The Altimeter - ATPL 022 — Instrumentation

The altimeter is the sealed aneroid capsule of the first lesson grown into the most consequential instrument on the panel: it converts static pressure into a height, and every vertical clearance, procedure altitude and flight level stands on that conversion. This lesson covers the instrument itself, the vocabulary of the altitudes it can be asked to show, its errors, the correction tables that tame the worst of them, and what to do when its static supply or its honesty fails.

Feet and metres

Altimeters read in feet in most of the world; some states (China among them) operate in metres. The relationship: 1 metre = 3.28 feet, so 1,000 m is about 3,280 ft and 10,000 ft about 3,048 m. Crossing between the two conventions is a conversion-table task on the flight deck: misapplying the factor turns a metric level into an error of thousands of feet, which is why the table, not mental arithmetic, does the job.

The vocabulary of vertical position

Height — vertical distance above a specific datum, in practice the aerodrome: what the altimeter shows with QFE set.
Altitude — vertical distance above mean sea level: what it shows with QNH set.
Indicated altitude — whatever the instrument displays with the pressure currently set in its subscale (the pressure-setting window); true altitude is the aircraft's actual vertical distance above mean sea level. The two agree only when the setting is right and the air column matches ISA.
Pressure altitude — the altitude read with 1013.25 hPa set: the aircraft's level in the standard atmosphere, the basis of flight levels.
Density altitude — pressure altitude corrected for non-ISA temperature: the altitude at which the standard atmosphere would have the day's actual air density. It is a performance number (the engine and wing feel density), not something the altimeter displays.

The barometric references

QNH — the aerodrome pressure reduced to mean sea level: set it and the altimeter reads altitude, showing the aerodrome's elevation on touchdown.
QFE — the pressure at the aerodrome itself: set it and the altimeter reads height above that aerodrome, zero on touchdown.
1013.25 hPa (the standard setting) — set above the transition altitude (the published altitude where the reference changes from QNH to standard), making every aircraft's altimeter count from the same imaginary surface: readings become flight levels, comparable between aircraft regardless of the weather below.

How the instrument works

The case is fed static pressure; inside sits the sealed capsule, partially evacuated, with a spring keeping it from collapse. Climb, and the static pressure around the capsule falls, so the capsule expands; a linkage and gear train magnify that small expansion into pointer rotation. The subscale knob rotates the mechanism's datum, which is how one instrument can answer in QNH altitude, QFE height or standard-setting flight level: the capsule always measures the same static pressure, and the knob chooses which pressure counts as zero.

Three altimeter designs

Simple altimeter — a single capsule and direct linkage: cheap, robust, coarse. Adequate where a few tens of feet do not matter.
Sensitive altimeter — a stack of capsules adds their expansions together, giving a finer, more repeatable movement, a multi-pointer display and a subscale. The classic mechanical altimeter of instrument flying.
Servo-assisted altimeter — the capsule no longer drives the display at all: an electrical pick-off senses its tiny movement and a servo motor drives the counters and pointer. Friction and lag largely disappear, and accuracy at altitude improves decisively, which is the reason particular airspace demands the design: at high levels a given pressure change spans many more feet, and the reduced vertical separation of RVSM airspace is built on altimetry only servo-class accuracy can deliver.

Reading the displays

Four presentations, in rising order of clarity:

Single pointer — one needle and one revolution per thousand feet, simple and unambiguous within its range.
Multi-pointer — three needles for hundreds, thousands and tens of thousands: compact, and notorious, because misreading which needle is which has put aircraft ten thousand feet from where the crew believed, the classic misread of instrument-flying history.
Drum-pointer — a rolling digital drum for the thousands with a single pointer for the hundreds: the misreading removed.
Vertical straight scale — the tape of electronic displays, the current standard, with the exact value in a window on the tape.

The error catalogue

Static system error — the pitot-static lesson's position error arriving here: the port's location distorts the sensed static pressure, so the altimeter inherits the distortion.
Instrument error — tolerances, friction and hysteresis in the mechanism; calibrated down, never to zero.
Barometric error — the subscale holds a wrong or stale pressure. Each hectopascal is worth about 27 feet near sea level: fly from high pressure toward low with the old QNH set and the altimeter flatters you, indicating higher than you truly are.
Temperature error — the altimeter's conversion assumes the ISA air column. In colder-than-ISA air the column is compressed, pressure falls faster with height, and the aircraft holding an indicated altitude is truly lower than indicated. Cold and low pressure fail in the same direction, which is what the saying compresses: from high to low, look out below.
Lag — the mechanism needs time to follow a change of height, so during climbs and descents the reading trails reality slightly. Servo-assisted designs all but remove it.

Using the correction tables

Because the temperature error is systematic, it is corrected by table or by rule rather than accepted. The working rule behind the published tables: the error is about 4% of the height above the aerodrome for every 10 °C below ISA, added to the altitudes that must be protected (minimum sector, procedure and decision altitudes) so that flying the corrected indication puts the aircraft at the intended true altitude.

The position-error correction table in the flight manual works the same way for the static system error: enter it with the configuration and speed, read the correction, apply it to the indicated altitude. Worked once: gear down at 130 kt, the table row reads +30 ft, so an indicated 1,970 ft is a corrected 2,000 ft. On modern aircraft the air-data computer applies this one silently; the table remains for the standby instruments and for the exam.

Blockage and leakage of the static line

A blocked static line (ice, water, insects, or tape left over the port) traps the pressure of the moment of blockage: the altimeter freezes at the altitude where the blockage occurred and ignores every climb and descent after it, the full deception treated with the ASI's blockages in the next lesson. A leak in the static line lets the line take up the pressure around the leak instead of the port's. In an unpressurised airframe that means slightly low cabin-like pressure and a slightly high reading, the alternate-static story again. In a pressurised fuselage the consequence is severe: the line takes the cabin's pressure, and the altimeter reads something near the cabin altitude — perhaps 8,000 ft while the aircraft cruises at 36,000 ft, an under-read of nearly the whole cruise altitude.

The GPS cross-check and its limits

When an altimeter indication is suspect, satellite navigation offers an independent opinion: GPS altitude is geometric, computed from satellite ranges with no pressure involved, so it survives every pressure-system failure and setting blunder, and a gross disagreement points squarely at the barometric side. Its limits keep it a cross-check, not a replacement. GPS altitude is measured against a mathematical model of the Earth, so it can sit tens of feet or more from true altitude even when healthy; its vertical accuracy is inherently poorer than its horizontal accuracy; and above all, ATC separation and terrain procedures are built on barometric altimetry: every aircraft in the same air makes the same barometric errors, so the safe comparison between aircraft is barometric, and flying GPS altitude instead would step outside the system everyone else is using.

What the exam asks

The barometric references, the cold-air correction arithmetic and the blockage behaviours are the bank's favourites here. The overview, following the learning objectives:

List the altimeter's units and their relationship

Define height, altitude, indicated, true, pressure and density altitude

Define QNH, QFE and the standard setting

Explain the altimeter's operating principles

Describe and compare the simple, sensitive and servo-assisted designs and why airspace may require one

Give examples of the display types

Describe the static-system, instrument, barometric, temperature and lag errors

Demonstrate the use of correction tables for temperature and position errors

Describe the effects of static-line blockage and leakage

Describe the GPS-altitude cross-check and its limitations