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.
VSI Metering Unit
Provides a definite pressure difference for any ROC or ROD
Uses an orifice and capillary to allow readings to remain correct over a wide range of temperatures
Pressure Compensation (Height)
The pressure difference across an orifice for a given ROC decreases with increasing height.
The pressure difference across a capillary type metering unit at a constant rate of climb increases with increasing height.
Therefore a combination compensates for both issues.
VSI Lag
There is a delay before the pointer settles on the correct ROC/ROD due to the pressure difference developing however a trend is immediately indicated
VSI Pressure Error
Due to the movement of air around the pitot/static heads it may incorrectly indicate during a considerable change in airspeed
Static Line Blockage VSI
Becomes unserviceable
If breaking VSI glass, due to the metering device the VSI will work in reverse and the altimeter will lag
VSI Instrument Error
Small errors can be taken out with the zero adjustment screw
Instantaneous VSI
Minimises lag by using a Dash Pot
Works to pressurise the system when a climb or descent is initiated causing the correct Trent to show instantly
Dash Pot
Consists of an inertial operated air pump reacting to pitch and accelerations
IVSI Errors
Same as VSI except lag
Direct Reading Compass
Magnetic compass which aligns itself with Earth’s magnetic field and may not sit horizontal
This is called compass dip which increases as latitude increases
Causes compass centre of gravity to move away from the compass pivot point
Compass Errors
SAND (Max on E/W headings)
ONUS (Max on N/S headings)
DG Rigidity
Rigidity of the gyroscope causes the heading indicating to change whilst the card remains still
Axis reference will gradually shift due to real and apparent precession
Apparent drift
Movement across the Earth’s surface and the Earth’s rotation
Real Drift
Gyro imperfections and bearing drag
Remote Magnetic Indicator (RMI)
A DG which maintains its ow alignment with the aircraft heading
An electric heading reference signal from a flux valve and a motor on the instrument card to realign the card to the correct heading
Flux Valve
Remotely located to decrease the effect of magnetic deviation
Consists of 3 electrical coils generating electrical signals when passed through the Earth’s magnetic field
Signals are then fed to the control unit
Main Error of a Flux Valve
Is still deviation! Just to a smaller degree
Flux Valve Control Unit
Where flux signals are compared to a reference
If signals are synchronised gyro is aligned
Minimises turning and acceleration errors
Bootstrapping
Left flux valve supplies heading reference for the left RMI and right HSI
Horizontal Situation Indicator (HSI)
An RMI with a full VOR/ILS display
