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DGCA Technical General · 32 questions

INS, IRS and FMS: DGCA Technical General Questions and Answers

32 practice questions on INS, IRS and FMS 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 INS, IRS and FMS for the DGCA Technical General paper. Below are the key ideas first, then 12 practice questions with answers and explanations, picked from the 32 questions in the TrueHeading bank for this topic.

In short: Accelerometers measure acceleration; integrating twice gives distance and position.

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

Key concepts: INS, IRS and FMS

3 ideas to know cold.

How does inertial navigation work?

Accelerometers measure acceleration; integrating twice gives distance and position.

  • Needs alignment on the ground, stationary, with known position
  • IRS uses ring-laser gyros (strapdown)
  • Position error grows with time, roughly 1–2 NM per hour
  • Schuler period 84.4 minutes

What does the FMS do?

The flight management system holds the navigation and performance databases.

  • Computes position from IRS, GPS and radio updates
  • Provides LNAV and VNAV guidance to the autopilot
  • Optimises speed using the cost index

How does an inertial reference system work?

Accelerometers measure acceleration; integration gives velocity and position. Gyros give orientation.

  • Alignment: needs position and stillness (~10 min)
  • Drift: 1–2 NM/h typical
  • Schuler period 84.4 min
  • Ring laser gyro: Sagnac effect

Practice questions: INS, IRS and FMS

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

Q1An inertial navigation system (INS) is based on:

  1. magnetic field
  2. radio signals
  3. accelerometers and gyros
  4. star tracking
Show answer

Answer: C. accelerometers and gyros

Acceleration is integrated twice to give position.

Q2An INS needs to be aligned on the ground because:

  1. it needs to compare with GPS
  2. its memory is empty
  3. it needs fuel data
  4. it needs its initial position and the direction of true north and the vertical
Show answer

Answer: D. it needs its initial position and the direction of true north and the vertical

Alignment takes about 10 minutes (gyrocompassing).

Q3The INS alignment cannot be completed if:

  1. the aircraft is moved during alignment
  2. the doors are shut
  3. the aircraft is stationary
  4. the radios are on
Show answer

Answer: A. the aircraft is moved during alignment

Movement during alignment will cause a fault.

Q4The INS error in position increases with:

  1. radio signals
  2. pressure altitude
  3. time
  4. distance from the equator
Show answer

Answer: C. time

Typically about 1–2 NM per hour for older systems.

Q5Schuler tuning ensures that:

  1. the platform stays level as the aircraft moves over the curved earth, using an 84-minute pendulum period
  2. the radio is tuned
  3. the batteries are charged
  4. the gyro remains aligned with north
Show answer

Answer: A. the platform stays level as the aircraft moves over the curved earth, using an 84-minute pendulum period

84.4 minutes is the period of a pendulum of earth-radius length.

Q6The FMS (flight management system) combines:

  1. only a compass
  2. radio communications only
  3. navigation, performance and flight guidance functions
  4. weather data only
Show answer

Answer: C. navigation, performance and flight guidance functions

It uses a navigation database to build the flight plan.

Q7The FMS navigation database is updated every:

  1. 7 days
  2. 365 days
  3. 28 days (AIRAC cycle)
  4. 5 years
Show answer

Answer: C. 28 days (AIRAC cycle)

AIRAC cycles are 28 days.

Q8The AIRAC cycle ensures that:

  1. aircraft are maintained
  2. airlines are paid
  3. changes to aeronautical information are made on common effective dates
  4. crews are rested
Show answer

Answer: C. changes to aeronautical information are made on common effective dates

Information is published in advance.

Q9The FMC performance database provides:

  1. ATC clearances
  2. optimum speeds, altitudes and fuel predictions
  3. crew roster
  4. runway lights
Show answer

Answer: B. optimum speeds, altitudes and fuel predictions

Cost index sets the economy speed.

Q10The cost index in the FMS is:

  1. a hotel cost
  2. the price of fuel
  3. the ratio of time cost to fuel cost
  4. a ticket price
Show answer

Answer: C. the ratio of time cost to fuel cost

A higher cost index gives higher speeds.

Q11RNP (required navigation performance) requires:

  1. a radar altimeter
  2. a weather radar
  3. on-board performance monitoring and alerting
  4. a VOR on board
Show answer

Answer: C. on-board performance monitoring and alerting

It is performance-based navigation.

Q12GPS RAIM is used to:

  1. detect traffic
  2. correct the time
  3. measure altitude
  4. verify the integrity of the GPS signals
Show answer

Answer: D. verify the integrity of the GPS signals

Receiver autonomous integrity monitoring.

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More DGCA Technical General topics

Aerodynamics & Performance181 QFlight Controls & Stability38 QAirframe & Structures25 QPowerplant79 QPropellers32 QAircraft Systems116 QPitot Statics35 QBarometric Altimeters61 QRadio Altimeters37 QVertical Speed Indicators (VSI)32 QAirspeed Indicators24 QAirspeeds41 QMach55 QGyroscope52 QAttitude Indicators20 QTurn and Slip50 QEFIS24 QAutoflight23 QFlight Director21 QEICAS / ECAM24 QEngine Instruments29 QTemperature Indicators34 QFuel Systems35 QCompass54 QWarning & Recording Systems33 QTCAS39 Q
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