Pressure Instruments

Question 1

Define static pressure

Answer 1

• Pressure exerted by stationary air, equal in all directions
• Decreases with decrease in air density
• Sensed in aircraft through static port

Question 2

Define dynamic pressure

Answer 2

• Pressure of the air caused by movement
• Depends on the density and velocity of the air

Formula: 1/2pV^2
- 1/2 =-0.5 constant that comes from the kinetic energy formula
- p (Rho) = symbol for air density
- V^2 = velocity of air squared

Question 3

Define total pressure

Answer 3

Static pressure + dynamic pressure

AKA pitot pressure

Question 4

Pitot-Static system

Answer 4

• Altimeter, vertical speed indicator, airspeed indicator
• VSI, ASI, and altimeter = static pressure
• ASI = total pressure

Question 5

Pitot head

Answer 5

Combination of pitot tube and static vent

Question 6

Drain hole

Answer 6

Removes water from the system

Question 7

Pitot heat

Answer 7

Remove ice, and prevent build

Question 8

Alternate static source

Answer 8

In case the main static vent becomes blocked, not all systems have an alternate source.

Question 9

Air Speed Indicator (ASI) Operation

Answer 9

Measures dynamic pressure and gives a reading in Knots

• Input from static port and pitot tube in order to work.
• Air from pitot tube enters the diaphragm, causing it to expand.
• Air from the static port enters the instrument outside the diaphragm but inside the instrument case, pushes against the diaphragm and cancels out the static pressure inside the diaphragm.
• As the diaphragm expands, the needle on ASI moves

Question 10

IAS/TAS relationship

Answer 10

For a constant TAS, IAS will increase with an increase in density, and will decrease with a decrease in density.

The opposite can be said, climbing at a constant IAS, TAS will increase and vice versa.

IAS = the speed felt by the aircraft

Question 11

Indicated airspeed (IAS)

Answer 11

Airspeed displayed by the ASI

Question 12

Calibrated airspeed (CAS)

Answer 12

IAS corrected for pressure errors

Question 13

Equivalent airspeed (EAS)

Answer 13

CAS corrected for compressibility error

Question 14

True airspeed (TAS)

Answer 14

EAS corrected for density error

Question 15

Position error

Answer 15

If the aircraft does not enter the pitot tube from straight on.

• Flying at high angles of attack, slip or skid.

Question 16

Compressibility error

Answer 16

When the aircraft is travelling at a high speed.

ASI overestimates the true airspeed of the aircraft.

Over reads at high speeds

Question 17

Blockage and leaks

Answer 17

• Icing
• Foreign matter

Question 18

Pitot tube blockage

Answer 18

ASI will read low, regardless of attitude and altitude.

Question 19

Pitot tube leak

Answer 19

ASI will under-read

Question 20

Static vent blockage

Answer 20

• ASI under reads with a gain in altitude
• ASI overestimates reads as the aircraft descends

Question 21

Static vent leak

Answer 21

• ASI will over read as the static pressure inside the cabin is slightly less than outside.

Alternate static port can be used.

Question 22

PUDSUC

Answer 22

Pitot blocked Under-read, in Descent, Static blocked, Under-read, in Climb

Question 23

ASI Serviceability checks

Answer 23

Before flight:
- All ports and vents are clear and not contaminated.
- Check ASI is not damaged

In flight:
- Check ASI needle begins indicated on take-off roll
- Check ASI indicated appropriate speed at lift off

Question 24

Principle of altimeter operation

Answer 24

• Inside aneroid capsule - sealed.
• Manufactured to have internal pressure of 1013.25hPa
• Static pressure allowed inside instrument case, exerts pressure on the aneroid capsule
• High pressure compresses the capsule, causing needle to expand and contract.

Question 25

QNH

Answer 25

- Ambient pressure at MSL - Measures the surface pressure at an airfield datum, then converting that to a MSL using ICAO standard atmosphere. Gives an airfield’s elevation above MSL. Used in NZ at all altitudes below 13,000ft

Question 26

QFE

Answer 26

- Current pressure level at any datum other than MSL. - Used at air shows - Set altimeter to read zero feet when on ground at display aerodrome. Shows height above aerodrome

Question 27

QNE

Answer 27

Standard altimeter setting of 1013.25hPa (or 29.92 inches of Mercury). - Reads pressure altitude - In NZ used at all flight levels from FL150 - Set when climbing through 13,000ft AMSL

Question 28

Altimeter position error

Answer 28

Side-slipping and disrupted air around the static source, affects the altimeter reading.

Question 29

Altimeter instrument error

Answer 29

- Wear and tear - Need to be calibrated every two years

Question 30

Altimeter lag

Answer 30

Happens if the pilot “changes” the altimeter. Difficulty staying at the desired altitude.

Question 31

Altimeter blockages and leaks

Answer 31

- Altimeter only uses static port - Any blockage will mean the altimeter will not record any change in altitude - Will sense static pressure at any leak. - Unpressurised cabin - altimeter will overhead as the cabin pressure is slightly lower than the OAT. - Pressurised cabin - altimeter will match the cabin altitude.

Question 32

Alternate static air

Answer 32

- Fitted in some aircraft - Used with the static port gets blocked

Question 33

Barometric setting

Answer 33

- Accurate reference for accurate reading. - QNH = aerodrome elevation

Question 34

Changes in pressure (altimeter)

Answer 34

- Altimeter displays the difference between the pressure at a reference level and the static pressure at the level flown by the aircraft. - If you are flying towards an area of high pressure, the MSL pressure will increase

Question 35

Changes in temperature (altimeter)

Answer 35

- In warmer air, the pressure decreases slower with an increase in altitude - In colder air, the pressure decreases faster with an increase in altitude From high to low, read high be low From low to high read low be high

Question 36

Altimeter serviceability checks

Answer 36

- Instrument glass is intact - Sub-scale is increased, the altitude also increases and vice versa

Question 37

Vertical speed indicator (VSI)

Answer 37

- Measures the rate of change of air pressure to gauge whether or not the aircraft is climbing or descending. - Receives air only from the static port. - The rate of change in air pressure is how the VSI determines vertical speed - In level flight the air pressure inside the diaphragm and inside the VSI case is equal

Question 38

VSI position error

Answer 38

Anything which affects air entering the static source must source must affect the VSI - side-slipping and situated air around the static source.

Question 39

VSI instrument error

Answer 39

- Wear and tear - Need to be calibrated every two years

Question 40

VSI lag

Answer 40

- Uses lag to determine the rate of climb or descent - Normal to expect slight lag in the reading compared to what is actually happening

Question 41

VSI blockages and leaks

Answer 41

- Any blockage in the static port will not record any change in air pressure and so the needle will remain at zero. - Any leak in the line between the static port and the VSI, it will respond to the pressure (and any changes) at that site of that leak

Question 42

Alternate static air VSI

Answer 42

- If fitted, can be used by the VSI

Question 43

Serviceability checks VSI

Answer 43

- Instrument glass is intact and needle is pointing to zero - In flight, VSI indicates logically - Can time a steady climb or descent and check if VSI matches calculations.