Pressure Instruments

Question 1

Define static, dynamic and pitot pressure

Answer 1

Static - pressure exerted by atmosphere at rest
Dynamic - pressure due to movement
Pitot - total pressure (static+dynamic)

Question 2

Explain the pressure requirements of the three basic pressure instruments.

Answer 2

Operate by sensing changes in pressure of the air which is then converted into a measurement.

Question 3

Describe a typical pitot-static system

Answer 3

Arrangement of intakes and tubing that obtain pitot pressure and static pressure which then guide theses pressures as appropriate to the pressure instruments.

Question 4

Explain the difference between a single pitot (pressure) head and separate pitot tube and static vent system.

Answer 4

Single pitot head only collects dynamic pressure whereas a seperate pitot tube and static vent system collects dynamic and static.

Question 5

Explain the function of pitot heat.

Answer 5

To prevent blockage from ice.

Question 6

Explain the following errors affecting an ASI:

(a) density error (IAS/TAS relationship);
(b) position (pressure) error;
(c) compressibility error.

Answer 6

(A) variations in air density, so climbing at a constant IAS TAS will progressively increase
(B) position of pitot head, static vent doesn’t let some air in or at weird angles meaning incorrect readings
(C) causes instruments to overread, correction is always deduct from the CAS and airspeed called EAS.

Question 7

Principle of operation of ASI

Answer 7

A diaphragm with total pressure form pitot tube is fed to one side and static pressure from static vent fed to another.

Diaphragm expand adopting a position depending on the difference of pressure which is sent to the instrument pointer.

Question 8

Colourcoding of ASI

Answer 8

VSO - bottom of white arc
VFE - top of white arc
VS1 - bottom of green arc (flaps up and undercarriage)
VNO - top of green arc
VNO - yellow arc 
VNE - red
VYSE - blue
VMCA - red line

Question 9

What’s the IAS/TAS relationship

Answer 9

If an aircraft climbed at a constant IAS, TAS must progressively increase as it gains altitude for a constant dynamic pressure to be measured by the airspeed indicator.

Question 10

Describe the pre-flight and in-flight serviceability checks for an ASI.

Answer 10

Pre flight - covers removed and clear of impurities.
Check operation of pitot heat and check ASI isn’t damaged and is reading zero

Inflight - check needle comes off zero shortly after takeoff and is indicating properly.

Question 11

What happens with blockage and leaks of ASI

Answer 11

Pitot tube block or leak - ASI will underread or be low

Static vent block - underread in climb, overread in descent

Static vent leak - overread

Question 12

Principle of operation of altimeter

Answer 12

Contains a sealed aneroid capsule that’s exposed within the the instrument case. As aircraft climbs static pressure decrease and capsule expands and drive the pointers.

Question 13

Explain the meanings of the subscale settings QNH and QFE and QNE

Answer 13

QNH - reads the pressure at mean sea level

QFE - current pressure level at any given datum other then MSL. When set will give a reading of its height above the datum.

QNE - pressure altitude at 1013.2hPa

Question 14

Explain the effect of pressure setting error on altimeter indication.

Answer 14

Hi - Lo - hi - Lo (flying high to low, altimeter reads high but you’re actually low)
Lo - hi - Lo - hi (flying low to high, altimeter reads low but you’re actually high)

Question 15

Explain the effect of air mass temperature, at constant QNH, on altimeter indication.

Answer 15

Flying from high temp to low, altimeter will read high, but aircraft is lower then expected.

Flying from low temperature to high temperature, altimeter will read low, but aircraft is higher then expected.

This is as pressure lapse rate is less in warm air than cold air.

Question 16

Explain the following errors affecting an altimeter:

(a) instrument error;
(b) position error;
(c) lag.

Answer 16

Instrument - manufacturing differences, and general wear and tear

Position - static sources slightly pressurised due to unbalance flight

Lag - takes a second or two to respond

Question 17

Describe altimeter blockages and leakages

Answer 17

Blockages
•Static – Air trapped so no change in altitude recorded

Leaks
•Static – unpressurised – cabin air lower OVERREADS
•Static – pressurised – cabin air higher – UNDERREADS

Question 18

Describe the serviceability checks and the accepted indication tolerances of an altimeter.

Answer 18

Check instrument glass isn’t broken or cracked

Make sure altimeter is within +30 and -45ft limits of correct altitude.

Question 19

Explain the principle of operation of a vertical speed indicator (VSI).

Answer 19

Instrument case of VSI contains a flexible capsule which is vented to the surrounding outside static pressure via static vent and a metering unit which acts as a restricting electrical on the passage of air through it.

in climb. Outside pressure reduced and air vacates the instrument through the static lines and vent. As climb continues theres a greater pressure in the instrument case than in the capsule so capsule is compressed and this action is transferred to the VSI needle as an UP (rate of climb)

Aircraft in descent, pressure increase and air enters the instrument through the vent and line. The capsule expands and is transferred to the VSI needles as a DOWN (rate of descent)

Question 20

Explain the following errors affecting a VSI:

(a) position error;
(b) lag.

Answer 20

Position error - fluctuations in pressure sensing at static vent

Lag - as air is restricted it takes time to adjust

Question 21

Explain the serviceability checks for a VSI

Answer 21

VSI needle should lie horizontally showing no rate of climb or descent and the instrument is considered serviceable if the needle shows no more than 200ft/min within a temp range of - 20 degrees and + 50 degrees and up to +/- 300ft outside.

In flight - check for accuracy. Takes two minutes to gain 1000ft, should read 500ft/min climb

Question 22

Blockage and leak of VSI

Answer 22

Blockage - no air to enter or vacate instrument and VSI will remain static.

Leak - no effect on VSI since cabin air will equalise ambient outside pressure without much delay. Pressurised aircraft, VSI will show the rate of change of cabin pressure.

Question 23

Explain the function and operation of the alternate static system.

Answer 23

• This is used if there should be an issue with the normal static system
• Turning the alternate static system ‘on’ means that the normal static system will be bypassed

Question 24

During flight checks

Answer 24

Ice blockage on pitot tube - only ASI will be affected

Question 25

Errors of asi

Answer 25

Position
Density
Compressibility