DGCA Air Navigation: Machmeter Questions
97 exam-style Machmeter questions for the DGCA Air Navigation paper (CPL · ATPL · PPL). The first 12 are free below with answers and explanations — sign up free to practise all of them with progress tracking.
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Q1
What is the effect on the Mach number and TAS in an aircraft that is climbing with constant CAS?
- A Mach number increases, TAS increases
- B Mach number remains constant, TAS increases
- C Mach number decreases, TAS decreases
- D Mach number increases, TAS remains constant
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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.
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Q2
If OAT increases whilst maintaining constant CAS and flight level:
- A TAS decreases.
- B TAS remains constant.
- C Mach number remains constant
- D Mach number decreases.
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Correct answer: C. Mach number remains constant
Mach depends only on CAS and pressure altitude, so it stays constant despite the OAT change.
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Q3
During descent through a block of airspace of constant temperature and while flying at a constant mach no will cause the CAS to:
- A increase
- B decrease
- C remain constant
- D increase at a rate of 1·98°/1000 ft
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Correct answer: A. increase
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Q4
If OAT decreases while maintaining constant CAS and flight level:
- A Mach number increases.
- B TAS increases.
- C Mach number remains constant.
- D TAS remains constant.
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Correct answer: C. Mach number remains constant.
At constant CAS and flight level Mach is independent of temperature and remains constant.
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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:
- A 1, 3.
- B 3.
- C 1.
- D 1, 2, 3.
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Correct answer: A. 1, 3.
Mach-meter position error varies with angle of attack and speed (TAS).
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Q6
The Mach meter is subject to position error. This error results from:
- A Incorrect pressure sensing caused by disturbance of airflow around the pitot tube or the static ports.
- B Imperfect elasticity of the aneroid capsule.
- C Incorrect altimeter setting.
- D Non standard atmosphere conditions.
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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.
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Q7
An aircraft is descending at a constant mach number. If the aircraft is descending through an inversion layer, the CAS will:
- A Remain constant
- B Increase
- C Decrease
- D Decrease then decrease more slowly
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Correct answer: B. Increase
This Explanation is Coming Soon
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Q8
The Mach number is subject to position errors. The value of this error varies according to:
- A OAT only
- B TAS and OAT.
- C TAS only.
- D TAS and angle of attack.
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Correct answer: D. TAS and angle of attack.
The position-error magnitude varies with TAS and angle of attack.
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Q9
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
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Correct answer: B. an airspeed indicator with an altimeter capsule
This Explanation is Coming Soon
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Q10
The combined Machmeter/ASI is subject to the following errors:
- A Position, density, instrument, compressibility, manoeuvre induced
- B Those of the Machmeter only
- C Instrument, pressure and temperature only
- D Instrument and compressibility only
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Correct answer: A. Position, density, instrument, compressibility, manoeuvre induced
This Explanation is Coming Soon
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Q11
An aeroplane is flying at FL140, with a CAS of 260 knots in standard conditions. The mach number is:
- A 0.53.
- B 0.41.
- C 0.43
- D 0.51
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Correct answer: D. 0.51
TAS about 323 kt over LSS about 629 kt gives a Mach near 0.51.
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Q12
You are flying at a constant FL 290 and constant mach number. The total temperature increases by 5°. The CAS will:
- A remain approximately constant
- B increase by 10 kts
- C decrease by 10 kts
- D will increase or decrease depending on whether you are above or below ISA.
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Correct answer: A. remain approximately constant
This Explanation is Coming Soon
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