Chapter 2 - Pressure Altimeter Flashcards
What converts pressure changes into a mechanical movement?
Aneroid Capsule
What does an aneroid capsule contain?
Contains vacuum or very low pressure
Detail the different types of altimeters
Simple
Sensitive
Servo
What are advantages of servo altimeters?
Improved accuracy at higher altitudes
Friction in conventional linkage reduced
Digital readout / warnings are easily incorporated
What is an encoder Altimeter?
Digitizers send output to the transponder for transmission to ATC, only on 1013 hPa.
What is density altitude?
Density Altitude is the height in ISA conditions that has a density corresponding to the actual ambient density. Or in other words, it is Pressure Altitude corrected for ambient conditions.
What is the formula for working out density altitude?
Density Altitude = Pressure Altitude +/- 120ft x per 1oC change from ISA
- If colder than ISA, subtract from Pressure Altitude
- If warmer than ISA, add to Pressure Altitude
In an unpressurised A/C will the altimeter over or under read?
The altimeter will over-read due to the pressure in the cabin being lower than
ambient (aerodynamic suction). N.B. For EASA, assume alternate static port is within the cabin.
In a pressurised A/C will the altimeter over or under read?
The altimeter will show the (lower) cabin altitude rather than ambient altitude.
What is Lag error?
Response to height changes are not instantaneous:
- Climb – Under-Read
- Descent – Over read
Sensitive Altimeter - lag should not exceed 150ft.
Servo-assisted Altimeter - no lag for vertical speeds less than 10 000 ft/min Altitude and time spent at altitude / cold increases the lag.
What are causes of error in altimeter readings?
Barometric (atmospheric pressure differs from ISA), Temperature (temp different from ISA), Pressure, Instrument, Lag, Orographic, Blockages
What is the rule of thumb for correcting for density altitude?
Density Altitude = 120 ft change in Pressure Altitude per 1C change from ISA for density Altitude
