Revision QuestionsNavigation — Instrumentation — DGCA CPL practice questions
Question 1 of 15
A 2-axis gyro measuring vertical changes will have:
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Navigation — Instrumentation · DGCA CPL. The correct option is marked on each.
Q1. A 2-axis gyro measuring vertical changes will have:
- A.one degree of freedom, vertical axis
- B.two degrees of freedom, vertical axis✓
- C.one degree of freedom, horizontal axis
- D.two degrees of freedom, horizontal axis
Why: A 2-axis gyro has two degrees of freedom (it can precess in two planes). To measure vertical changes (pitch and roll — earth vertical reference), the spin axis must be vertical. This is the configuration used in the artificial horizon. Degrees of freedom = number of axes around which the gyro can precess freely. — DOF = number of planes in which the gyro can precess. Artificial horizon: 2 DOF, vertical spin axis. DGI: 2 DOF, horizontal spin axis. Turn indicator: 1 DOF, horizontal spin axis. These pairings are fundamental gyroscope exam knowledge.
Q2. The properties of a gyro are: (1) mass (2) rigidity (3) inertia (4) precession (5) rotational speed
- A.1, 2 & 3
- B.2 & 4✓
- C.2 & 3
- D.1 & 3
Why: A gyroscope has two fundamental properties: (1) Rigidity in space — tendency to maintain its spin axis direction in space when spinning at high speed; and (2) Precession — when a force is applied, the gyro responds at 90° to the applied force in the direction of rotation. Mass, inertia, and rotational speed contribute to the degree of rigidity but are not the "properties" per se. — Gyro Properties = Rigidity + Precession. Everything else (mass, rpm, rotor diameter) affects the degree of rigidity. This two-word answer appears in virtually every DGCA instrumentation exam.
Q3. The Machmeter consists of:
- A.an airspeed indicator with Mach scale
- B.an airspeed indicator with an altimeter capsule✓
- C.an altimeter corrected for density
- D.a VSI and altimeter combined
Why: The Machmeter has two capsule systems: (1) a pitot-static capsule that measures dynamic pressure (like an ASI), and (2) an aneroid (altimeter) capsule that senses ambient (static) pressure as a proxy for altitude/density. The ratio of these two pressures gives Mach number: M = √(dynamic pressure / static pressure function). The altimeter capsule corrects the speed reading for altitude. — Mach = TAS ÷ LSS. Since LSS varies with temperature (and thus altitude), the Machmeter needs both an ASI capsule (dynamic pressure) and an altimeter capsule (static pressure). The mechanical linkage between…
Q4. An aircraft is flying at an indicated altitude of 16,000 ft. The outside air temperature is −30°C. What is the true altitude of the aircraft?
- A.16,200 ft
- B.15,200 ft✓
- C.18,600 ft
- D.13,500 ft
Why: ISA temperature at 16,000 ft = 15 − (16 × 2) = 15 − 32 = −17°C. Actual OAT = −30°C. Temperature deviation = −30 − (−17) = −13°C colder than ISA. True altitude = indicated altitude × (actual temp / ISA temp in Kelvin). Using the 4 ft/°C/1000 ft rule: correction = −13 × 16 × 4 / 1000 ≈ −832 ft. True alt ≈ 16,000 − 832 ≈ 15,168 ft ≈ 15,200 ft . Cold temperatures make the aircraft lower than indicated — the altimeter over-reads. — Cold air = denser air = altimeter over-reads = true altitude is LESS than indicated. Warm air = less dense = altimeter under-reads = true altitude is MORE than indica…
Q5. What is the Schuler period?
- A.21 minutes
- B.84 minutes✓
- C.1 oscillation in azimuth
- D.63 minutes
Why: The Schuler period is 84.4 minutes — the natural oscillation period of an inertial navigation platform when tuned to Earth's radius. It is derived from the pendulum equation: T = 2π√(R/g), where R = Earth's radius (~6,371 km) and g = gravitational acceleration. An INS tuned to the Schuler period will not precess due to accelerations caused by motion over the Earth's curved surface. — The Schuler period of 84 minutes = same as the orbital period of a satellite at Earth's surface. An INS aligned to this period will maintain a stable earth-vertical reference despite vehicle acceleration. This…
Q6. At 50 feet AGL during an autoland, what happens to the glide slope signal?
