Pressure Instruments Flashcards
Define static pressure
- Pressure exerted by stationary air, equal in all directions
- Decreases with decrease in air density
- Sensed in aircraft through static port
Define dynamic pressure
- 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
Define total pressure
Static pressure + dynamic pressure
AKA pitot pressure
Pitot-Static system
- Altimeter, vertical speed indicator, airspeed indicator
- VSI, ASI, and altimeter = static pressure
- ASI = total pressure
Pitot head
Combination of pitot tube and static vent
Drain hole
Removes water from the system
Pitot heat
Remove ice, and prevent build
Alternate static source
In case the main static vent becomes blocked, not all systems have an alternate source.
Air Speed Indicator (ASI) Operation
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
IAS/TAS relationship
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
Indicated airspeed (IAS)
Airspeed displayed by the ASI
Calibrated airspeed (CAS)
IAS corrected for pressure errors
Equivalent airspeed (EAS)
CAS corrected for compressibility error
True airspeed (TAS)
EAS corrected for density error
Position error
If the aircraft does not enter the pitot tube from straight on.
- Flying at high angles of attack, slip or skid.
Compressibility error
When the aircraft is travelling at a high speed.
ASI overestimates the true airspeed of the aircraft.
Over reads at high speeds
Blockage and leaks
- Icing
- Foreign matter
Pitot tube blockage
ASI will read low, regardless of attitude and altitude.
Pitot tube leak
ASI will under-read
Static vent blockage
- ASI under reads with a gain in altitude
- ASI overestimates reads as the aircraft descends
Static vent leak
- ASI will over read as the static pressure inside the cabin is slightly less than outside.
Alternate static port can be used.
PUDSUC
Pitot blocked Under-read, in Descent, Static blocked, Under-read, in Climb
ASI Serviceability checks
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
Principle of altimeter operation
- 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.
QNH
- 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
QFE
- 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
QNE
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
Altimeter position error
Side-slipping and disrupted air around the static source, affects the altimeter reading.
Altimeter instrument error
- Wear and tear
- Need to be calibrated every two years
Altimeter lag
Happens if the pilot “changes” the altimeter. Difficulty staying at the desired altitude.
Altimeter blockages and leaks
- 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.
Alternate static air
- Fitted in some aircraft
- Used with the static port gets blocked
Barometric setting
- Accurate reference for accurate reading.
- QNH = aerodrome elevation
Changes in pressure (altimeter)
- 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
Changes in temperature (altimeter)
- 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
Altimeter serviceability checks
- Instrument glass is intact
- Sub-scale is increased, the altitude also increases and vice versa
Vertical speed indicator (VSI)
- 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
VSI position error
Anything which affects air entering the static source must source must affect the VSI - side-slipping and situated air around the static source.
VSI instrument error
- Wear and tear
- Need to be calibrated every two years
VSI lag
- Uses lag to determine the rate of climb or descent
- Normal to expect slight lag in the reading compared to what is actually happening
VSI blockages and leaks
- 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
Alternate static air VSI
- If fitted, can be used by the VSI
Serviceability checks VSI
- 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.
