Air Speed Indicator

Dynamic press = P – S (Pilot – Static)

Sea level gravity is 980 : 65 cm/sec2

  • Position Error: Static causes position error. Ground error causes static to read high because the pressure around the Aircraft is high. Therefore, causes A.S.I. to read low or A.S.I. under reads. Generally I.A.S. is more than IAS but close to Vne IAS is more than CAS. Whenever Alternate Static source is used the A.S.I. over reads because in a un pressurized a/c the static pressure is less than the outside pressure. Alternate static vents are vented inside the cabin.
  • Instrument Error: Due to defect in instrument.
  • Density: Because of difference from Standard Density Dynamic pressure depends upon Density therefore error comes. If Density is less than 1225 ASI will under read and if it is more than 1225 ASI will over read CAS/RAS when corrected for the Density error gives True Air Speed

A Thumb rule for this calculation is as follows, since it gives a conservative side of the ASI value-

TAS increases @ 1.75% of RAS / 1000 Feets
Alternate Increasing of IAS Increasing of IAS in %
5,000 32 9%
10,000 42 16%
15,000 52 25%
20,000 62 36%
25,000 72 49%
30,000 82 64%
35,000 92 81%
40,000 102 100%

We calculate TAS for Navigation purposes (because we are covering distance on ground with the help of TAS).

At higher altitude we require more power because at higher altitude TAS is more and to maintain this higher TAS we require more power. Above 20,000 feet we use Mach number and not IAS. With increase in altitude as pressure drops the Static pressure drops and static vent measures less pressure and A.S.I. over reads. Therefore IAS is higher than CAS/RAS generally at lower altitude IAS is less than CAS/RAS because of higher static pressure.

Therefore, we can say that at lower altitude A.S.I. always under read. Even on T/o A.S.I. under reads due to Ground effect, where the Static pressure is high near the immediate vicinity of the Aircraft due to closure of the ground (Pressure Compressed).

When is TAS less than IAS. When a/c is on ground, when pressure is standard and temperature is less than standard sea level.

  • Compressibility Error: At speeds only above 300 kts. At speed above 300 kts. The a/c gets compressed. Therefore Dynamic pressure measured by Pilot tube is more than actual pressure. Therefore,  A.S.I. over reads. While flying IAS can never be above TAS, Compressibility error causes A.S.I. to over read, but not above TAS. Therefore Compressibility error is always subtracted. Compressibility error increases with increase in Air speed. As altitude increases pressure decreasing therefore air is easier to compress. Compressibility of air is a function of speed of sound, as speed of sound decreases compressibility error increases.

As the airspeed increases there is a greater drop in pressure in the immediate vicinity of the a/c. Therefore the Static pressure is measured less by the Static parts and therefore the Static pressure starts decreasing significantly above a particular speed and also CAS<IAS, IAS becomes more than CAS.

Generally at low speeds CAS> IAS, CAS is more that IAS.

Therefore, as we know speed of sound is a function of temperature.

We can say Compressibility Error is also a function of temperature.

  • If Pitot blocks, Air Speed Indicator acts as Altimeter
    • On Climbing it shows more than actual reading.
    • On Descending it shows less than actual reading.
    • If Static blocks, we use Alternate Static source.
    • If no Alternate Static source is available, it is advised to break the VSI glass because in such a condition it is the least important equipment.
    • In a pressurized a/c if V.S.I. is broken the Cabin press will leak into it and
    • Altimeter will show a descent and indicate the maintained Cabin Altitude.
    • Altimeter and power will indicate air speed if V.S.I. is not working.
    • On using Alternate Static source A.S.I. will show over read, because the Alternate Static source is located in the Cabin, where static pressure is low.
    • In level flight which instrument shows extend of power being used-A.S.I.
    • Attitude and power will indicate airspeed if A.S.I. fails.
    • It is a normal practice to turn the Pitot head heat and Windscreen heat ON for Takeoff and Landing, since the impact strength of the Windscreen increases by using the Windscreen heat, that is very beneficial in case of a bird hit.
    • The heat to the Pitot head and Windscreen is Electro statically controlled.

Ground Effect: A/c finds difficulty in climbing after T/o. or A/c tends to settle down after T/o. With in ground effect you feel nose heavy and back pressure on the control column is required because with in ground effect downwash of the tail plane decreases, therefore tail plane effectiveness decreases, this decreases down load being produced by the fail plane to become airborne.

A/c floats on landing. A/c gets airborne early due to the Ground Effect with in ground effect (up to one Wing Span of the A/c) there are no Wing Tip  vortices, because air does not get a chance to go around .

Therefore, there is no Induced Drag in the Ground Effect.

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A.S.I.

In warm region A.S.I. under reads less that actual.

In cold region A.S.I. over reads shows more than actual.

As altitude increases atmosphere becomes rarer and density decreases therefore the dynamic pressure falls and indicator shows reading which is too low for a given airspeed

                                                  Dynamic = Pilot – Static

                                                  Pilot = Dynamic + Static

As altitude increases TAS increases because of density decreasing.

A.S.I. will read correct only if flying in an air masses of such density as would produce if the prevailing pressure was 1013.2 and temperature is + 150 c (ISA conditions)

TAS increases as follows
Alternate Increasing of IAS Increasing of IAS in %
5,000 32 9%
10,000 42 16%
15,000 52 25%
20,000 62 36%
25,000 72 49%
30,000 82 64%
35,000 92 81%
40,000 102 100%

For more accurate calculations at high altitude (TAS increasing 1.75% of RAS per 1000 feet) Or (2% increasing TAS of RAS per 1000 feet) can also be taken.
Compressibility error is Applicable to a/c flying above 300 knots.
Compressibility error always cause A.S.I. to over read and the correction applied is always negative.
Only error that is always subtracted.

ASI Errors

  1. Instrument
  2. Position
  3. Compressibility.
  4. Density
  5. Blockage

Speeds

I.A.S Corrected for instrument and position gives
CAS/RAS corrected for Compressibility error gives
EAS corrected for Density error gives
TAS corrected for Wind Velocity gives
G/S
If Pitot Blocks, ASI works like altimeter
On Climbing ASI, over reads, shows more than actual.
On Descent ASI, under reads, shows less than actual.
If Static Blocks ASI-
On Climbing ASI, under reads, shows less than actual.
On Descending ASI, over reads, shows more than actual, which can be very dangerous?