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BAK – Pressure Instruments

Three Instruments

1.  Airspeed Indicator (ASI)

2.  Vertical Speed Indicator (VSI)

3.  Altimeter

1.  Airspeed Indicator

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-pitot tube faces airflow

-collects both dynamic and static pressure - the “total pressure” 

-static vent

-contains only the still air pressure- the “static pressure” 

-Airspeed Indicator

-shows difference between total pressure and static pressure – the “dynamic pressure” 

-may have small errors in mechanism called “instrument error” 

-non ideal collection of the total pressure or static pressure causes errors called “position error” 

Airspeed Indicator Markings

• White arc - commonly referred to as the flap operating range since its lower limit represents

the full flap stall speed and its upper limit provides the maximum flap speed. Approaches andlandings are usually flown at speeds within the white arc. 

• Lower limit of white arc (Vso) - the stalling speed or the minimum steady flight speed in the

landing configuration. In small aircraft, this is the power-off stall speed at the maximum landing

weight in the landing configuration (gear and flaps down). 

• Upper limit of the white arc (Vfe) - the maximum speed with the flaps extended. 

• Green arc—the normal operating range of the aircraft. Most flying occurs within this range.

• Lower limit of green arc (Vs1) - the stalling speed or the minimum steady flight speed

obtained in a specified configuration. For most aircraft, this is the power-off stall speed at the

maximum takeoff weight in the clean configuration (gear up, if retractable, and flaps up). 

• Upper limit of green arc (Vno) - the maximum structural cruising speed. Do not exceed thisspeed except in smooth air.

• Yellow arc - caution range. Fly within this range only in smooth air, and then, only with

caution.

• Red line (Vne) - never exceed speed. Operating above this speed is prohibited since it may

result in damage or structural failure.

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2.  Vertical Speed Indicator (VSI)

-measures the rate at which static pressure changes

-the case is connected through a choke (calibrated leak) to static pressure

-the diaphragm drives the pointer and is connected directly to static pressure

-diaphragm expands or contracts with changes to static pressure (altitude) until case pressure equalizes

-an instantaneous VSI (IVSI) has small weight in linkage to remove pointer lag

3.  Altimeter

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-a sealed diaphragm (aneroid wafers) containing a fixed amount of air

-case vented to static pressure

-wafers expand or compress according to static pressure changes (altitude or atmospheric)

-linkage attached to wafers moves pointers

-instrument indicates height above reference pressure set by barometric adjustment knob

-its up to the pilot to make the altimeter readings meaningful by setting the barometric knob

Operational considerations

-some light aircraft and all larger aircraft have alternate static source

-a second source of static air pressure

-on small aircraft it may be cockpit pressure

-if so then selecting alternate static will result in:

-Altimeter indicates slightly higher than actual

-ASI indicates higher airspeed than actual

-VSI shows momentary climb then stabilizes

-blocked static vent

-select alternate static source if available

-climbing IAS decreases

-descending IAS increases

-VSI shows constant zero

-Altimeter shows constant height at which static vent blocked

-blocked pitot tube

-partial blockage

-airspeed reduced or drops to zero

-complete blockage

-pressure caught in pitot system

-changes due only to changes in static system

-descending IAS will drop

-ascending IAS will increase