Pressure Sensing

Question 1

Pressure formula

Answer 1

Pressure = Force/Area

Question 2

Absolute pressure

Answer 2

Absolute pressure is measured relative to a perfect vacuum

Question 3

Differential pressure

Answer 3

Differential pressure is measured relative to another pressure source. Eg. gauge pressure is differential pressure between the measured source and ambient

Question 4

What are the units of pressure?

Answer 4

Pascal (Pa) = 1 newton per square metre
Hectopascal (hPa) = 100Pa
1hPa = 1 millibar (mb)

Inches of mercury (inHg) and pounds per square inch (psi) are also used

Question 5

What is the ISA sea level pressure?

Answer 5

1013.25 hPa
1013.25 mb
29.92 inHg
14.7 psi

Question 6

Static air pressure

Answer 6

Is measured by a partially evacuated aneroid capsule. It is the pressure of the air when stationary

Question 7

Dynamic air presssure

Answer 7

Is the pressure of air, entirely attributable to its movement:

Dynamic = 1/2 ρ (V^2) - V is TAS
Pitot = Static + Dynamic
Dynamic = Pitot + Static

Question 8

Pitot tube

Answer 8

Is an open tube pointing into the air flow. Requires electrical anti-icing for most purposes. Has a water drain at the bottom of the probe.

Question 9

Static ports

Answer 9

Ports are located at right angles to the air flow, usually on opposite sides of the fuselage. Air must not be turning over the port. Manoeuvre errors, especially yaw, are reduced by a pair of ports. The alternate static source is the backup for light unpressurised aircraft. Causes instruments to over-read because cabin pressure is slightly lower than ambient

Question 10

Combined pitot/static ports

Answer 10

May be used on light aircraft but are less accurate

Question 11

Errors

Answer 11

-Instrument errors
-Manoeuvre errors: too many variables to calibrate
-Position error: can be corrected manually or by computer
-Static system error: reduced by converting air data to digital data at or near the static port, or by reducing the length of pipe runs between the static port and the instrument

Question 12

What systems use the pitot/static system as a source

Answer 12

Flight instruments, pressurisation system and flight control systems

Question 13

CAT aircraft pressure systems

Answer 13

CAT aircraft have multiple systems and data interchangeability to give greater system redundancy

Question 14

Heating of pitot/static systems

Answer 14

Is essential to protect against ice build-up, which prevents the systems working properly

Question 15

Alternate static sources

Answer 15

Are used in the event of static port ice build-up
Cabin pressure is substituted for static pressure. Slightly lower pressure causes instruments to over-read

Question 16

Solid state pressure sensors

Answer 16

CAT aircraft have solid-state pressure sensors, with digital output via a data bus, and data is available to all systems using it eg.:
-ADC
-FADEC
-Flight control synthetic feel system
-Aircraft pressurisation system