Air Data Sources

Question 1

what is static pressure?

Answer 1

the environmental pressure which surrounds use and results from the weight of the column of the air above which represents the force exerted by the atmosphere at rest.

Question 2

What is dynamic pressure?

Answer 2

the pressure due to the atmosphere impacting on a body.

Question 3

what is pitot pressure?

Answer 3

total pressure, static + dynamic pressure.

Question 4

what are the factors affecting dynamic pressure?

Answer 4

the velocity of the body relative to the air

the density of air.

Question 5

how are pressures sampled more accurately?

Answer 5

using separate pitot and static vents. static vents located on both sides of the fuselage but feeding to the same system will minimise manoeuvre errors.

Question 6

the greater the airspeed the greater the what?

Answer 6

the mass of air impacting on the aircraft per unit time and therefore the greater the dynamic pressure.

Question 7

what is VNE?

Answer 7

never exceed speed (red line)

Question 8

what is VNO?

Answer 8

maximum cruise speed (top of green arc)

exceed this speed only in smooth air

Question 9

what is VFE?

Answer 9

maximum speed for flying with flaps fully extended (top of white arc)

flap damage may results if you exceed this speed with flaps down.

Question 10

what is VS1?

Answer 10

Stall speed with undercarriage and flaps up (bottom of green arc)

Question 11

what is VS0?

Answer 11

Stalling speed with undercarriage and flaps down (bottom of white arc)

Question 12

How does an ASI work?

Answer 12

air from the pitot tube (total pressure) enter the diaphragm causing it to expand moving the needle on the ASI.

The air from the static port enters the instrument outside the diaphragm but inside the instrument case the static pressure pushes against the diaphragm and in this way cancels out the static pressure inside.

Question 13

how can airspeeds be remembered and what they correct?

Answer 13

ICE T and PCD

• Indicated Airspeed (IAS)
• Calibrated Airspeed (CAS) which is IAS corrected for Pressure Errors
• Equivalent Airspeed (EAS) which is CAS corrected for compressibility error
• True Airspeed (TAS) is EAS corrected for density error

P = pressure
C = Compression
D = density

Question 14

ASI is calibrated to indicate what?

Answer 14

TAS only under ISA conditions and only at MSL.

Question 15

what is groundspeed used for?

Answer 15

navigation

Question 16

what is the relationship between IAS, TAS, and GS

Answer 16

• IAS is the aerodynamic speed of the aircraft. in lowest levels of the atmosphere, IAS is usually close to TAS
• TAS is the actual airspeed. Can be calculated by applying a correction for air density to IAS
• GS is the speed of the aircraft relative to the ground. can be calculated by applying wind velocity to the aircraft heading and TAS.

Question 17

what are the three main pressure instruments in an aircraft?

Answer 17

ASI, VSI and altimeter.

Question 18

what do pitot tubes do?

Answer 18

the pitot tube is designed to measure the pressure of the air as the plane flies through it.

Question 19

what do static ports do?

Answer 19

measure the static pressure of the air.

Question 20

what does the drain hole do?

Answer 20

remove water from the pitot tube system

Question 21

what does the heater do?

Answer 21

remove ice and prevent a build-up

Question 22

what does an alternate static source do?

Answer 22

used in case the main static vent becomes blocked.

Question 23

what is density error? and how is this fixed?

Answer 23

the inability of the ASI to take account of variations in air density, i.e. to give a true airspeed called density error.

a small rotatable scale, which provides compensation for density error allowing the pilot to obtain the TAS as well as the IAS.

Question 24

what causes instrument error?

Answer 24

usually some slight variatiosn in the readings obtained from individual instruments, through age, minor differences in manufacture and the application of particular ASI to the type of aircraft.

only amount to no more then 1-2 knots so typically ignored.

Question 25

what causes position error?

Answer 25

by disrupted airflow as it passes the static vents.

also occurs if the airflow doesn’t enter the pitot tube from straight on, or if airflow around the static vent is affected by the way the aircraft is being flown. (flying at high AOA or with slip and skid)

Question 26

what is the most common cause of a blockage?

Answer 26

icing followed by foreign matter and bugs in the pitot tubes

Question 27

what are the symptoms of blockages and leaks? (4)

Answer 27

pitot tube blockage - ASI read low regardless of attitude and altitude

Pitot tube leak - ASI will under-read

Static Vent blockage - ASI will under-read with a gain in altitude

Static Vent leak - ASI will over-read (unless pressurised and aircraft draws air from cabin ASI will under-read)

Question 28

how can blocks and leakages be remembered?

Answer 28

PUDSUC

Pitot Blocked
Underread
In Descent
Static blocked
Underread
In Climb

Question 29

What are preflight checks of ASI? (4)

Answer 29

• ensure pitot cover and any static vent blanks are removed and pitot head isn’t damaged
• check pitot and static intakes are clear of contamination
• check operation of pitot heat. don’t leave it on for longer then a few seconds when checking on the ground
• in cockpit check ASI isn’t damaged and is indicating 0

Question 30

what are in-flight checks of ASI? (3)

Answer 30

• Check ASI needle begins indicating airspeed on the take-off roll.
• check that the ASI indicates the appropriate speed at lift off
• Airspeed indications should be as expected for the power setting and attitude flown

Question 31

what are the after landing checks of the ASI?

Answer 31

• fit pitot and static covers on if you have

- ensure pitot heat has been turned off