Chapter 3 - Pitot/static instruments Flashcards
What are the three Q codes?
QNE - ICAO standard mean sea level pressure
QFE - Ambient static pressure existing at mean airfield level at any particular time of the day
QNH - Mean sea level pressure at any particular time of day
How does a pilot obtain values for QFE and QNH?
ATC
What will show on the instrument when QNH is input while the aircraft is on the ground?
Height of the airfield above mean sea level
What should all aircraft instruments be set to above a transition altitude?
QNE
How can an approximate true altitude be obtained locally?
By setting the altimeter to the local QNH
What is the absolute altitude?
Height above ground level (AGL)
What is true altitude?
Height above mean sea level (AMSL)
How is absolute altitude calculated?
Absolute altitude = true altitude - elevation of the ground
What is the only system capable of directly indicating the absolute altitude?
Radar altimeter
What is indicated altitude?
Indicated reading which is seldom accurate as standard conditions are rarely met and errors may be present in the instruments and pitot/static systems
What is calibrated altitude?
Indicated altitude correct for instrument error and pressure errors
How will increases in temperature affect indicated altitude?
Drop in indicated altitude
How will an increase in atmospheric pressure affect indicated altitude?
Shows a drop in indicated altitude
What might cause a sensitive altimeter case to misread?
The instrument case is sealed, therefore leaks in a pressurised aircraft will cause the instrument to misread (altitude will read lower)
Why is beryllium copper often used as the material of which the stack of three capsules is made within sensitive altimeter cases?
Natural springiness of the material
How is capsule elasticity compensated for temperature changes?
Invar steel/brass bi-metallic ‘U’ shaped strip around the capsule stack
How is reading of the altimeter dial simplified?
Number of different pointer lengths and shapes have been used
What are servo altimeters?
Pointer and counter mechanism are driven using a servo (motor) system to overcome torque requirements of operating counter mechanisms
Where is the input excitation supplied in a servo assisted altimeter?
AC supply is fed to the centre leg of the ‘E’ bar
How are the secondary coils connected in the ‘E’ bar of a servo assisted altimeter?
The voltages in each leg are 180° out of phase and equal and opposite voltages
Why should the pressure setting in a servo altimeter only be moved with electrical power supplied to the instrument?
The E&I bar might become out of balance by a large amount, so that when power is reapplied a large error voltage is created, and may drive the system very fast and damage internal components
What scale of error is accepted in cabin altimeters?
± 500 ft
How does the cabin altimeter sense cabin pressure?
The instrument’s case is vented directly to atmosphere
The vent is usually protected by a renewable filter
How does a dual altimeter and differential pressure gauge display differential pressure?
Both atmosphere and cabin altitudes are displayed using different pointers
The difference between the two pointers is used to indicate pressure differential in psi
What is the altitude capsule in the cabin differential pressure gauge connected to?
Aircraft static system
What are the safety precautions to be taken when pressure testing the cabin with regards to the cabin altimeter and differential pressure gauge?
The unit is only designed to operate with less than atmospheric pressure, therefore damage to the aneroid capsules and the transmission could result
It can be temporarily removed, or the cabin vent must be blanked
What is the term used to describe the airspeed shown on the instrument?
Indicated airspeed
How does indicated airspeed become calibrated airspeed?
Indicated airspeed has to be corrected for instrument error and aircraft pressure error, during calibration of by the pilot
How does calibrated airspeed become true airspeed?
A correct for air density is carried out using outside air temperature to obtain TAS
What are colour arcs and radial lines known as on an ASI?
Range markings
What will red radial lines signify?
Maximum and minimum limits of airspeed (VNE: velocity never exceed)
What will a yellow arc signify?
Precautionary ranges (VNO: velocity normal operating)
What will a green arc signify?
Normal operating ranges
What will a white arc signify?
Ranges in which the flaps may be lowered
How is backlash or wear in the gearing absorbed in a typical ASI?
Hairspring on the pointer
What are the typical uses of an airspeed switch?
To switch on electrical circuits which are only needed in flight
To switch off electrical circuits which are only needed on the ground
How is temperature compensation handled in an ASI?
Small strip of invar steel is laminated to the ranging bar
The bar will be straight at 15°C but will bend when the temperature changes
How does the capsule expansion change with airspeed?
Small at low speeds and large at high speeds
What is the banana slot?
Slot in the sector gear which is so shaped so that there is some lost motion as the scale increases
What is square law correction?
Taking allowances for the uneven capsule expansion due to the fact that pressure increases four times as speed doubles, to give a nearly linear scale
What are fitted to British made VSIs?
Mechanical stops to prevent pointer movement exceeding the maximum scale reading
They limit the push rod travel but allow the capsule to expand and contract beyond its normal range
What are included in VSIs to compensate for density variations?
Metering unit
What are the two types of metering units in common use in VSIs?
- Ceramic type
- Capillary and hole (orifice)
In the capillary and hole type of metering unit in a VSI, how does air density affect the component?
- A decrease in air density will increase turbulence through the capillary and therefore increase the pressure drop across it
- An increase in air density will decrease the turbulence through the orifice and therefore decrease the pressure drop across it
What are used instead of a tapered screw to achieve zero adjustment in some other VSIs?
Eccentric shaft
At what value of percentage is the required zero adjustment of a VSI considered unserviceable?
More than 10% of the maximum reading
What is a variometer?
Instantaneous vertical speed indicator
Piston on a spring is affected by inertia, so pumps air out or draws air in at the moment a vertical change in displacement is effected, which gives an instantaneous reading on the VSI
What is the speed of sound at sea level and at 40,000 feet?
Sea level - 760 mph
40,000 feet - 650 mph
What is the simple expression for Mach number?
(Pitot - Static)/Static
What will happen to the reading of a Mach meter when altitude is increased but airspeed is maintained constant?
The gauge will indicate a higher Mach number
What is the ‘Barber’s pole’ on a combined ASI and machmeter?
Indication of the onset of high-speed buffet due to the aircraft reaching its critical Mach number
The ‘Barber’s pole’ is linked to altitude and speed and is the setting of aircraft speed in knots which is the equivalent of the critical Mach number at the altitude the aircraft is flying at
