TrueHeading
Log inEnrol
DGCA Technical General · 37 questions

Radio Altimeters: DGCA Technical General Questions and Answers

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

Practise all 37 questionsFlashcards

About this topic

Use this page to revise Radio Altimeters for the DGCA Technical General paper. Below are the key ideas first, then 12 practice questions with answers and explanations, picked from the 37 questions in the TrueHeading bank for this topic.

In short: It transmits a radio signal downward and measures the time delay or frequency difference of the reflection.

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

Key concepts: Radio Altimeters

3 ideas to know cold.

How does a radio altimeter measure height?

It transmits a radio signal downward and measures the time delay or frequency difference of the reflection.

h=c Δt2h=\frac{c\,\Delta t}{2}
  • 4.2–4.4 GHz FM-CW
  • Range up to about 2500 ft above ground level
  • Used for DH, GPWS and autoland
transmitreflectedh = c × Δt ÷ 24.2–4.4 GHz FMCWheight above groundaccurate up to ~2500 ft AGL, used for autoland / GPWS

Radio altimeter versus barometric altimeter?

The radio altimeter reads height directly above the terrain and responds to sudden changes; the barometric reads altitude above the pressure datum.

Tip: Steep terrain makes the radio altimeter jump.

How does a radio altimeter work?

It transmits an FM (or pulse) signal to the ground and measures the time delay or beat frequency of the reflection.

h=c t2h=\dfrac{c\,t}{2}
  • 4.2–4.4 GHz
  • Range −20 to 2 500 ft AGL
  • Used for DH in CAT II/III and GPWS

Practice questions: Radio Altimeters

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

Q1Radio waves travel about 984 ft per microsecond. The round-trip delay for a radio altimeter at a true height of 50 ft is approximately:

  1. 203 ns
  2. 102 ns
  3. 51 ns
  4. 1016 ns
Show answer

Answer: B. 102 ns

Delay = 2h / c (the signal travels down and back).

Q2Radio waves travel about 984 ft per microsecond. The round-trip delay for a radio altimeter at a true height of 100 ft is approximately:

  1. 203 ns
  2. 2033 ns
  3. 102 ns
  4. 407 ns
Show answer

Answer: A. 203 ns

Delay = 2h / c (the signal travels down and back).

Q3Radio waves travel about 984 ft per microsecond. The round-trip delay for a radio altimeter at a true height of 250 ft is approximately:

  1. 254 ns
  2. 508 ns
  3. 1016 ns
  4. 5081 ns
Show answer

Answer: B. 508 ns

Delay = 2h / c (the signal travels down and back).

Q4Radio waves travel about 984 ft per microsecond. The round-trip delay for a radio altimeter at a true height of 750 ft is approximately:

  1. 3049 ns
  2. 1524 ns
  3. 762 ns
  4. 15244 ns
Show answer

Answer: B. 1524 ns

Delay = 2h / c (the signal travels down and back).

Q5Radio waves travel about 984 ft per microsecond. The round-trip delay for a radio altimeter at a true height of 1 500 ft is approximately:

  1. 30488 ns
  2. 3049 ns
  3. 6098 ns
  4. 1524 ns
Show answer

Answer: B. 3049 ns

Delay = 2h / c (the signal travels down and back).

Q6Radio waves travel about 984 ft per microsecond. The round-trip delay for a radio altimeter at a true height of 2 500 ft is approximately:

  1. 2541 ns
  2. 10163 ns
  3. 5081 ns
  4. 50813 ns
Show answer

Answer: C. 5081 ns

Delay = 2h / c (the signal travels down and back).

Q7A radio altimeter measures a round-trip delay of 1000 ns (c ≈ 984 ft/µs). The height is approximately (ft):

  1. 246
  2. 1968
  3. 984
  4. 492
Show answer

Answer: D. 492

Height = c × t / 2, because the measured time covers the path down and back.

Q8A radio altimeter measures a round-trip delay of 3000 ns (c ≈ 984 ft/µs). The height is approximately (ft):

  1. 738
  2. 2952
  3. 1476
  4. 5904
Show answer

Answer: C. 1476

Height = c × t / 2, because the measured time covers the path down and back.

Q9A radio altimeter measures:

  1. altitude above the aerodrome using QNH
  2. altitude above sea level
  3. pressure altitude
  4. the height of the aircraft above the terrain directly beneath it
Show answer

Answer: D. the height of the aircraft above the terrain directly beneath it

It measures the true height above ground level (AGL).

Q10The measuring range of a typical radio altimeter is:

  1. 0 to 500 ft only
  2. 0 to 50 000 ft
  3. −20 ft to 2 500 ft AGL
  4. 1 000 to 40 000 ft
Show answer

Answer: C. −20 ft to 2 500 ft AGL

Used for the final approach and landing.

Q11The decision height is set on the radio altimeter for:

  1. take-off
  2. oceanic flights
  3. en-route cruise
  4. CAT II and III approaches and low-visibility operations
Show answer

Answer: D. CAT II and III approaches and low-visibility operations

DH is based on the radio altitude.

Q12Radio altimeter accuracy is affected by:

  1. pitch and bank angle (slant range) and terrain profile
  2. the temperature
  3. the aircraft weight
  4. the pressure
Show answer

Answer: A. pitch and bank angle (slant range) and terrain profile

Large attitudes or rough terrain may degrade accuracy.

Practise all 37 questionsTake a Technical General mock test

More DGCA Technical General topics

Aerodynamics & Performance181 QFlight Controls & Stability38 QAirframe & Structures25 QPowerplant79 QPropellers32 QAircraft Systems116 QPitot Statics35 QBarometric Altimeters61 QVertical Speed Indicators (VSI)32 QAirspeed Indicators24 QAirspeeds41 QMach55 QGyroscope52 QAttitude Indicators20 QTurn and Slip50 QINS, IRS and FMS32 QEFIS24 QAutoflight23 QFlight Director21 QEICAS / ECAM24 QEngine Instruments29 QTemperature Indicators34 QFuel Systems35 QCompass54 QWarning & Recording Systems33 QTCAS39 Q
All DGCA questionsCPL exam syllabusHow to prepareTechnical General formulas

Learn Technical General one-on-one with Nikunj

Unlimited personal sessions, weekly subject tests and lifetime access to the student website. First-attempt pass guarantee, or your money back.

Enquire & enrolDGCA Technical General classes
Call WhatsApp