ASI ERRORS Density Error.

Unless the air round the aircraft is at the calibration density of 1225 grammes per cubic metre, which can only occur near sea level, the ASI cannot correctly indicate TAS.

The formula in Paragraph 4.1. shows that dynamic pressure is proportional to density, so at altitude, where density is less, the dynamic pressure generated by a given TAS will be less than for the same TAS in flight at sea level. ASI capsule expansion will be proportionately less and the speed indicated will be less than the true speed.

Summarising, the ASI under-reads the true speed at altitude, the discrepancy being called ‘density error’. If below MSL, the ASI will over read the true speed.

The navigational computer is commonly used to correct for density error, computing true airspeed from CAS (the ‘Calibrated’ Air Speed obtained by applying corrections for instrument and position errors to the reading of the ASI). In the Airspeed window, set pressure altitude against Corrected Outside Air Temperature (COAT); then read off TAS on the outer scale against CAS on the inner scale.

This computation allows for the height of the aircraft above the calibration 1013.25 mb pressure level (which should be set on the altimeter subscale) and also for temperature deviation from standard conditions.

Summary:

CAS + correction for density error (nearly always + ) = TAS Problem

If the temperature at FL 100 is ISA minus 15°C (SAT = - 20°C), will the TAS for a given CAS be greater or smaller than in standard conditions?

Solution

TAS will be smaller in the lower temperature conditions. If the CAS is kept the same as in standard conditions, the dynamic pressure must be the same.

So ½ ρV2 _{is unchanged and since a lower temperature must increase the air density, then V, the}

TAS, must be less to preserve the balance of the equation (Dy = ½ ρV2_).

Instrument Error

Manufacturing imperfections and usage result in small errors which are determined on the ground under laboratory conditions by reference to a datum instrument.

A correction card can be produced for the speed range of the instrument. Position Error

Alternatively known as ‘pressure’ error, this arises mainly from the sensing of incorrect static pressure, and is described more fully in the section entitled Pressure Heads.

Position errors throughout the speed range are determined by the aircraft manufacturer during the test flying programme for a particular aircraft type.

It is not unusual to compile a joint correction card for position and instrument errors and place it in the aircraft near the ASI concerned.

Summary:

IAS (indicated air speed) ± P and I correction = CAS Manoeuvre-Induced Errors.

These are associated chiefly with manoeuvres involving change in angle of attack, giving transient errors and a lag in the indication of changes in airspeed.

Compressibility Error

Air is compressible and except at true airspeeds below about 150 knots where the effect is negligible, the pressure produced in the pitot tube is higher than it would be for an ideal incompressible fluid, for which the dynamic pressure is ½ ρV2_.

The ASI is calibrated to this ideal incompressible flow formula instead of to a more complex compressible flow formula.

Because of this, the instrument will over-read, IAS and CAS will be too high, and a subtractive compressibility correction will have to be applied preferably before correcting for density error, i.e. to CAS giving what is known as EAS (equivalent airspeed).

The correction, which exceeds 20 knots if TAS is near the speed of sound, can be obtained from graphs or tables, or it can be applied by most high speed navigation computers.

With true airspeeds of less than 300 knots the error is small enough to be ignored in the calculation of TAS from IAS.

With most ASIs, compressibility error appropriate to IAS mean sea level conditions is allowed for in the calibration of the instrument.

Order of Correction

1. Apply P and I correction to IAS, giving CAS.

2. At high speeds, apply the subtractive compressibility correction to CAS to give EAS.

3. From CAS or EAS, obtain TAS by use of the computer (or a special chart) to correct for density error.

Air Speed Definitions

IAS(Indicated Air Speed) = indicated reading on instrument

CAS(Calibrated Air Speed) = IAS ± correction for instrument & position error EAS(Equivalent Air Speed) = CAS ± compressibility corrections

TAS(True Air Speed) = CAS ± density error ± compressibility error or EAS ± density error

More ASI Definitions

V_S0 = The stall speed or the minimum steady flight speed in the landing configuration. V_S1 = The stall speed or the minimum steady flight speed in a specified configuration. V_FE = The maximum Flap Extension speed

V_NO = The maximum normal operating limit speed. V_NE = The Never Exceed speed

V_LE = The maximum speed Landing Gear Extended speed V_YSE = Best rate of climb when Single Engine (2 eng a/c)

Some ASI=s incorporate coloured markings on the dial - these >range markings’ consist of coloured arcs and radial lines.

The White Arc denotes the flap operating range, from stall at maximum AUW in the landing configuration (full flap, landing gear down, wings level, power-off) up to V_FE (maximum flaps extended speed).

The Green Arc denotes the normal operating speed range, from stall speed at maximum all-up weight (flaps up, wings level) up to V_NO (normal operating limit speed or maximum structural cruising speed) which should not be exceeded except in smooth air. Operations at IASs in the green arc should be safe in all conditions, including turbulence.

The Yellow Arc denotes the caution range, which extends from V_NO (normal operating limit speed) up to V_NE (the never exceed speed). The aircraft should be operated at IASs in the caution range only in smooth air.

A Red Radial Line denotes V_NE, the never exceed speed. Some ASIs have blue radial lines to denote certain important speeds, (e.g. best single-engines speed for a light twin-engined aeroplane).

Optionally for piston engined light twins:

A Blue Radial Line denotes the best rate of climb speed for one engine out, maximum weight, at mean sea level (V_YSE).

A RedRadial Line denotes the minimum control speed at maximum weight (V_MCA). TOLERANCE - (JAR 25) - ± 3% or 5 Kts whichever is the greater

V_SO V_S1 V_MCA V_YSE V_FE V_NE V_NO