Provides measurement of the total pressure created by the forward motion of the aircraft and the static pressure of the air surrounding the aircraft.

Total energy at the stagnation point is the sum of Potential Energy + Kinetic energy + Pressure Energy

Mostly affected by pressure of position error - which is reduced considerably by removing the static from the probe and installing static vents

Pressure instruments are calibrated to a known set of atmospheric conditions (ISA)

lapse rate of 1.98C per 1000 ft up to 36090 ft where the temperature remains constant at -56,5C

From 20KM to 32KM temp increase at 1C per KM or 0,3C per 1000ft

Air data is the term used to describe the static and pitot pressure

The transmission of static & pitot pressures is achieved via very small pipelines, which in large aircrafts with ore instruments requires substantial plumbing

A system is used which transmits the pressures only as far as an Air Data Computer(ADC) which then converts the pressure into electrical signals which are transmitted to the various instruments

This eliminates mechanical linkage errors and corrections can be automatically applied.

Warning flags are provided in each indicator - activated by the loss of data signals and annunciator lights on the computer

The ASI measures the difference between Pitot Pressure (P) and Static Pressure (S)

A stationary aircraft is subject to atmospheric pressure which is Static Pressure (S), due to the weight of the column of air above the aircraft

In flight, the aircraft is subject to Dynamic Pressure (D) due to movement through the air

Pitot Pressure is measured by the Pitot head and is the sum of (D+S)

D = P - S (we must subtract static pressure from Pitot pressure to obtain dynamic pressure or airspeed.

ASI has a capsule which is sensitive to changes of Pitot Pressure which is supplied from the Pitot head.

Capsule is mounted inside an airtight case which is fed with static pressure from the static vent. and now that static pressure is inside and outside the capsule. they cancel out leaving only dynamic pressure (aka Airspeed)

an error arises when the actual air temperature differs from ISA due to speed of sound varying with temperature.

Instrument error is small due to minor imperfections from manufacturing or wear & tear

Incorrect pressures being sensed by the pitot static probe or static vent

Instrument and Pressure errors are usually combined and a correction card placed near the ASi

Random error due to changes of aircraft attitude or when the pitot head is not aligned with airflow (climb and Descent)/ changes of aircraft configuration i.e flaps or landing gear being lowered

The ASI is calibrated for ISA, in such conditions, RAS = TAS. Under ANY other conditions, a density error will occur

Air density lower than standard will indicate an airspeed lower than TAS

Air is compressible, at high speeds (TAS 300 +) air is compressed and causes higher dynamic pressure than normal.

As air is compressed, its temperature rises and will have to be corrected before the correction is made on the nav computer

Permissible speed range with flaps extended - Low speed end - stalling speed with flaps down

If the pitot line from the pitot head to the ASI develops a leak, the ASI will underread (pitot pressure too low)

If the pitot head becomes blocked in level flight the ASI pressure line will hold the pressure and ASI reading will maintain the last recorded airspeed.

If the static source becomes blocked in level flight, the ASI will continue to read correctly.