Pressure Instruments Flashcards
Dynamic Pressure Formula
KE (dynamic pressure) = 0.5pV ²
P = local air density
V = true airspeed
Altimeter
Equilibrium created by the pressure of the atmosphere and the tension of a spring
Closed capsule mounted in a case fed by the static source
Altimeter instrument Error
Due to capsule movements small irregularities are impossible to avoid
Error increases with altitude
Altimeter Pressure Error
Atmospheric pressure not accurately transmitted to the instrument due false static pressure created in the vicinity of the pressure head due airflow over it
Negligible at low altitudes and speeds
Correction applied at indicated height
Altimeter Time Lag
Response not instantaneous therefore a lag causing an under-read on the climb and over-read on descent
Proportional to rate of descent
Altimeter Hysteresis Error
Capsule under stress exhibits an imperfect elastic response
Non-linear response of bellows
Noticeable after sharp climbs and descents
Altimeter Barometric Error
Actual sea level barometric pressure differs from that assumed and calibrated by the standard atmosphere set on the sub scale
High to low pressure = overreads
Altimeter Temperature Error
Atmospheric conditions differ to standard atmosphere
Cold air = decreased pressure and altimeter will read high
ISA +, the altimeter will underread
Altimeter Static Vent Blockage
No change to altimeter readout
Calculate Aircraft True Altitude Using Wiz Wheel
- Calculate pressure height
- Determine OAT at aircraft pressure height
- Using flight computer set Pressure height against OAT
- Find calibrated altitude and across will be the true altitude
Calculate Aircraft True Altitude
4% of indicated height AGL (local QNH) per 10 degrees of ISA variation
4% of indicated altitude AMSL (area QNH) per 10 degrees of ISA variation
Pressure Height
Elevation + 30(1013 - QNH)
Airspeed Indiactor
Measures dynamic pressure in a capsule enclosed by static pressure
TAS
Is equal to IAS at ISA conditions
If conditions vary a correction is required
Airspeed Instrument Error
Manufacturing tolerances in the construction
Determined using calibration and any correction is combined with that for pressure error
Airspeed Pressure Error
Disturbances in static pressure around the aircraft due to movement through air
Pressure Error Correction (PEC)
Tabulated with Instrument Error Correction (IEC) on a correction card and applied to IAS to obtain CAS or rectified air speed (RAS)
Airspeed Compressibility Error
The compressibility of air is significant at higher speeds if different at all from ISA
At higher altitudes, less dense air is more easily compressed resulting in greater dynamic pressure and causing the ASI to overread
Compressibility increases with speed
Equivalent Air Speed (EAS)
Application of compressibility error by the flight computer (f factor) to CASA
Above 250-300kts
Airspeed Density Error
Dynamic pressure varies with airspeed and density
As altitude increases, density decreases so IAS and EAS will progressively become lower than TAS
For the same capsule deflection as that at MSL the aircraft must be travelling faster
Correction calculated by flight computer
Blockages (PUDSUC)
Pitot blocked: no changes in level flight
Static blocked: overread at lower altitudes and underread at higher altitudes than that at which the blockage occurred.
Mach Meter (IMN)
ASI with altitude sensing capsule to compensate for pressure height
Mach Meter Errors
Instrument and pressure errors
True Mach Number (TMN) accounts for these errors
Vertical Speed Indicator
Records rate of change of atmospheric pressure
As the aircraft climbs, pressure escapes the capsule faster and contracts causing the pointer to indicate a rate of climb and vice versa.
