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DGCA Air Navigation: Machmeter Questions

97 exam-style Machmeter questions for the DGCA Air Navigationpaper (CPL & ATPL). The first 12 are free below with answers and explanations — sign up free to practise all of them with progress tracking.

  1. Q1

    What is the effect on the Mach number and TAS in an aircraft that is climbing with constant CAS?

    • AMach number increases, TAS increases
    • B Mach number remains constant, TAS increases
    • C Mach number decreases, TAS decreases
    • D Mach number increases, TAS remains constant
    Show answer

    Correct answer: A. Mach number increases, TAS increases

    Climbing at a constant calibrated airspeed, true airspeed increases because air density falls, and the Mach number also increases because the local speed of sound decreases with the falling temperature. Both quantities rise together as the aircraft climbs.

  2. Q2

    If OAT increases whilst maintaining constant CAS and flight level:

    • ATAS decreases.
    • BTAS remains constant.
    • CMach number remains constant
    • DMach number decreases.
    Show answer

    Correct answer: C. Mach number remains constant

    Mach depends only on CAS and pressure altitude, so it stays constant despite the OAT change.

  3. Q3

    During descent through a block of airspace of constant temperature and while flying at a constant mach no will cause the CAS to:

    • Aincrease
    • Bdecrease
    • Cremain constant
    • Dincrease at a rate of 1·98°/1000 ft
    Show answer

    Correct answer: A. increase

    This Explanation is Coming Soon

  4. Q4

    If OAT decreases while maintaining constant CAS and flight level:

    • AMach number increases.
    • BTAS increases.
    • CMach number remains constant.
    • DTAS remains constant.
    Show answer

    Correct answer: C. Mach number remains constant.

    At constant CAS and flight level Mach is independent of temperature and remains constant.

  5. Q5

    The Mach meter is subject to position error. This error varies according to: 1. Angle of attack. 2. OAT. 3. TAS. The combination regrouping all of the correct option is:

    • A1, 3.
    • B3.
    • C1.
    • D1, 2, 3.
    Show answer

    Correct answer: A. 1, 3.

    Mach-meter position error varies with angle of attack and speed (TAS).

  6. Q6

    The Mach meter is subject to position error. This error results from:

    • AIncorrect pressure sensing caused by disturbance of airflow around the pitot tube or the static ports.
    • BImperfect elasticity of the aneroid capsule.
    • CIncorrect altimeter setting.
    • DNon standard atmosphere conditions.
    Show answer

    Correct answer: A. Incorrect pressure sensing caused by disturbance of airflow around the pitot tube or the static ports.

    Position error comes from disturbed airflow giving incorrect pressure sensing at the pitot/static.

  7. Q7

    An aircraft is descending at a constant mach number. If the aircraft is descending through an inversion layer, the CAS will:

    • ARemain constant
    • BIncrease
    • CDecrease
    • DDecrease then decrease more slowly
    Show answer

    Correct answer: B. Increase

    This Explanation is Coming Soon

  8. Q8

    The Mach number is subject to position errors. The value of this error varies according to:

    • AOAT only
    • BTAS and OAT.
    • CTAS only.
    • DTAS and angle of attack.
    Show answer

    Correct answer: D. TAS and angle of attack.

    The position-error magnitude varies with TAS and angle of attack.

  9. Q9

    The machmeter consists of:

    • Aan airspeed indicator with mach scale
    • Ban airspeed indicator with an altimeter capsule
    • Can altimeter corrected for density
    • Da VSI and altimeter combined
    Show answer

    Correct answer: B. an airspeed indicator with an altimeter capsule

    This Explanation is Coming Soon

  10. Q10

    The combined Machmeter/ASI is subject to the following errors:

    • APosition, density, instrument, compressibility, manoeuvre induced
    • BThose of the Machmeter only
    • CInstrument, pressure and temperature only
    • DInstrument and compressibility only
    Show answer

    Correct answer: A. Position, density, instrument, compressibility, manoeuvre induced

    This Explanation is Coming Soon

  11. Q11

    An aeroplane is flying at FL140, with a CAS of 260 knots in standard conditions. The mach number is:

    • A0.53.
    • B0.41.
    • C0.43
    • D0.51
    Show answer

    Correct answer: D. 0.51

    TAS about 323 kt over LSS about 629 kt gives a Mach near 0.51.

  12. Q12

    You are flying at a constant FL 290 and constant mach number. The total temperature increases by 5°. The CAS will:

    • Aremain approximately constant
    • Bincrease by 10 kts
    • Cdecrease by 10 kts
    • Dwill increase or decrease depending on whether you are above or below ISA.
    Show answer

    Correct answer: A. remain approximately constant

    This Explanation is Coming Soon

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