What is altimeter scale error and how does it affect altitude readings?

Altimeter scale error occurs when the mechanical linkage between the aneroid capsule and the pointer produces inaccurate readings at certain portions of the altitude range, even though the instrument may appear correct at other points. This means the altimeter could display a precise reading at one altitude while being significantly off at another, posing a serious safety risk. The FAA requires altimeter calibration checks across multiple points on the scale to detect and correct these non-linear mechanical inaccuracies.

How do you calibrate an aircraft altimeter and what standards must be met?

Altimeter calibration is performed by subjecting the instrument to controlled pressure changes in a certified test bench that simulates various altitudes, then comparing the instrument's indications against known pressure values. Per 14 CFR Part 43, Appendix E, altimeters used in instrument flight must be tested and inspected within the preceding 24 calendar months by an appropriately rated repair station or the aircraft manufacturer. The calibration must confirm that scale error does not exceed the tolerances specified in the Appendix E test-altitude table, which allow a smaller margin at lower altitudes and progressively wider margins at higher altitudes.

What's the difference between altimeter calibration error and altimeter setting error?

Altimeter calibration error is an internal mechanical inaccuracy caused by imperfect linkages, worn components, or manufacturing tolerances within the instrument itself, meaning it will produce incorrect readings even when the correct altimeter setting is applied. Altimeter setting error, by contrast, occurs when the pilot or technician dials in an incorrect barometric pressure reference using the Kollsman window, shifting all readings uniformly up or down. As explained in the Pilot's Handbook of Aeronautical Knowledge, correcting altimeter setting error is as simple as entering the proper local altimeter setting from ATIS or ATC, whereas calibration error requires maintenance action by a qualified AMT or repair station.

Altimeter Calibration and Scale Error

The altimeter is one of the most safety-critical instruments aboard any aircraft. It translates atmospheric pressure into a displayed altitude reading, giving pilots the situational awareness they need to terrain avoidance, instrument approaches, and regulatory compliance. But like every mechanical and aneroid device, altimeters are subject to drift, wear, and manufacturing imperfections that can cause them to read incorrectly. For the Aviation Maintenance Technician (AMT) working on airframe systems, understanding altimeter calibration and scale error is not simply a knowledge-test topic — it is a genuine airworthiness responsibility rooted in 14 CFR Part 91 and the standards published in the FAA's Aviation Maintenance handbooks.

This article walks through how an altimeter works, what causes scale error, how calibration is performed and documented, and what the regulations demand before an aircraft with a pitot-static altimeter system can be returned to service for instrument flight rules (IFR) operations.

How the Altimeter Works

An altimeter is an aneroid barometer calibrated to read in feet (or meters) of altitude rather than units of pressure. Inside the instrument case, one or more sealed, corrugated aneroid wafers — sometimes called capsules — expand and contract in response to changes in static air pressure delivered through the aircraft's static port. As the aircraft climbs, ambient pressure decreases, the capsules expand, and a series of mechanical links, levers, and gears translate that expansion into a clockwise rotation of the altimeter's pointers. Descent compresses the capsules and reverses the pointer movement.

A manually adjustable kollsman window allows the pilot to dial in the current altimeter setting (local sea-level pressure in inches of mercury, or in hectopascals outside the United States). This effectively references the instrument to a known pressure datum. Standard atmosphere at sea level is 29.92 in Hg (1013.25 hPa); each 1,000-foot change in altitude corresponds to roughly 1 in Hg change in pressure under standard conditions.

What Is Scale Error?

Scale error — sometimes called calibration error or scale factor error — occurs when the mechanical linkage inside the altimeter does not translate pressure changes into pointer movement with uniform accuracy across the full range of the instrument's scale. In other words, the altimeter may be accurate at one altitude but read high or low at another. This non-linearity can arise from several sources:

Worn or fatigued aneroid capsules: Over time, the metal wafers lose elasticity and do not respond with the same sensitivity they had when new, causing compression of scale markings at some altitudes.
Mechanical linkage wear: Gear teeth, sector arms, and hairsprings can develop wear, slop, or corrosion that introduces variable friction into the pointer movement.
Manufacturing tolerances: Even new altimeters have small deviations between the theoretical and actual pressure-to-altitude conversion built into the aneroid stack and gear train.
Temperature effects: Metals expand and contract. Temperature-induced dimensional changes in gears and capsule material can alter the effective scale at altitude extremes.
Hysteresis: The capsule material exhibits hysteresis — a lag where the expansion path differs from the compression path — so readings obtained while climbing may differ from those obtained while descending through the same pressure level.

Scale error is distinct from position error (caused by the static port location in disturbed airflow) and instrument error from friction or jewel defects, though all three can occur simultaneously and compound each other.

Regulatory Requirements for Altimeter Testing

Under 14 CFR §91.411, no person may operate an aircraft under IFR in controlled airspace unless, within the preceding 24 calendar months, the aircraft's altimeter system and altitude reporting equipment have been tested and inspected in accordance with appendix E to Part 43. This means an IFR-equipped aircraft needs a fresh altimeter system check at least every two years. The test must be performed by either the manufacturer, a certificated repair station, or a certificated mechanic with an airframe rating who has the equipment and expertise to do so correctly.

