Altimeter simulator

Free tool · Meteorology · DGCA CPL and ATPL

The red aircraft is where you are; the dotted one is where the altimeter says you are. Make them disagree.

What will your altimeter read?

Fly the altimeter

An altimeter is an aneroid barometer marked in feet. Turn the subscale, move the aircraft, change today's pressure and temperature, and watch the needles. The red aircraft is where you really are; the dotted one is where the altimeter says you are.

  • Sea-level field
  • Field at 600 ft
  • Delhi 778 ft
  • Leh 10,700 ft
  • On the ground: QFE
  • The teacher's example: field elevation 600 ft, QFE 990 hPa. With QFE set, the altimeter reads 0 on the field (the 'zero setting').
  • On the ground: QNH
  • QNH = QFE + 600 ÷ 30 = 990 + 20 = 1010 hPa. With QNH set, the altimeter reads the field elevation, 600 ft.
  • On the ground: 1013
  • With 1013 set it reads the pressure altitude of the field: (1013 - 990) × 30 = 690 ft, not the elevation.
  • High to low: look out below
  • You set 1013 and fly into an area where the pressure has fallen to 1000 hPa. At the same true altitude the altimeter now reads about 390 ft too high. Keep the reading constant and you will descend: High to Low, look out below.
  • Low to high
  • Flying from low to high pressure without resetting, the altimeter under-reads: you are higher than it says.
  • A cold day
  • 20 °C colder than ISA at 10,000 ft: the altimeter over-reads by about 7% (1% for every 3 °C). Warm to cold, look out below.
  • A hot day
  • 15 °C warmer than ISA: the altimeter under-reads by about 5%. You are higher than it says.
  • Leh: QFE off the scale
  • At a very high field the QFE (about 656 hPa here) is below the subscale's range, so it cannot be set. That is why QNE is used at high-altitude airfields.

Memory hook: QFE: zero on the field. QNH: field elevation on the field. 1013: flight levels. High to low (or warm to cold), look out below.

Takeaway: The altimeter only measures pressure and turns it into feet above whatever pressure is set on the subscale. Set QFE and it reads height above the field; set QNH and it reads altitude above MSL; set 1013.25 and it reads pressure altitude. If the pressure falls, or the air is colder than ISA, it over-reads: you are lower than it says.

Will the altimeter over-read or under-read?

Over or under?

Tap a situation, then what the altimeter does.

  • Flying from high to low pressure
  • Flying from low to high pressure
  • Air colder than ISA
  • Air warmer than ISA
  • Wind from port (NH)
  • Wind from starboard (NH)
  • Subscale set higher than the real QNH
  • Subscale set lower than the real QNH

Memory hook: High to Low, or Hot to cold: look out below (over-reads).

Takeaway: Pressure falling, or air colder than ISA: over-reads, you are lower than shown. Pressure rising, or air warmer than ISA: under-reads, you are higher than shown.