Inertial Navigation SystemsNavigation — Instrumentation — DGCA CPL practice questions

All 8 questions — Inertial Navigation Systems

Navigation — Instrumentation · DGCA CPL. The correct option is marked on each.

1. Q1. INS errors are classified as "bounded errors" and "unbounded errors". Which statement is correct?

   • A.An "unbounded error" increases with time; example: distance gone error due to a ground speed error✓
   • B.An "unbounded error" increases with time; example: increasing ground speed error due to platform not being levelled correctly
   • C.A "bounded error" is subject to sudden unpredictable random changes, most notable during pitching manoeuvres
   • D.A "bounded error" is "tied" to the real wander rates of the gyros on the platform

   Why: Unbounded errors are cumulative errors that grow with time. A distance gone error due to a ground speed error (e.g., from azimuth misalignment or levelling gyro wander) keeps accumulating — it never "comes back" within an 84.4-min cycle. Bounded errors are those that oscillate and return to zero within the 84.4-minute Schuler cycle.

2. Q2. Two checks for correctly entered sequential waypoints are:

   • A.Select DSR.TK/STS and check status <4; select DIS/TIME and check time agrees with flight plan
   • B.Select DIS/TIME and check distance agrees with flight plan; then check time agrees with flight plan
   • C.Select DIS/TIME and check distance agrees with flight plan; select DSR.TK/STS and check track agrees with flight plan✓
   • D.Select DIS/TIME and check distance agrees with flight plan; select HDG/DA and check heading agrees

   Why: The two standard cross-checks are: (1) select DIS/TIME and verify distance matches the flight plan leg distance, and (2) select DSR TK/STS and verify the desired great-circle track matches the flight plan. This confirms both the correct waypoints AND correct lat/long entries.

3. Q3. In an INS the E/W accelerations are converted into E/W speed at the first stage of integration, and into E/W distance (departure) at the second stage. To convert departure to d'long (min) for longitude update:

   • A.Departure × cosine of present latitude
   • B.Direct d'long (min) without conversion
   • C.Departure × secant of present latitude✓
   • D.Departure × sine of present latitude

   Why: Departure (NM) = d'long (min) × cos(lat). Therefore d'long (min) = departure ÷ cos(lat) = departure × sec(lat). At the equator (lat=0), cos=1 so departure = d'long exactly. At higher latitudes, the meridians converge, so more departure is needed for the same change in longitude. Memory hook: "Departure × SEC(lat) = d'long". The word "secant" has to "expand" the longitude value because degrees of longitude get shorter as you move from the equator toward the poles.

4. Q4. At the second stage of integration, E/W speed is converted into E/W distance (departure). To convert departure into change of longitude it must:

   • A.Be divided by secant of the latitude
   • B.Be multiplied by secant of the latitude✓
   • C.Be divided by tangent of the latitude
   • D.Be multiplied by cosine of the latitude

   Why: Same principle as Q3: d'long = departure × sec(lat). The answer here is stated directly: multiply by secant of latitude.

5. Q5. The amber ALERT light on an INS control and display unit:

   • A.Illuminates steadily 2 minutes, in AUTO mode, before reaching the next waypoint✓
   • B.Starts flashing 2 minutes before reaching the next waypoint and goes out at 30 seconds to run
   • C.Illuminates if power from the aircraft bus bar has been lost and the system is on standby battery
   • D.Illuminates steadily after passing a waypoint in manual mode, until the next leg is programmed in

   Why: In AUTO mode, the ALERT light illuminates steadily at 2 minutes before the waypoint and extinguishes as the track automatically changes overhead the waypoint. There is no flashing in AUTO mode.

6. Q6. With reference to INS, the functions of the integrators are: (i) At second stage of integration to suppress unbounded errors (NAV mode) (ii) At first stage of integration to convert acceleration → speed (NAV mode) (iii) At second stage of integration to convert speed → distance gone (NAV mode) (iv) To align the platform (level and align modes)

   • A.All four statements are true
   • B.Only (ii), (iii) and (iv)
   • C.Only (i), (ii) and (iii)
   • D.Only (ii) and (iii)✓

   Why: Statement (ii) and (iii) are correct — these are the two integration stages: acceleration→speed, and speed→distance. Statement (i) is wrong: integrators do not suppress unbounded errors — they actually propagate them (second-stage integrator errors ARE a source of unbounded errors). Statement (iv) is wrong: the platform is aligned by gyro compassing and torquing, not by integrators.

7. Q7. The computer of a north-referenced INS in flight provides compensation for:

   • A.Aircraft manoeuvres, real wander, apparent wander, transport wander
   • B.Coriolis, real wander, apparent wander, transport wander
   • C.Earth rotation, transport wander, coriolis✓
   • D.Transport wander, apparent wander, coriolis, magnetic variation

   Why: INS computers compensate for: (1) Earth rotation rate (apparent wander = earth rate compensation); (2) Transport wander (transport rate compensation = V/R); (3) Coriolis and centrifugal effects. "Real wander" is an imperfection of the physical gyro and cannot be compensated by the computer (it is a random inherent error). Magnetic variation is irrelevant — INS operates in True reference. Remember: INS compensates for predictable, mathematically-determinable phenomena. Real wander (from bearing imperfections) is random and unpredictable — it cannot be computed away.

8. Q8. During initialization of an INS the aircraft must not be moved until:

   • A.The ramp position has been inserted and checked
   • B.The platform is levelled
   • C.The gyros and accelerometers are in the "null" position
   • D.The green "READY NAV" light has been illuminated and the mode selector switch has been set to the "NAV" position✓

   Why: The INS must complete the full alignment sequence — warm-up, coarse levelling, coarse azimuth, fine levelling, and gyro compassing — before the aircraft moves. READY NAV illuminates when this is complete. Once in NAV mode, the aircraft may taxi. Moving before READY NAV and NAV selection destroys the alignment and means the INS cannot be re-aligned in flight.