The MachmeterNavigation — Instrumentation — DGCA CPL practice questions
Question 1 of 9
The local speed of sound is equal to: (K = Constant)
All 9 questions — The Machmeter
Navigation — Instrumentation · DGCA CPL. The correct option is marked on each.
Q1. The local speed of sound is equal to: (K = Constant)
- A.K √ temperature (°F) knots
- B.K √ temperature (K) knots✓
- C.K √ temperature (°C) knots
- D.K √ temperature (K) metres per second
Why: The formula LSS = 38.95 × √T requires T in Kelvin (absolute temperature) and gives LSS in knots . Using °C or °F would give incorrect results because negative Celsius values would make the square root imaginary. See Section 2 . — Always convert to Kelvin: T(K) = T(°C) + 273. This is non-negotiable for all LSS calculations.
Q2. At FL350 with a JSA deviation of −12, the true airspeed when flying at M 0.78 is:
- A.460 kt
- B.436 kt✓
- C.447 kt
- D.490 kt
Why: JSA FL350 temperature = −70°C (JSA lapse 2°/1000ft, 35×2=70, MSL +15 −70 = −55°C... wait, let me recalculate: JSA at FL350 = 15 − (35×2) = 15 − 70 = −55°C. Deviation −12 → actual temp = −55 − 12 = −67°C = 206K. LSS = 38.95×√206 = 38.95×14.35 = 559 kt. TAS = 0.78×559 ≈ 436 kt. See Section 2 and Section 12 . — JSA lapse rate is 2°C per 1000 ft with no tropopause (unlike ISA which stops at FL360). Always state whether you're using ISA or JSA in exam solutions.
Q3. When climbing at a constant Mach number below the tropopause through an inversion:
- A.the CAS and TAS will both increase
- B.the CAS and TAS will both decrease
- C.the CAS will decrease and the TAS will increase✓
- D.the CAS will increase and the TAS will decrease
Why: In an inversion, temperature rises with altitude → LSS increases. For constant Mach number, TAS = MN × LSS → TAS must increase. Despite TAS increasing, air density decreases faster during the climb, so CAS (which is density-dependent) decreases. See Section 10.2 . — Constant Mach climb: CAS ALWAYS decreases. TAS behaviour depends on the temperature profile — in an inversion it increases, in ISA it decreases, in isothermal it stays constant.
Q4. When descending below the tropopause under normal conditions (increasing temperature) at a constant CAS:
- A.both TAS and Mach number will decrease✓
- B.both TAS and Mach number will increase
- C.the TAS will decrease and the Mach number will increase
- D.the TAS will increase and the Mach number will decrease
Why: This is the mirror image of constant CAS climb. In a constant CAS descent under ISA conditions, density increases → TAS decreases (TAS = CAS × density correction, lower altitude = denser air = lower TAS). LSS increases (higher temperature) but TAS falls faster → Mach number decreases. See Section 8 . — Descent at constant CAS is the mirror image of climb at constant CAS. Climb: TAS↑ Mach↑. Descent: TAS↓ Mach↓.
Q5. Cruising at FL390, M 0.84 is found to give a TAS of 499 kt. The ISA deviation at this level will be:
- A.−17
- B.+17✓
- C.+19
- D.−19
Why: LSS = TAS/MN = 499/0.84 = 594 kt. √T = 594/38.95 = 15.25 → T = 15.25² = 232.6K = −40.4°C ≈ −40°C. FL390 is above the ISA tropopause (ISA temp = −56.5°C). Deviation = actual − ISA = −40 − (−56.5) = +16.5 ≈ +17 . See Section 12 . — Process: TAS/MN → LSS → (LSS/38.95)² → T(K) → T(°C) → compare with ISA. Practice this sequence until automatic.
Q6. The errors to which the Machmeter is subject are:
- A.instrument error, position error, compressibility error and manoeuvre induced error
- B.instrument error, position error and manoeuvre induced error✓
- C.instrument error, position error, barometric error, temperature error and manoeuvre induced error
- D.instrument error, position error, density error and manoeuvre induced error
Why: The Machmeter is calibrated to the ratio of dynamic to static pressure. Density and temperature cancel from the calculation, so no density or temperature error. Compressibility is inherently calibrated out by design. Only instrument, position, and manoeuvre-induced errors remain. See Section 5 . — The Machmeter's immunity to temperature, density, and compressibility errors is its key advantage over the ASI. This is a very common exam question.
Q7. The relationships between TAS, Mach number (MNo) and local speed of sound (LSS) is:
- A.LSS = MNo/TAS
- B.MNo = LSS/TAS
- C.TAS = MNo × LSS✓
- D.MNo = LSS × TAS
Why: By definition, Mach Number = TAS/LSS. Rearranging: TAS = MNo × LSS. This is the fundamental Mach relationship. See Section 2 . — Start from MN = TAS/LSS and rearrange as needed. This single formula underpins all Mach calculations.
Q8. The Machmeter gives an indication of Mach number by measuring the ratio:
- A.pitot pressure / static pressure
- B.static pressure / dynamic pressure
- C.dynamic pressure / pitot pressure
- D.dynamic pressure / static pressure✓
Why: Mach number ∝ √(Dynamic Pressure / Static Pressure) where Dynamic Pressure = Pitot − Static. The altitude capsule measures static pressure (denominator), the airspeed capsule measures dynamic pressure (numerator). See Section 3 . — Dynamic pressure = pitot − static. Mach ∝ √(Dp/Ps). The altitude capsule compensates for changing Ps at different altitudes.
Q9. An aircraft is flying at FL350 with a JSA deviation of +8. The Mach No. is 0.83 and the TAS 485. If the aircraft descends to FL300 and maintains the same Mach No. and TAS, the JSA deviation will now be:
- A.+8
- B.−2✓
- C.+2
- D.−18
Why: Same Mach (0.83) and same TAS (485) → LSS unchanged → temperature unchanged. From FL350 calculation: LSS = 485/0.83 = 585 kt → T = (585/38.95)² = 225K = −48°C. JSA FL350 = 15−70 = −55°C; deviation +8 → −55+8 = −47°C ≈ −48°C (consistent). At FL300: JSA = 15−60 = −45°C. Actual temp is still −48°C → deviation = −48−(−45) = −3 . Nearest answer is (b) −2 (rounding differences in calculation). The key insight: same Mach + same TAS = same temperature = isothermal layer, so the deviation changes as standard temperature changes with altitude. See Section 12, Problem 5 . — Same MN + same TAS at diffe…