12.36.10(b) . subscale settings and the meaning of QNH and QFE;
– designed so a range of pressure values can be set on a subscale using the rotatable knob – displays the height above whatever pressure value has been set on the subscaleperations below 13000 ft AMSL arfe conducted with reference to mean sea level and so the MSL is always set on the subscale – this pressure setting is the QNH
QFE – is the current pressure level at any given datum other than MSL When QFE is set on the subscale the altimeter will give a reading of its vertical distance or height above the datum selected
12.36.10(a) . basic principle of operation and serviceability checks;
Altimeter – the most important instrument for vertical navigation – relates the static pressure at the level of the aircraft to a height inthe International Standard Atmosphere (ISA) – The Standard (ISA) pressure at Mean Sea Level MSL is 1013 hPa
Serviceability checks – during pre-start checks turn the altimeter subscale setting knob until the aerodrome elevation is indicated – when QNH is obtained set it on the subscale and check the altimeter reading is within +30 and -45 ft of aerodrome elevation – if not request a repeat QNH – if altimeter remains outside the limits it is deemd to be unserviceable – refer to instructor or service engineer
12.36.10. With respect to the altimeter, describe the:
Alt
(a) . basic principle of operation and serviceability checks;
Altimeter – the most important instrument for vertical navigation – relates the static pressure at the level of the aircraft to a height inthe International Standard Atmosphere (ISA) – The Standard (ISA) pressure at Mean Sea Level MSL is 1013 hPa
Serviceability checks – during pre-start checks turn the altimeter subscale setting knob until the aerodrome elevation is indicated – when QNH is obtained set it on the subscale and check the altimeter reading is within +30 and -45 ft of aerodrome elevation – if not request a repeat QNH – if altimeter remains outside the limits it is deemd to be unserviceable – refer to instructor or service engineer
(b) . subscale settings and the meaning of QNH and QFE;
– designed so a range of pressure values can be set on a subscale using the rotatable knob – displays the height above whatever pressure value has been set on the subscaleperations below 13000 ft AMSL arfe conducted with reference to mean sea level and so the MSL is always set on the subscale – this pressure setting is the QNH
QFE – is the current pressure level at any given datum other than MSL When QFE is set on the subscale the altimeter will give a reading of its vertical distance or height above the datum selected
(c) . errors affecting the altimeter, including subscale setting error.
Altimeter Errors
Instrument error – small imperfections arising from the manufacture, installation, mintenance, age of the individual altimeter Instrument lag – altimeter takes a second or two to respond to rapid pressure changes Position error – occurs for same reason as on ASI except only the static vent is involved (ie if flown with slip or skid) Subscale setting errors – QNH must be set accurately on the subscale – setting must be updated whe significant changes in MSL pressure occur
12.36.8(d) . errors affecting the ASI, and how position error correction is applied.
Density Error – this is the inability of the ASI to take account of the air density variations. – some ASIs have a small rotatable scale which provides compensation for its density error
Position Error – if in a high nose attitude the pitot tube and static vents will be at an unusual angle to the oncoming airflow and the way in which they register the pressure may be changed. Any position error from a high nose attitude is allowed for in the callibration of the Vso and Vs1 marks on the ASI A posiiton error correction PEC is usually found in the POH (Pilot operating Handbook) – applied to the ASI readout to give the CAS (callibrated airspeed)
Instrument Error – sometimes wear and tear can cause slight variations in the readings – only usually amounts to 1-2 knots of error
12.36.8(a) . basic principle of operation and serviceability checks;
– provides a reading of the airspeed referred to as the Indicated Airspeed IAS – measures dynamnic pressure acting on the aircraft
Checks During pre-flight external inspection – check the pitot cover is removed Chech pitot head and static vents for damage and blockage ASI should read zero
12.36.8. With respect to the airspeed indicator, describe the:
(a) . basic principle of operation and serviceability checks;
- provides a reading of the airspeed referred to as the Indicated Airspeed IAS - measures dynamnic pressure acting on the aircraft
Checks During pre-flight external inspection - check the pitot cover is removed Chech pitot head and static vents for damage and blockage ASI should read zero
(b) . colour coding, and the meaning of VSO, VS1, VFE, VNO and VNE;
Green arc - normal operating speed range Yellow arc - caution range White arc - flaps operating range Red radial line - never exceed speed
VSO - stalling speed - landing gear down, flaps lowered power off VS1 - stall speed - landing gear up, flaps up, power off VFE - maximum speed flaps extended VNO - normal operating limit speed VNE - never exceed speed
12.36.8(c) . IAS/TAS/groundspeed relationship;
IAS is the "aerodynamic" airspeed of the aricraft At low levels of the atmosphere IAS is close to TAS
TAS is the actual speed of the aircraft through the air It can be calculated by applying a correction for air density to IAS
GS is the speed of the aircraft relative to the ground It can be calculated by applying W/V to the aircraft heading and TAS
(d) . errors affecting the ASI, and how position error correction is applied.
Density Error - this is the inability of the ASI to take account of the air density variations. - some ASIs have a small rotatable scale which provides compensation for its density error
Position Error - if in a high nose attitude the pitot tube and static vents will be at an unusual angle to the oncoming airflow and the way in which they register the pressure may be changed. Any position error from a high nose attitude is allowed for in the callibration of the Vso and Vs1 marks on the ASI A posiiton error correction PEC is usually found in the POH (Pilot operating Handbook) - applied to the ASI readout to give the CAS (callibrated airspeed)
Instrument Error - sometimes wear and tear can cause slight variations in the readings - only usually amounts to 1-2 knots of error
– enables the pressure inside the cabin to be sensed in the event of ice or other stuff blocking the external vents – in unpressurised aircraft the cabin pressure is only slightly less than the external atmospheric pressure so the instrument readings will be only slightly errored if the alternate source is selected