Directional Gyro Indicator (DGI)Navigation — Instrumentation — DGCA CPL practice questions
Question 1 of 10
A directional gyro indicator is basically a:
All 10 questions — Directional Gyro Indicator (DGI)
Navigation — Instrumentation · DGCA CPL. The correct option is marked on each.
Q1. A directional gyro indicator is basically a:
- A.horizontal axis earth gyro
- B.horizontal axis tied gyro✓
- C.vertical axis earth gyro
- D.vertical axis tied gyro
Why: The DGI employs a tied gyro (not a free/earth gyro) because its rotor axis is maintained in the yawing plane of the aircraft by the erection system (air jets + wedge plate). The rotor axis is horizontal in level flight (not vertical). See Section 2 . — The distinction between "earth gyro" and "tied gyro" is critical. Earth gyro = spin axis free. Tied gyro = spin axis maintained in a specific plane by an erection system. The DGI ties its axis to the yawing plane; the AH ties its axis to the vertical.
Q2. Apparent drift may be corrected in a DGI by:
- A.causing the gyro to precess in a clockwise direction (in the northern hemisphere)
- B.attaching a bias weight to the inner gimbal which makes the gyro precess in azimuth in the same direction as apparent wander
- C.correcting wander by means of air jets
- D.attaching a bias weight to the inner gimbal which makes the gyro precess in azimuth in the opposite direction to apparent wander✓
Why: The latitude nut (rider nut) on the inner gimbal introduces a real drift that is equal and opposite to the apparent drift at the chosen latitude. In the northern hemisphere, apparent drift causes the reading to decrease (negative), so the nut is set to produce a positive (increasing) drift to cancel it. See Section 10 . — The latitude nut is an example of "fighting fire with fire" — introducing a controlled real drift to cancel an uncontrolled apparent drift.
Q3. An air driven DGI is corrected for apparent wander at 56°N. If the aircraft is maintaining constant DGI readings:
- A.when flying north from 56°N the true heading of the aircraft will decrease
- B.when flying east from 56°N the true heading will decrease
- C.when flying south from 56°N the true heading will decrease✓
- D.when flying west from 56°N the true heading will increase
Why: The DGI is corrected (latitude nut set) for 56°N, meaning the nut produces a +12.9°/h real drift (to counter the apparent −12.9°/h at 56°N). When the aircraft flies south, the apparent drift rate reduces (lower latitude, lower sin value), but the latitude nut still applies its fixed +12.9°/h over-correction. The net drift is positive (reading increases on the DGI). If the DGI reading is held constant, the pilot is correcting for the increase by turning the aircraft onto a heading that is west of north. West of north means the true heading is decreasing. See Section 10 . — This question test…
Q4. The formula used to calculate apparent wander of a directional gyro in the northern hemisphere is:
- A.+15 sine latitude in degrees for the time of running
- B.+15 sine latitude in degrees per hour
- C.−15 sine latitude in degrees per hour✓
- D.15 sine latitude in degrees per hour increasing
Why: In the northern hemisphere, the DGI reading decreases due to earth's rotation — hence the negative sign. The formula is: Apparent Drift Rate = −15 × sin(latitude) °/hour. See Section 9 . — The sign is the whole point here. Southern hemisphere = +15 sin lat (reading increases). Northern hemisphere = −15 sin lat (reading decreases).
Q5. Errors of the directional gyro are:
- A.acceleration error, turning error, altitude error, transport wander, rotor speed error
- B.gimballing error, random wander, apparent wander, rotor speed error, transport wander✓
- C.gimballing error, looping error, rolling error, rotor speed error, transport wander
- D.transport wander, apparent wander, latitude error, turning error, acceleration error
Why: These are the five principal errors listed in the textbook. The DGI has no magnetic element, so it has no acceleration or turning errors in the compass sense. See Section 6 . — Memorize the 5 DGI errors: GRATRs — Gimballing, Random, Apparent, Transport, RPM (rotor speed).
Q6. The spin axis of a directional gyro is maintained in ....... by means of ...... in an air driven gyro and by means of a ....... in an electrically driven gyro.
- A.the horizontal plane; air jets; wedge plate
- B.the vertical plane; air jets; torque motor
- C.the yawing plane; air jets; torque motor✓
- D.the yawing plane; air jets; wedge plate
Why: The DGI rotor axis is maintained in the yawing plane (not simply "horizontal plane"). Air-driven DGIs use air jets (and wedge plate as fine adjustment). Electrically driven DGIs use a torque motor instead of air jets and wedge plate. See Section 3 . — Air-driven = air jets (+ wedge plate fine adjustment). Electrical = torque motors. The yawing plane is the key term for the DGI, contrasted with the AH which uses the vertical.
Q7. The purpose of the caging knob is:
- A.to prevent the gyro toppling
- B.to reset the heading
- C.to reset the heading and to prevent toppling✓
- D.to prevent apparent wander
Why: The caging knob serves two purposes: (1) locking the inner gimbal to prevent toppling during synchronization or violent manoeuvres, and (2) allowing the scale to be rotated for synchronization with the compass. See Section 4 . — Classic "incomplete answer" trap. Both functions are needed for full credit.
Q8. In an air driven directional gyro the air jets are attached to:
- A.the inner gimbal
- B.the outer gimbal✓
- C.the instrument casing
- D.the rotor axis
Why: Suction is applied to the instrument case, and replacement air is ducted to jets on the outer gimbal ring which act on 'buckets' cut in the rotor. The jets are mounted on the outer gimbal. See Section 3 . — Think of the outer gimbal as the "frame" that carries the jets aimed at the rotor. The jets are part of the erection system, not just the drive system.
Q9. The limits of pitch and roll for a modern directional gyro are respectively:
- A.55° and 85°
- B.85° and 55°
- C.55° and 55°
- D.85° and 85°✓
Why: Modern DGIs have limits of 85° in both pitch and roll. Older air-driven DGIs had limits of only 55° . The question specifies "modern directional gyro" — answer is 85° for both. See Section 5 . — Modern DGI = 85° (both). Old air-driven = 55° (both). Do not mix them.
Q10. Gimballing error:
- A.will disappear after a turn is completed✓
- B.will remain until the gyro is reset
- C.will only occur during a 360° turn
- D.will be zero on only two headings during a 360° turn
Why: Gimballing errors are transient — they exist because of gimbal geometry during bank, but disappear as soon as level flight is resumed. They do not accumulate. See Section 7 . — "Gimballing errors disappear when level flight is resumed" — this distinguishes them from drift errors, which accumulate over time.