B - The Altimeter (17-05-18) Flashcards
What is the altimeter calibrated to?
ISA
What pressure lapse rate should be used in instrument calculations?
30ft/hPa unless otherwise stated.
What happens to altimeter accuracy with altitude?
The accuracy decreases
What errors are altimeters subject to?
Instrument error
Maneouvre error
Position/pressure error
Barometric error
Lag
Temperature Error
IMP BLT
How do we reduce the lag of an altimeter?
Buy a more expensive one.
What is barometric error?
When the altimeter has the wrong MSL pressure set as it’s reference
What is temperature error in an altimeter?
Where the outside air temperature is different to ISA so the altimeter over/underreads.
What causes lag in an altimeter?
friction in gears and linkages
What is the effect of a total blockage of the altimeter static line?
The altimeter reading will freeze.
What is the effect of a total blockage of one of an altimeter’s static ports?
There will no effect unless the aircraft is side slipping
What is the effect of a total blockage of an altimeter’s upstream static port in a side slip?
Only the static port subject to suction will provide a static reading.
This reading will therefore be lower and the altimeter will over read.
What is the effect of a total blockage of an altimeter’s downstream static port in a side slip?
Only the static port subject to increased pressure due to dynamic pressure will provide the static pressure reading.
This will increase the overall static reading causing the altimeter to underread.
What will be the effect of a leak in the static line of an altimeter in an unpressurised aircraft?
The static port will read the same pressure as the aircraft cabin, which in this case is slightly lower thean outside (due to the suction effect at any orifices).
Therefore the altimeter will over read.
What will be the effect of a leak in the static line of an altimeter in a pressurised aircraft?
The static pressure will be read as the same as cabin pressure, which in this case will be much higher than outside air pressure.
Therefore the altimeter will most likely under read.
What are the 3 types of altimeter?
Simple altimeter
Sensitive altimeter
Servo altimeter
What are the features of a simple altimeter?
Has 1 needle
Range is typcally 0-10000ft
Can’t set reference pressure
What are the features of a sensitive altimeter?
You can set reference pressure
Has warning flags
More accurate than simple altimeter
Has a knocking device to reduce lag by vibrating mechanical linkages
Has 3 needles
How does a servo altimeter work?
An E bar has it’s middle prong turned into an electromagnet.
The 2 outer prongs have wire wrapped around them in opposite directions.
An I bar is placed close to the E bar and an AC current is run through the midd prong of the E bar wiring.
This induces a magnetic field in 2 circuits, one involving the top prong and one with the bottom
As the aneroid capsule expands or contracts this moves the I bar and makes the distance from the E bar different from top to bottom
This changes the magnetic fields and induces a current which is amplified and sent to a motor which turns the needles.
The E bar is connected to the motor which then rotates it back to neutral position which stop the motor from continuously turning the needles.
What are the advantages of a servo altimeter?
It is more accurate at high altitude
Electrical output can be fed to other components
Lag is virtually eliminated
What is density altitude?
The altitude in the ISA at which the prevailing/occuring density would occur
