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DGCA Technical General · 61 questions

Barometric Altimeters: DGCA Technical General Questions and Answers

61 practice questions on Barometric Altimeters with answers and explanations, plus the key concepts and formulas, from the TrueHeading question bank.

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About this topic

Use this page to revise Barometric Altimeters for the DGCA Technical General paper. Below are the key ideas first, then 12 practice questions with answers and explanations, picked from the 61 questions in the TrueHeading bank for this topic.

In short: A stack of aneroid capsules expands as static pressure falls. Gears turn the pointers.

Open the full set in the student zone to answer every question, track your accuracy and take timed mock tests.

Key concepts: Barometric Altimeters

3 ideas to know cold.

How does a barometric altimeter work?

A stack of aneroid capsules expands as static pressure falls. Gears turn the pointers.

  • Calibrated to ISA
  • Three pointers: 10 000 ft, 1000 ft, 100 ft
  • Subscale sets the reference pressure

List altimeter errors.

Instrument, barometric, temperature, position and lag errors.

  • Temperature error: about 4 ft per 1000 ft per °C
  • Hysteresis and lag after a rapid climb or descent
  • Mountain effect: Venturi in strong winds over ridges can lower static pressure

What are the errors of a barometric altimeter?

Instrument (manufacturing), position (static source), lag, temperature (non-ISA), pressure (setting) and blockage.

  • Pressure error 27–30 ft per hPa
  • Temperature error 4 ft per 1 000 ft per °C

Practice questions: Barometric Altimeters

Tap “Show answer” after you have tried each one.

Q1The correct QNH is 1013 hPa but the pilot sets 1003 hPa. Taking 30 ft per hPa, the altimeter reads:

  1. 600 ft higher than actual altitude
  2. 300 ft higher than actual altitude
  3. 300 ft lower than actual altitude
  4. 150 ft lower than actual altitude
Show answer

Answer: C. 300 ft lower than actual altitude

A higher pressure setting makes the altimeter read higher, so a setting 10 hPa below the correct value gives 300 ft lower than the truth. Setting too high is dangerous (aircraft lower than indicated).

Q2The correct QNH is 1013 hPa but the pilot sets 1019 hPa. Taking 30 ft per hPa, the altimeter reads:

  1. 90 ft higher than actual altitude
  2. 180 ft lower than actual altitude
  3. 360 ft lower than actual altitude
  4. 180 ft higher than actual altitude
Show answer

Answer: D. 180 ft higher than actual altitude

A higher pressure setting makes the altimeter read higher, so a setting 6 hPa above the correct value gives 180 ft higher than the truth. Setting too high is dangerous (aircraft lower than indicated).

Q3The correct QNH is 1009 hPa but the pilot sets 1013 hPa. Taking 30 ft per hPa, the altimeter reads:

  1. 120 ft lower than actual altitude
  2. 240 ft lower than actual altitude
  3. 60 ft higher than actual altitude
  4. 120 ft higher than actual altitude
Show answer

Answer: D. 120 ft higher than actual altitude

A higher pressure setting makes the altimeter read higher, so a setting 4 hPa above the correct value gives 120 ft higher than the truth. Setting too high is dangerous (aircraft lower than indicated).

Q4The correct QNH is 1009 hPa but the pilot sets 1019 hPa. Taking 30 ft per hPa, the altimeter reads:

  1. 600 ft lower than actual altitude
  2. 300 ft higher than actual altitude
  3. 150 ft higher than actual altitude
  4. 300 ft lower than actual altitude
Show answer

Answer: B. 300 ft higher than actual altitude

A higher pressure setting makes the altimeter read higher, so a setting 10 hPa above the correct value gives 300 ft higher than the truth. Setting too high is dangerous (aircraft lower than indicated).

Q5The correct QNH is 1023 hPa but the pilot sets 1019 hPa. Taking 30 ft per hPa, the altimeter reads:

  1. 240 ft higher than actual altitude
  2. 60 ft lower than actual altitude
  3. 120 ft lower than actual altitude
  4. 120 ft higher than actual altitude
Show answer

Answer: C. 120 ft lower than actual altitude

A higher pressure setting makes the altimeter read higher, so a setting 4 hPa below the correct value gives 120 ft lower than the truth. Setting too high is dangerous (aircraft lower than indicated).

Q6The correct QNH is 1001 hPa but the pilot sets 1009 hPa. Taking 30 ft per hPa, the altimeter reads:

  1. 480 ft lower than actual altitude
  2. 240 ft higher than actual altitude
  3. 240 ft lower than actual altitude
  4. 120 ft higher than actual altitude
Show answer

Answer: B. 240 ft higher than actual altitude

A higher pressure setting makes the altimeter read higher, so a setting 8 hPa above the correct value gives 240 ft higher than the truth. Setting too high is dangerous (aircraft lower than indicated).

Q7Indicated altitude 4 000 ft with QNH set outside air temperature −15 °C from ISA. Using 4 ft per 1 000 ft per °C the true altitude is approximately (ft):

  1. 3760
  2. 3520
  3. 4240
  4. 4000
Show answer

Answer: A. 3760

Correction = 4 × (−15) × 4 = −240 ft. Colder than ISA → true altitude lower than indicated; warmer → higher.

Q8Indicated altitude 6 000 ft with QNH set outside air temperature −10 °C from ISA. Using 4 ft per 1 000 ft per °C the true altitude is approximately (ft):

  1. 6000
  2. 5520
  3. 6240
  4. 5760
Show answer

Answer: D. 5760

Correction = 4 × (−10) × 6 = −240 ft. Colder than ISA → true altitude lower than indicated; warmer → higher.

Q9Indicated altitude 8 000 ft with QNH set outside air temperature −10 °C from ISA. Using 4 ft per 1 000 ft per °C the true altitude is approximately (ft):

  1. 7360
  2. 8320
  3. 8000
  4. 7680
Show answer

Answer: D. 7680

Correction = 4 × (−10) × 8 = −320 ft. Colder than ISA → true altitude lower than indicated; warmer → higher.

Q10With QNH set, the altimeter reads on landing:

  1. zero
  2. the aerodrome elevation
  3. the pressure altitude
  4. the elevation plus 1 000 ft
Show answer

Answer: B. the aerodrome elevation

QNH is the pressure that gives elevation on the ground.

Q11The altimeter lag error is greatest in:

  1. a stable approach
  2. level flight
  3. a standard rate turn
  4. a fast climb or descent
Show answer

Answer: D. a fast climb or descent

Friction and inertia delay the response.

Q12The SSEC (static source error correction) is applied to:

  1. correct the compass
  2. correct the lag
  3. correct temperature errors
  4. compensate for position error automatically
Show answer

Answer: D. compensate for position error automatically

Applied in the ADC.

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