- A.It continues to be actioned
- B.It is disconnected✓
- C.It is factored for range
- D.It is used to flare the aircraft
Why: At 50 ft AGL during autoland, the glide slope signal is disconnected. Below this height, the flare is initiated using the radio altimeter signal. The GS signal is unreliable at very short ranges due to the extreme geometry (near vertical approach to the GS transmitter). The flare law takes over, using radio altimeter height to initiate the pitch-up and thrust reduction. — Autoland flare sequence: GS controls approach → at 50 ft, GS disconnected → radio altimeter triggers flare → at 15 ft, autothrottle retards to idle (flare complete) → touchdown via roll-out mode.
Q7. If only a single A/P is used to climb, cruise and approach, following a failure:
- A.it is fail-passive with redundancy
- B.it is fail-operational and will not disconnect
- C.it is fail-soft and will not disconnect
- D.it is fail-safe and will disconnect✓
Why: A single autopilot system, when it fails, must disconnect safely — this is called fail-safe . It disconnects on failure, alerting the crew to take manual control. Fail-passive systems (used in autoland) also disconnect but in a neutral position. Fail-operational systems (dual/triple autopilots) can continue the approach after one failure. A single AP has no redundancy, so it must disconnect cleanly on failure. — Single AP = fail-safe (disconnects). Duplex AP = fail-passive (disconnects in neutral → CAT II). Triplex AP = fail-operational (continues after 1 failure → CAT III). These three cat…
Q8. The primary input to a basic stall warning system is:
- A.angle of attack✓
- B.IAS
- C.slat/flap position
- D.MNO
Why: The primary input to a stall warning system is angle of attack (AoA) . Stall is fundamentally an aerodynamic phenomenon that occurs at a critical AoA, not at a specific IAS. The AoA sensor (vane or probe on the fuselage) provides the primary signal. Configuration (flap/slat position) adjusts the stall threshold but is secondary. Note that a basic stall warning (e.g., a stall warning horn driven by AoA alone) uses only AoA. — AoA = primary stall warning input. Configuration adjusts the threshold. Weight/bank angle are factored in more sophisticated systems. The vane-type AoA sensor on the fu…
Q9. An FDR fitted to an aircraft of over 5,700 kg after April 1998 must record for:
- A.10 hours
- B.25 hours✓
- C.30 minutes
- D.60 minutes
Why: Aircraft above 5,700 kg MTOM require an FDR with a minimum recording time of 25 hours . Registration after 1 April 1998 makes this Case 1. The 10-hour recording time applies to turbine-powered aircraft below 5,700 kg MTOM but with more than 9 passenger seats registered after 1 April 1998. — FDR recording: >5,700 kg = 25 hrs. <5,700 kg (turbine, >9 seats) = 10 hrs. CVR recording: standard = 30 min; >5,700 kg post-Apr '98 = 2 hrs. These four figures are often cross-tested.
Q10. What type of sensor is used to measure the output of a low-pressure booster pump?
- A.Bourdon tube
- B.Aneroid capsule
- C.Bellows✓
- D.Differential capsule
Why: A bellows-type element is typically used to measure pressures such as the output of the low-pressure booster pump. Typical pairings: LP booster pump, bellows; oil pressure, Bourdon tube; altimeter, aneroid capsule.
