfall and limited visibility. During the approach, the captain failed to set the correct pressure for the barometric altimeters. The altitude indicated was therefore off by about 385 meters. The airplane struck the slope of a ravine 21 km short of the runway and broke up. This goes to show how dangerous it can be if altimeters are not set correctly.
FAA figure
An altimeter is an aneroid barometer that is calibrated to convert atmospheric pressure to an approximate altitude. In simpler terms, the altimeter is an instrument that displays an aircraft’s vertical distance above a fixed level. The altimeter works by taking the air pressure intake from a small opening on the exterior of the aircraft called the static port, which naturally collects air as the plane propels forward through the air. Located inside the altimeter, there is a small enclosed disc called the aneroid wafer. Air collected through the static port flows into the same area where the aneroid wafers are located. As the aircraft goes up in altitude, the pressure around the aneroid wafers decreases and the small device expands like a balloon. As an aircraft goes down in altitude, the pressure around the aneroid wafers increases, and the small devices deflate like a balloon. Pilots must always set the altimeter to the current pressure at an airport so that the altimeter displays the correct altitude while airborne.
Mechanical Altimeter Errors can be caused by the normal wear and tear within the instrument, which can result in discrepancies among the little mechanisms within the casing. This type of error is not constant nor is it predictable. It can however be checked for during pre-flight by using the barometric scale adjustment knob to input the local altimeter setting. Once the altimeter is set, the altitude reading should indicate within + 75 feet of the airport’s elevation. If the altimeter exceeds the limit, then the problem should be brought up to an appropriately rated mechanic.
Scale Error is also caused by discrepancies in the mechanisms within the casing. This error can be caused by irregular wafer expansion. With the barometric pressure scale at the standard 29.92 inches of mercury, the altimeter should be subjected successively to pressures corresponding to the standard pressure lapse rate: approximately 1’Hg per 1,000 ft in altitude.
FAA figure on atmospheric pressure and the standard atmospheric pressure
Installation/position Errors involve the static system itself and the ambient air against which the altimeters’ aneroids expand or contract. As the aircraft moves through the atmosphere, variations in the airflow past the static ports also cause variations in the displayed altitude. As such, the magnitude of any installation/position error varies with the type of aircraft, its speed and altitude. Some aircraft include a correction card the pilot can use to compensate for this error, and the manufacturer’s documentation will explain the conditions-different altitudes and airspeeds-under which it must be used.
Reversal Errors result from large-scale or temporary disruptions to the static pressure system. These changes generally only occur during abrupt and rapid pitch changes and result in momentary indications in the opposite direction. Most aircraft likely aren’t capable of the performance necessary to generate this error, so it is rare.
Hysteresis errors are lags in the instrument’s altitude indication. It’s created by the natural differences in elasticity among the altimeters’ various internal components and can occur after an aircraft has maintained a constant altitude for an extended period of time. During that time, the component’s inertia tends to resist movement. Introduce a large-scale altitude change and the altimeter may lag. A typical example results in altimeter readings higher than actual after a rapid descent. This type of error either does not exist or is minimal during climbs and slow descents or after leveling at a new altitude.
Many of these errors can be minimized or eliminated altogether by regular instrument maintenance and proper care. At the same time, these errors may only present themselves in high-performance aircraft. The 24-month altimeter/static system checks required for flying IFR are designed to uncover these errors and either correct or compensate for them.
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