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Airspeed Indicator Mechanics

This document discusses the construction and operation of an airspeed indicator (ASI). It explains that airspeed is proportional to dynamic pressure, which is measured by the pitot tube and converted to a pressure reading. It describes the basic components of a simple ASI including the pitot and static ports, capsule, pinion, and quadrant. It also covers sources of error in ASI readings including instrument errors, position errors, density errors, and compressibility errors. Temperature variations and calibration procedures are also summarized.

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Shyam Manocha
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231 views12 pages

Airspeed Indicator Mechanics

This document discusses the construction and operation of an airspeed indicator (ASI). It explains that airspeed is proportional to dynamic pressure, which is measured by the pitot tube and converted to a pressure reading. It describes the basic components of a simple ASI including the pitot and static ports, capsule, pinion, and quadrant. It also covers sources of error in ASI readings including instrument errors, position errors, density errors, and compressibility errors. Temperature variations and calibration procedures are also summarized.

Uploaded by

Shyam Manocha
Copyright
© Attribution Non-Commercial (BY-NC)
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PPSX, PDF, TXT or read online on Scribd
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Presentation

Presentation by:
by: Gp
Gp Capt
Capt SK
SK Manocha
Manocha (Retd)
(Retd)
AIRSPEED is proportional to DYNAMIC PRESSURE

PITOT Pressure = DYNAMIC Pressure + STATIC Pressure

D=P-S (Pitot Excess)

CONSTRUCTION of SIMPLE ASI


POINTER

STATIC
PRESSURE PINION

CAPSULE QUADRANT

PITOT
PRESSURE
POINTER

STATIC
PRESSURE PINION

CAPSULE QUADRANT

PITOT
PRESSURE
CALIBRATION:
ISA - MSL Conditions

Kinetic Energy converted to Pressure Energy

D = ½ ρV²

Air Density (ρ) Variation with Altitude:


Requires Correction
Square Law Compensation: D = ½ ρV²

CAPSULE EXPANSION Proportional to V²


Low Speed Reading : Difficult

Limits: Extent of High Speed Indication

Internal Compensation Device

SQUARE LAW CALIBRATION


of ASI DIAL
Square Law Compensation: D = ½ ρV²

CAPSULE EXPANSION Proportional to V²

Low Speed Reading : Difficult

Limits: Extent of High Speed Indication

Internal Compensation Device

TEMPERATURE VARIATION
TYPICAL
TYPICAL ASI
ASI
PRESENTATION
PRESENTATION
Bi-metallic Strip
INSTRUMENT ERROR
Friction and Tolerances
POSITION ERROR (or PRESSURE ERROR)
Fluctuations at Static Source: LG and Flaps Configuration
Position and Instrument Correction Card
DENSITY ERROR
Dynamic Pressure Proportional to Density.
For Given TAS Less Dynamic Pressure at Higher Altitude
COMPRESSIBILITY ERROR
Compressibility Effects above 300 Kts TAS: Over reads
Density and Compressibility Errors by Nav/ Flt Computer
MANOEUVRE-INDUCED ERROR
Transient Error and Lag with a/c configuration: angle of
attack, flaps, landing gear etc affecting static pressure
PITOT HEAD BLOCKED:
LEVEL CRUISE: Speed Changes NOT Registered
ALTITUDE CHANGE(CLIMB): IAS INCREASE in CLIMB
DECREASE in DESCENT
PITOT LINE LEAKAGE:
LEVEL CRUISE: IAS Gradually Decreases

STATIC HEAD or VENT BLOCKED:


ALTITUDE CHANGE (CLIMB):
ASI UNDER-READING in CLIMB
OVER-READ (Dangerous) in DESCENT

TOLERANCE: ± 3 % or 5 Kts (higher one)


ASIR ASI Reading
IAS (Indicated Air Speed)
IAS = ASIR + IEC
CAS (Calibrated Air Speed) (or RAS Rectified Air Speed)
ASIR corrected for ‘Instrument and Pressure Errors’ (PIC)
CAS = ASIR + PIC
EAS (Equivalent Air Speed)
CAS corrected for Compressibility Error (CEC)
EAS = CAS + CEC
TAS (True Air Speed)
EAS corrected for Density Error (DEC)
TAS = EAS + DEC
CAS vs TAS at ALTITUDE

CAS = 140 kts

ALTITUDE ISA ISA + 20

5,000 150.7 kts 155.9 kts

10,000 162.5 kts 168.5 kts

15,000 175.8 kts 182.4kts

THUMB RULE
2 % of CAS/ 1000 ft

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