PITOT-STATIC SYSTEMS

Question 1

Describe how Altitude is measured

Answer 1

Static pressure is fed into the case of the instrument.
With height the pressure inside the case decreases and the capsule inside it expands and drives a pointer to indicate higher altitude

Question 2

What is:

1. QFE
2. QNE
3. QNH

Answer 2

1. QFE = Altimeter reads zero at airfield elevation
2. QNE = Based on 1013 hPa
3. QNH = Barometric pressure corrected for sea level

Question 3

What errors are associated with an altimeter?

Answer 3

1. Instrument error
2. Pressure error due to disturbed airflow. Worse a higher speeds
3. Barometric error
   HI-LOW careful go
   LOW-HI clear the sky
   Pressure lines compact and expands due to pressure and temperature
4. Blockages
   SUCOD and POCUD

Question 4

What happens to the altimeter in a climb with a blocked static port?

Answer 4

It will under-read
The higher pressure will be trapped inside the capsule which makes the altimeter feel like it is lower than it actually is.

Question 5

Describe how Vertical Speed is measured?

Answer 5

Records rate of change of atmospheric pressure
A capsule is fed with static pressure
The air tight case is also fed via a restricted choke
If static pressure is changed the pressure surrounding the capsule changes at a slower rate than within the capsule

Question 6

How is a climb measured with a VSI?

Answer 6

The pressure in the capsule is less than that in the case so there’s a slight compression on the capsule to stop it behaving like an altimeter.

Question 7

How will blockages affect the VSI?

Answer 7

Blockages will cause the needle to return to zero

Question 8

What two different scales can a VSI have?

Answer 8

1. Linear, same distance between increments

2. Logaritmic, greater distance between increments at lower readings making it easier to read on an ILS

Question 9

What preflight checks do you do on a VSI?

Answer 9

+-200 ft in temp range of -20 to +50 deg C

Question 10

What does a pitot tube measure?

Answer 10

Total pressure which is Static pressure + Dynamic pressure

Question 11

Describe how airspeed is measured?

Answer 11

Pitot pressure is fed into a capsule. Static pressure is fed into the case
Dynamic pressure is: Total pressure - Static pressure
The static pressure is pushing back on the total pressure inside the capsule, effectively canceling out the Static Pressure inside the capsule and leaving us with dynamic pressure

Question 12

How does a Mach meter function?

Answer 12

Measures the ratio of Dynamic pressure to Static Pressure which is TAS/LSS

Question 13

What error are associated with a Machmeter?

Answer 13

Instrument error and Position error
Doesn’t suffer from density or compression error
Density as it affects Dynamic- and Static pressure equally and is effectively cancelled out