True Airspeed -Cont.

To convert IAS to TAS, first determine Calibrated Airspeed (CAS). CAS is the result of IAS

corrected for instrument error. This instrument error is recorded in the cockpit in the form of

an Airspeed Calibration Card which gives what the airspeed indicator reads (IAS) and what

it should read (CAS).

For a given IAS ("X"), TAS will generally increase with an increase in altitude.

PRESSURE ALTITUDE

Pressure Altitude (PA) is the measurement of atmospheric pressure from the "Standard

Datum Plane." To find Pressure Altitude, first determine the Calibrated Altitude. Calibrated

Altitude is Indicated Altitude PLUS or MINUS instrument error. Example follows:

Indicated Alt

= 10,000 feet

Altimeter error

0 feet

Calibrated Alt

= 10,000 feet

Next, find the difference between the given altimeter setting and the Standard Datum

Plane. If the local altimeter setting was 31.12" and the Standard Datum Plane is 29.92", the

problem would look like this:

Local Altimeter

31.12"

Standard Datum Plane

- 29.92"

1.20" (pressure difference)

Then, change the pressure difference (1.20") to altitude (feet) using the standard lapse rate

of 1" Hg (mercury) = 1,000 feet. A difference of 1.20" Hg would equal 1,200 feet.

Finally, you must either ADD or SUBTRACT the pressure difference (1,200 feet) from the

Calibrated Altitude (10,000 feet). If the given altimeter setting is less than 29.92", you ADD.

If the given altimeter is greater than 29.92", (like the above example) you SUBTRACT:

Calibrated Alt

10,000 feet

Pressure Difference

- 1,200 feet

Pressure Altitude

8,800 feet

NOTES:

To assist in determining whether to add or subtract the pressure difference, apply

the term "LAGS" which stands for:

- If the given altimeter setting is Less (than 29.92"), then ADD

- If the given altimeter setting is Greater (than 29.92"), then SUBTRACT

L ess

A dd

G reater

S ubtract

4.6-97