An altimeter is a barometer wearing a height scale: it measures pressure and merely claims to measure height. This lesson is about the gap between the claim and the truth: the family of settings and readings the claim comes in, the transition procedures that organise them, and the corrections for pressure, temperature and terrain that turn a reading into a real distance from the ground. The ISA (International Standard Atmosphere) numbers of the previous lesson get spent here.
Three questions from this lesson — one straightforward, one that applies it, one that pushes a little further. Nothing is recorded, and you can draw a new set as often as you like.
The transition level is:
Descending toward an aerodrome, transition level FL 55, QNH (sea-level setting) 998 hPa. Passing FL 55 (5,500 ft of pressure altitude) the pilot sets QNH. The reading becomes about:
Indicated altitude 12,000 ft on the correct QNH (sea-level setting); the air is 20 °C colder than ISA (the International Standard Atmosphere). Using the 4-per-cent rule, the true altitude is about:
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Every altimeter reading is a distance from a chosen datum, and the setting knob chooses the datum. Wind a pressure into the subscale and the altimeter reads zero wherever that pressure is found; everything above is measured from there. Three settings matter:
QFE — the pressure at aerodrome level. Set it and the altimeter reads height: the vertical distance above that aerodrome. On its runway, QFE reads zero.
QNH — the aerodrome pressure reduced to mean sea level (MSL) using ISA assumptions. Set it and the altimeter reads altitude: the vertical distance above MSL. On the runway, QNH reads the aerodrome's elevation: the vertical distance of that fixed point above MSL.
The standard altimeter setting, 1,013.25 hPa — no reduction at all, just the ISA's own sea-level pressure. Set it and the altimeter reads pressure altitude: the altitude in the ISA at which today's ambient pressure would be found. Expressed in hundreds of feet it becomes a flight level (FL): FL 85 is 8,500 ft of pressure altitude.
Here is the separation the previous lesson promised: QNH reduces the station pressure to MSL with ISA assumptions; QFF does the same with the actual temperatures. The weather chart wants the atmosphere as it really is, so its isobars are drawn in QFF; the altimeter is built and calibrated as an ISA instrument, so the cockpit gets QNH. Two reductions of the same measurement, one for the meteorologist and one for the pilot.
Three more terms complete the ladder, and none of them can be set on the subscale:
True altitude — the actual vertical distance above MSL.
True height — the actual vertical distance above the ground directly below.
Pressure level — the previous lesson's isobaric surface: where any given pressure actually lies today.
The first two are the real distances the settings only approximate: the altimeter's readings equal them only when the atmosphere happens to match the assumptions behind the setting. The rest of this lesson measures the mismatch.
Transition altitude (TA) — the altitude at or below which position is controlled by altitudes on QNH. It is fixed and published for the aerodrome or the state.
Transition level (TL) — the lowest flight level usable above the transition altitude. Because flight levels stand on the 1013 datum while the TA stands on QNH, the TL is not fixed: it shifts with the day's QNH.
Transition layer — the airspace between TA and TL. It is crossed, not cruised: aircraft climb and descend through it but do not level off inside it.
Terrain clearance — the vertical distance between the aircraft and the terrain beneath it: a true height, which no setting reads directly.
Lowest usable flight level — the lowest FL that still guarantees the required terrain clearance, once the day's QNH and temperature have been accounted for.
The associated altimeter-setting procedure writes itself from the definitions: depart on QNH (or QFE where local procedure uses height), climb through the transition altitude and set 1,013.25; cruise on flight levels; descend through the transition level and set QNH, passed by the controller or the aerodrome broadcast with the descent. The swap points differ deliberately: each crossing happens on the setting being left behind.
Converting between the readings is the first calculation family, and it runs entirely on the previous lesson's rule of thumb: 1 hPa is worth about 30 ft near MSL. The datums are stacked 30 ft apart per hectopascal of difference between their settings, and every reading is measured from its own datum. With a QNH below 1,013, the 1013 surface lies below MSL, so flight-level readings sit higher than the altitude by 30 ft per hectopascal of difference; and with a high QNH the sign reverses.
On the ground the same logic gives the three parked readings: QFE reads zero, QNH reads the elevation, and the standard setting reads the aerodrome's pressure altitude — the elevation shifted by 30 ft for every hectopascal between QNH and 1,013.25. An airfield at 1,200 ft elevation with QNH 1,003 hPa parks its aircraft at a reading of 1,200 + (1,013 − 1,003) × 30 = 1,500 ft with 1013 set.
The altimeter converts pressure to height using the ISA lapse rate, so temperature is baked into the conversion. Cold air is denser than the ISA expects: pressure falls faster on the climb, each hectopascal is used up over fewer feet, and the level the altimeter calls 10,000 ft arrives lower down. In colder-than-ISA air the altimeter over-reads: true altitude is below indicated; in warm air it under-reads and the aircraft is actually higher. The same squeeze acts between any two flight levels: two pressure levels 1,000 ft apart in the ISA lie closer together in a cold column — the packed isobaric surfaces of the previous lesson's cross-section.
Pressure plays the identical trick sideways. Fly at constant indicated altitude from high pressure toward low and the whole QNH datum sinks beneath the aircraft faster than the subscale is updated: true altitude falls although the needle never moves — the old warning runs from high to low, look out below, and it works for temperature as well as pressure. Flying toward high pressure or warm air, the error is on the safe side.
The temperature correction has an exam-sized rule: true altitude differs from indicated by about 4 per cent for every 10 °C of deviation from ISA — subtract in cold air, add in warm. The rule works in both directions, from indicated to true and from true back to indicated, and it is an approximation by design: near enough for the exam and for terrain sums, not a precision instrument. It treats heights exactly as it treats altitudes: the same percentages apply above any datum. At 10,000 ft indicated in air 10 °C below ISA, the true altitude is about 10,000 − 400 = 9,600 ft; to really be at 10,000 ft in that air, indicate about 10,400 ft.
The corrections stack into one chain: the calculation that decides the lowest usable flight level over high ground. Work from the rock upward: the terrain plus the required clearance gives the true altitude the aircraft must really have; the temperature correction turns that into the altitude the altimeter must show; the QNH-to-1013 step turns the altitude into a pressure altitude; and the first available flight level at or above it is the lowest usable one.
One last thief of clearance is the terrain itself. Wind forced over a ridge accelerates, and accelerating air trades static pressure for speed: the Bernoulli effect. The altimeter senses that locally lowered pressure and converts it, as always, into height: over the crest it over-reads exactly where the terrain is closest. The effect is qualitative exam material, but it stacks onto cold air and falling pressure as the third reason mountain clearances are smaller than the needle claims.
Altimetry is the most calculated corner of the whole subject: the definitions are recall, but nearly everything else is arithmetic with the 30 ft and 4 per cent rules. The overview, following the learning objectives:
Define height, altitude, pressure altitude, FL, pressure level, true altitude, true height, elevation, QNH, QFE and the standard setting, relate them, and name the setting behind each reading
Describe transition altitude, level and layer, terrain clearance and the lowest usable flight level
Apply the setting procedures: 1013 climbing through TA, QNH descending through TL
Calculate readings under different settings, on the ground and through the transition, at 30 ft per hPa
Explain the temperature and pressure-area effects on true altitude and apply the 4-per-cent rule in both directions
Calculate terrain clearance and the lowest usable flight level for given temperature and pressure
Describe qualitatively how accelerated airflow over topography affects the altimeter