Altimeter Flashcards
Altitude units
feet or meters
3.28ft= 1m
Height is…
height above ground
QFE used for height above aerodrome
Altitude is…
height above mean sea level
QNH used. may vary between regions with pressure and temperature
Pressure altitude is…
height above standard datum line (1013)
STD-flight levels
indicated altitude
reading on altimeter when set to local QNH
True altitude
actual elevation above MSL
How the altimeter works
purely on static pressure
partially evacuated aneroid capsule
measures change from set reference datum(pilot set QFE or QNH etc)
Calibrated to ISA
descent= more static= aneroid compress= indicate reduction in ALT
Altimeter construction
sealed tube static inlet partially evacuated aneroid capsule leaf spring metal linkage scale
ISA model
Rate of change of pressure change feet per hPa in the atmosphere is non linear
density altitude
ISA altitude equivalent to pressure altitude
= pressure alt + (isa deviation x 120ft)
Simple altimeter
not used 1 needle no pressure setting knob single aneroid basic adjustment to allow 0 feet to be set coarse scale- 0-10000ft range
sensitive altimeter
common in light larger range 3 needles pressure setting knob aneroid stack jewelled bearings vibrator (reduces lag) digitiser (for mode C height encoding) easier to read
Servo altimeter
more accurate virtually no lag aircraft with no ADC E and I bars (magnets) mechanical linkages removed 3 needles pressure setting knob aneroid stack stack connected to I bar relies on electromagnet moving- based on equal gaps between E & I bars more accurate
servo altimeter operation- Climb
capsules expand
I moves closer to E at bottom
imbalance in gaps
current induced and voltage to servo motor(turns needles)
worm gear and follow up cam re position E bar to equal the air gaps again and stop current
GPS altitude
receivers generate gps derived alt value for the receiver based on trilateration of the received gps satellite signals
not affected by temp or pressure
does not provide same separation from aircraft
large aircraft do not display GPS alt