Q11. During the take-off run, the effect of increasing airspeed is to cause the EPR indication to:
- A.increase, due to ram rise
- B.fall, due to increase of intake pressure relative to jet pipe pressure✓
- C.remain constant as the jet pipe and intake pressures increase at the same rate
- D.fall, then gradually return to the original setting as V2 is reached
Why: As airspeed increases during the take-off roll, the engine intake pressure increases due to the ram effect (more dynamic pressure). The jet pipe pressure initially does not increase proportionally at low airspeed. Therefore EPR (= jet pipe pressure ÷ intake pressure) falls . This is the well-known EPR apparent drop. Standard procedure: set EPR before 60 kt , do not increase power after that speed. — EPR drop on T/O is an APPARENT fall — not a real thrust loss. The standard answer is: set EPR before 60 kt, then do not touch throttles. After V2, as speed increases further, the ram pressure pa…
Q12. A modern radio altimeter uses the frequency band:
- A.VHF — 30–300 MHz
- B.SHF — 3,000 MHz–30 GHz✓
- C.UHF — 300 MHz–3 GHz
- D.HF — 3 MHz–30 MHz
Why: Radio altimeters (radar altimeters) operate in the SHF (Super High Frequency) band, typically around 4.2–4.4 GHz . This frequency provides the required short wavelength for accurate height measurement at low altitudes (0–2,500 ft). FM-CW (Frequency Modulated Continuous Wave) technique is used. — Radio altimeter = SHF (~4.3 GHz). This is in the microwave/centimetric wavelength range. The FM-CW technique allows measurement of height from 0 ft to 2,500 ft AGL with high accuracy.
Q13. An aircraft is travelling at 120 kt. What angle of bank would be required for a rate-one turn?
- A.30°
- B.12°
- C.19°✓
- D.35°
Why: Rate 1 turn = 3°/second (360° in 2 minutes). Bank angle for rate 1 ≈ TAS/10 + 7. At 120 kt TAS: 120/10 + 7 = 12 + 7 = 19° . More precisely: tan(bank) = (rate × TAS) / (g × 180/π). Rate = 3°/s = 0.0524 rad/s; TAS = 120 kt = 202 ft/s. tan(bank) = (0.0524 × 202) / 32.2 = 10.58/32.2 = 0.329 → bank = arctan(0.329) ≈ 18.2° ≈ 19° . — Quick formula: Rate 1 bank angle ≈ TAS/10 + 7. At 120 kt: 12+7=19°. At 180 kt: 18+7=25°. At 240 kt: 24+7=31°. This formula is approximate but accurate to within 1–2° for typical airspeeds.
Q14. An aircraft is travelling at 100 kt forward speed on a 3° glide slope. What is its rate of descent?
- A.500 ft/min✓
- B.300 ft/min
- C.250 ft/min
- D.500 ft/sec
Why: Standard formula: Rate of descent = groundspeed (kt) × 5 × glide slope angle (°). At 100 kt and 3°: ROD = 100 × 5 × 3 / 3 = 500 ft/min. More precisely: ROD = GS × tan(3°) × 101.3 ft/min per kt ≈ 100 × 0.0524 × 101.3 ≈ 531 ft/min, but the standard answer using the 5× rule gives 500. The rule-of-thumb: ROD = GS × (GPA × ~5/3) ≈ GS × 5 for 3° ≈ 500 ft/min at 100 kt. — Memorise: 3° GS at 150 kt ≈ 750 ft/min; at 120 kt ≈ 600 ft/min; at 100 kt ≈ 500 ft/min. These benchmarks cover the most common exam speeds. The general rule: ROD (ft/min) ≈ 5 × GS (kt) for a 3° slope.
Q15. What correction is given by TCAS?
- A.Turn left or right
- B.Climb or descend✓
- C.Contact ATC on receipt of a resolution advisory
- D.Climb or descend at 500 ft/min
Why: TCAS II provides Resolution Advisories (RAs) that give vertical manoeuvre guidance only — it tells the pilot to climb or descend (and at what rate). TCAS does NOT give horizontal manoeuvre guidance (turn left/right). The vertical-only guidance is why TCAS RAs specify climb/descend rates (e.g., 1,500 or 2,500 ft/min) rather than headings. — TCAS RA = vertical guidance only (climb or descend). The correct response: smoothly and immediately comply with the RA. Then notify ATC. NEVER follow ATC instructions that conflict with an active RA. This is the critical safety rule.