The check encompasses the entire pitot-static system — not just the altimeter face — including all connecting lines, fittings, and the static source itself. After any maintenance that could affect system integrity (such as an avionics installation or repair of a static line), an additional leak check is required before the aircraft is returned to IFR service.

Calibration Standards and Test Points

Appendix E to 14 CFR Part 43 specifies the altimeter system test criteria in detail. During calibration, a certified technician connects precision test equipment to the static system, evacuates the system to simulate various altitudes, and compares the altimeter reading to the known test altitude. The key performance standards include:

Scale error limits: Appendix E specifies a table of allowable tolerances that start smaller at low test altitudes (on the order of ±20 to ±30 feet) and increase progressively as test altitude increases, up to roughly ±80 feet near 20,000 feet pressure altitude — it is not a single flat ±75-foot tolerance across that entire range. Above 20,000 feet, the allowable tolerance broadens further per the Appendix E table.
Hysteresis check: The instrument is brought up to a test altitude, held there, then brought back down through the same point. The allowable difference between the ascending and descending readings at a given pressure level is defined by the specific tolerances in the Appendix E test-altitude table, rather than a single flat value applied uniformly at or below 10,000 feet.
After effect: After returning to the starting pressure, the altimeter must return to within 30 feet of its original reading to demonstrate acceptable capsule recovery.
Friction: When the instrument case is gently tapped (simulating vibration), the pointer must not move more than 25 feet from its pre-tap reading.
Case leak: Any leak in the altimeter's own case must not cause a reading drift exceeding 100 feet per minute once the system is stabilized.
Kollsman window range: The barometric correction mechanism must be able to be adjusted across a range of at least 28.00 to 31.00 in Hg, ensuring pilots can set realistic local altimeter settings at either pressure extreme.

All findings — including which test altitudes were checked, the observed errors, and whether the instrument passed or failed — must be documented in the aircraft maintenance records per 14 CFR §43.9, with the date, description of work, the name and certificate number of the person performing the inspection, and an airworthiness statement or notation of the defect found.

Performing the Calibration Check in Practice

A properly equipped avionics or pitot-static test bench applies precisely controlled, traceable vacuum to the static port connection. The technician reads the altimeter at each required test point, compares it against the known reference altitude on the test equipment, and records the deviation. If an altimeter fails any test criterion, it must be removed from service and sent to a certificated repair station or the manufacturer for internal adjustment or overhaul. An AMT airframe technician does not typically perform internal altimeter repairs — the watch-like precision required for gear train adjustment is beyond normal airframe maintenance — but the technician is responsible for the system-level check and for making the airworthiness determination.

When reconnecting the static system after any test, extreme care is required to ensure all fittings are secure and no condensation or debris has entered the lines. A final leak check of the completed system is mandatory. Even a small static system leak can cause the altimeter to read higher than actual altitude in a pressurized system or introduce erratic readings as pressure changes with altitude.

Why Altimeter Calibration Matters for Safety

An altimeter that reads 200 feet high on an ILS approach could place an aircraft well below the published decision altitude before the pilot believes they have reached it. At night or in instrument meteorological conditions, that silent error could be the difference between a safe missed approach and controlled flight into terrain. Similarly, an altitude reporting transponder that disagrees with the aircraft's altimeter by more than the allowable tolerance creates conflicts with ATC and Traffic Alert and Collision Avoidance System (TCAS) logic in other aircraft. Accurate, calibrated altimeters are therefore a foundational element of the National Airspace System's safety architecture.

Key Numbers and Rules

24 calendar months — maximum interval between altimeter system tests for IFR flight under 14 CFR §91.411.
±20 to ±80 feet (progressive by altitude) — allowable scale error per the Appendix E test-altitude table, increasing from lower altitudes up to roughly 20,000 feet pressure altitude, rather than a single flat tolerance.
±30 feet — maximum after-effect allowance (pointer must return within 30 feet of initial reading after a full test cycle).
25 feet — maximum friction error allowable when the instrument case is gently tapped.
100 feet per minute — maximum allowable leak drift rate within the altimeter case.
28.00 to 31.00 in Hg — minimum required range of the kollsman barometric adjustment window.
14 CFR §43.9 — documentation requirements for maintenance records after any inspection or repair.
14 CFR Part 43, Appendix E — the specific test criteria and tolerances governing the altimeter system test.

Common Test Traps

Confusing the inspection interval with a hard expiration: The 24-month rule is a calendar month interval, not a flight-hour limit. An aircraft that flies rarely still needs the check if 24 calendar months have passed.
Assuming the altimeter alone is the system: The regulation requires inspection of the entire pitot-static system, including lines and static ports. Replacing just the altimeter face and calling the system compliant is incorrect.
Misidentifying who may perform the test: An airframe-rated certificated mechanic may perform the system-level test if properly equipped; internal altimeter repair requires a certificated repair station or manufacturer.
Mixing up error types: Scale error (mechanical linkage inaccuracy) is not the same as position error (static port placement in airflow). Exam questions sometimes present both in the same scenario.
Forgetting post-maintenance leak checks: Any time the static system is opened — even for an unrelated avionics job — a leak check is required before IFR flight. Technicians sometimes overlook this when the altimeter itself was not the focus of the work.