Air Data Instruments & the Atmosphere

Question 1

Describe an Air Data System

Answer 1

Pitot tube informs of static and total pressure.
Temperature probe informs of temperature.
Static & total pressure used to calculate airspeed indicator
Static pressure used to calculate altimeter and vertical speed indicator

Question 2

List sensed and derived parameters of an air data system

Answer 2

Sensed: Pt (total air pressure), Ps (static air pressure), Ps (rate of change of static pressure), Tt (total air temperature)
Derived: ALT (altitude), IAS (Indicated AirSpeed), VS (Vertical Speed), M (Mach number), SAT (Static Air Temperature)

Question 3

What is calibrated air speed?

Answer 3

Airspeed obtained from calibration process applied to directly sensed qualities. Also corrected for installation effects.

Question 4

What is IAS?

Answer 4

Indicated AirSpeed
Displayed by the ASI
Calibrated for sea level conditions
IAS = CAS + X where X is instrument error arising from the physical properties of the indication device

Question 5

What is True airspeed?

Answer 5

CAS corrected for altitude and temperature effects.

It is the airspeed implied by direct application of Bernoulli’s equation

Question 6

Detail the structure of the standard atmosphere

Answer 6

Troposphere
- sea level to 11km
- temp gradient: T=T(sl)-Lh
- lapse rate: L=6.5C/km
Tropopause
-at 11km: -56.5C
Stratosphere (isothermal zone)
-11km to 20km
-constant temp of -56.5C