The Air Speed Indicator

Question 1

Units used for airspeed

Answer 1

Nautical miles per hour: knots (kt)
Statute miles per hour: mph
Kilometres per hour: km/h

Question 2

Operating principle

Answer 2

• metal capsule is connected to pitot pressure
• instrument case is connected to static pressure
• difference between static and pitot pressure (dynamic pressure) determines the extent to which the capsule expands
• linkage of arms and gears connects the capsule to a pointer hand against a scale calibrated to indicated airspeed

Question 3

Indication types

Answer 3

• Pointer hand (traditional)
• Vertical straight scale (EFIS)
• Numerical (on sime HUDs)

Question 4

Errors affecting ASIs

Answer 4

• Pitot/static system errors: relatively small
• Instrument error: relatively small
• Position error: varying magnitude, depending on aircraft configuration. On some (small) aircraft adjustments must be made to indicated airspeed.
• Compressibility error: negligible below 300kt, but can cause significant under-reading at higher speeds. Use a navigational computer to correct for this. ADCs automatically correct.
• Density error: air density decreases rapidly as altitude increases, resulting in indicated airspeed being significantly below TAS at high altitude

Question 5

ASI correction tables

Answer 5

Are used on some older, small aircraft to account for position error

Question 6

Pitot system leaks

Answer 6

Cause under-reading because some of the total pressure is lost.

• Static leak (pressurised aircraft): cabin pressure causes ASI to under-read
• Static leak (unpressurised aircraft): slightly lower cabin pressure causes ASI to slightly over-read

Question 7

Blockage in the static or pitot system

Answer 7

During descent:

• Static blockage results in over-reading ASI
• Pitot blockage results in under-reading ASI

During climb:

• Static blockage results in under-reading ASI
• Pitot blockage results in over-reading ASI

Mnemonic: Down SOUP

DESCENT: Static Overread, Underread Pitot

Question 8

Colour codes used on ASI Displays

Answer 8

White arc: flap operating range
Green arc: normal operating speed range
Yellow arc: caution speed range(s)
Red line: never exceed speed - VNE
Barber’s pole: maximum operating speed (VMO) or Mach number (MMO)
Blue line: MEP best rate of climb speed with one engine inoperative

Question 9

How can an unreliable ASI be identified when there are multiple ASIs in the cockpit?

Answer 9

• Indicated airspeed differences between ASIs
• Unexpected aircraft behaviour e.g. control response not as expected
• Unexpected buffet (can be caused by high or low airspeed)
• Aircraft system warnings
• Aircraft’s attitude not as expected for phase of flight

Question 10

Methods of judging approximate speed

Answer 10

• wind noise around the cockpit, a possible indicator of gross airspeed deviations
• GPS speed only useful if wind is already known (noted or on flight plan)
• Pitch and power settings: quite accurate and reliable