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Altimetry
Meteorology · Chapter 9

Altimetry

The altimeter and its settings

6 min read
Written from Joshi 2, Pressure Settings Oxford Vol 9, 9, The Altimeter and Altimeter Settings

An altimeter is a pressure gauge with a dial marked in feet. It cannot sense height directly. It reads the outside pressure and converts it to an altitude using a fixed rule, the ISA. Everything that goes wrong in this chapter comes from the real atmosphere not matching that rule.

How it works

The instrument is an aneroid capsule that expands as pressure falls. It is calibrated to the ICAO Standard Atmosphere, so every altimeter shows the same altitude for the same pressure. A subscale lets the pilot set which pressure counts as the datum, moving the needle to suit.

The four words

TermMeaning
AltitudeVertical distance above mean sea level
HeightVertical distance above a stated datum, such as the aerodrome
ElevationVertical distance of a fixed point, an aerodrome or an obstacle, above mean sea level
Flight levelA constant pressure surface referenced to 1013.25 hPa, quoted in hundreds of feet. FL350 is 35 000 ft on the standard setting

The settings

  • QFE. Aerodrome datum pressure. Reads zero on the ground and the approximate height above the aerodrome when airborne. Rounded down to the whole hectopascal.
  • QNH. QFE reduced to mean sea level by the ISA. Reads aerodrome elevation on the ground and the approximate altitude when airborne. Rounded down to the whole integer.
  • Standard, 1013.25 hPa. Reads pressure altitude. Used above the transition altitude, where levels are flight levels.
  • Regional, or Forecast, QNH. The lowest QNH forecast for an area for the next hour. Because the real pressure will be higher, the altimeter reads low, which is the safe side.
A relationship that is examined At an aerodrome at mean sea level, QFE = QNH = QFF. At an aerodrome below mean sea level, you drop further to reach the sea level datum, so pressure rises and QFE is greater than QNH.

Pressure, temperature and true altitude

7 min read
Written from Joshi 2, altimetry rules, L H L and H L H Oxford Vol 9, 9, altimeter responding to pressure and temperature

Flying at a constant indicated altitude means flying along a surface of constant pressure. If that surface tilts, your true altitude changes while the needle does not move. Two things tilt it: a change of surface pressure, and a change of air temperature.

Pressure

Fly towards lower pressure without resetting the subscale and the pressure surfaces dip, so your true altitude falls. Fly towards higher pressure and they rise, so your true altitude rises.

Exam wording Higher pressure, true altitude greater than indicated. Lower pressure, true altitude less than indicated. The old rule, from high to low, look out below.

Temperature

Cold air is dense, so pressure falls quickly with height and the surfaces sag. Warm air is thin, so they bulge up. Fly into colder air at a constant indicated altitude and your true altitude falls. Fly into warmer air and it rises.

Exam wording Warmer than ISA, true altitude greater than indicated. Colder than ISA, true altitude less than indicated.
Put the two together Anything that makes the air heavier below you, low pressure or cold air, pushes the pressure surface down and your true altitude below the reading. High pressure or warm air does the opposite. Pressure altitude equals true altitude only in full ISA conditions.
Interactive Flying along a constant indicated altitude
Indicated altitudeConstant
True altitudeRising
True vs indicatedTrue above indicated
Terrain riskLow
The green dashed line is what the altimeter shows, held constant. The orange line is where the aircraft actually is. Falling pressure or colder air tilts the true path downward, towards the ground, while the needle never moves.

The two altimeter errors

7 min read
Written from Joshi 2, altimeter over reading and under reading Oxford Vol 9, 9, Altimeter Errors and Temperature Error Correction

Beyond ordinary instrument slop, two errors matter for the exam, and both can be worked with a single number.

Barometric error

Caused by setting the wrong pressure on the subscale.

The figure to use The whole meteorology syllabus works barometric error at 27 ft per 1 hPa. Set the subscale 1 hPa too high and the altimeter reads about 27 ft too high.

Temperature error

The altimeter is calibrated to the ISA. When the air is not at ISA temperature, the reading is wrong. The direction is the one that catches people out:

Exam wording When the air is colder than ISA, the true altitude is lower than the altimeter reading. On the ground with QNH set, the altimeter still reads the correct elevation whatever the temperature.
The correction 4 ft per °C of ISA deviation, per 1000 ft of indicated height above the setting datum. So at 3000 ft above the datum on a day 20°C below ISA, the error is 4 × 20 × 3 = 240 ft, and the true altitude is 240 ft below the reading.
Interactive Temperature error on the approach
indicated true
Indicated height3 000 ft
Temperature error−360 ft
True height2 640 ft
Colder than ISATrue is below the reading
On a cold approach the true height is the shorter orange bar. The gap is 4 ft per degree of ISA deviation for every 1000 ft above the datum, which is why cold weather corrections are added to published approach minima.

Terrain clearance and minimum safe flight level

6 min read
Written from Joshi 2, Standard Isobaric Levels Oxford Vol 9, 9, Terrain clearance to Transition Layer

Put the two errors together and you can find where the aircraft really is over high ground, and how high a flight level has to be planned to stay safe.

The combined correction

The formula true altitude = indicated altitude + (indicated altitude / 1000 × ISA deviation × 4) + 27 × (actual pressure − pressure setting)

Worked over Mont Blanc at FL180, mean sea level pressure 983 hPa, air 25°C colder than ISA: the pressure term is 30 × 27 = 810 ft down to 17 190 ft, and the temperature term is 4 × 25 × 18 = 1800 ft down to a true altitude of about 15 390 ft. The summit is 15 872 ft, so without the correction the aircraft would be flown into the mountain.

Minimum safe flight level

The minimum indicated pressure altitude, on 1013 hPa, that keeps the aircraft at or above the safety altitude for a route section. Because both errors are worst in cold, low pressure air, you plan with the lowest forecast QNH and the lowest temperature for each section, apply both corrections, then round up to a usable flight level.

Which conditions are safest A flight level clears the terrain by the greatest margin in warm air with a high QNH, because both put the true altitude above the indicated. Cold air with a low QNH is the dangerous combination.

Transition

TermMeaning
Transition altitudeThe altitude at or below which vertical position is set by QNH
Transition levelThe lowest flight level, on 1013 hPa, available above the transition altitude
Transition layerThe airspace between the two

Standard pressure levels

The pressure surfaces used on upper charts sit at roughly fixed ISA heights, worth carrying for the quiz:

Pressure levelApprox ISA height
850 hPa5 000 ft, FL50
700 hPa10 000 ft, FL100
500 hPa18 000 ft, FL180
400 hPa24 000 ft, FL240
300 hPa30 000 ft, FL300
200 hPa38 000 ft, FL380