The Machmeter

Question 1

Local Speed of Sound (LSS)

Answer 1

Is the speed at which sound propagates through the air in current conditions. It depends ONLY on ambient temperature

Question 2

How is the LSS calculated?

Answer 2

LSS=39√K

K is the temperature in Kelvin

Question 3

How is the Mach number calculated?

Answer 3

M is a dimensionless unit representing the ration between TAS and LSS.

M=TAS/LSS

Question 4

How can Mach number be measured?

Answer 4

M can also be measured by dividing pitot pressure by static pressure

Question 5

Machmeter principle of operation

Answer 5

• Pitot pressure moves a ratio arm, which pushes against a ranging arm fixed at one end to a shaft
• The shaft is connected to the instrument pointer to indicate Mach
• The ratio arm is moved up and down the ranging arm by the expansion and contraction of the altitude capsule. For any given deflection of the ratio arm, needle deflection increases with increasing altitude

Question 6

Compressibility error

Answer 6

Is proportional to the dynamic pressure/static pressure. Because this is precisely what the machmeter measures, compressibility has no effect.

Question 7

Machmeter displays

Answer 7

• A pointer against a scale (machmeter)
• A numerical drum indication in an analogue ASI dial
• A component of a vertical scale speed ‘tape’ of electronic instruments primary flight display
• A digital indication on the HUD

Question 8

Digital Mach numbers

Answer 8

Are derived from calculations performed by an ADC. Conventional machmeters are now obsolete

Question 9

Pitot and Static line blockages

Answer 9

Have the same effect as on an ASI.

• Static blockages: in a descent, case pressure is too low so the machmeter over-reads
• Pitot blockages: in a descent, pitot pressure is too low so the Machmeter under-reads

Question 10

Pitot and Static line leakage

Answer 10

Have the same effect as on an ASI.

• Static leaks:
  • pressurised aircraft: cabin pressure causes significant under-read
  • unpressurised aircraft: slightly lower cabin pressure causes slight over-read
• Pitot leaks: cause under-read because some speed capsule expansion is lost

Question 11

Relationship of EAS, CAS, TAS and M

Answer 11

Mnemonic ECTM: extra chicken tikka masala

• with constant EAS all others increase in the climb and decrease in descent
• With constant CAS, EAS reduced while the others increase in the climb - the reverse is true in descent - and so on

Question 12

TAS/M relationship with temperature

Answer 12

With changing temperature, TAS/M relationship changes but other are unaffected:

• Warmer air - for same M, TAS increases, CAS and EAS stay the same
• Colder air - for same M, TAS reduces, CAS and EAS stay the same

Question 13

Barber pole indicator

Answer 13

Indicates the more restrictive of VMO or MMO. VMO is usually dominant at lower altitudes and MMO at higher altitudes

Question 14

Climb Patterns

Answer 14

Climbing at constant IAS can result in MMO being exceeding, descending at constant M can result in VMO being exceeded.

In a climb at constant M, IAS reduces so if you follow M this may result in little or no margin above the stall or close to the aircraft’s ceiling.

Usually a mixed profile of 300kt/M.82 is used to avoid these problems. Switching between the two occurs when the two cross.