Chapter – 3
Principle of operation:
A simple altimeter consists of a thin corrugated metal capsule which is partially evacuated, sealed and prevented from collapsing completely by means of a leaf spring. In some cases complete collapsing is prevented by its own rigidity. The capsule is mounted inside a case. The case is fed with static pressure from aircraft static tube/ vent. As the aircraft climbs the static pressure in the case decreases allowing the spring to pull the capsule faces apart. If the aircraft descends the rise in pressure in the case compresses the capsule faces. The linear movement of the capsule face is magnified and transmitted via a system of gears and linkages to a pointer moving over a scale graduated in feet according to the standard atmosphere. This calibration is according ICAO standard atmosphere. This is also called translation of pressure into vertical height. A simple altimeter is set to indicate height above the 1013.25 mb pressure. The dial adjusting knob allows the indicator needle to be moved away from datum. By this an altimeter is set on ground to read air field elevation so that it will there after indicate height above mean sea level. If the altimeter is set zero before takeoff it will read height above air field.
The sensitivity of altimeter can be increased by adding two or more capsules. So that it’s linear displacement is increased and it can detect small changes of pressure. Such directly operated capsule altimeters will lack accuracy above 60,000 ft. Normally, change of pressure of 1mb at sea level is indicates only 30 feet change ot height, whereas at 60,000 feet a similar change relates to a height change of 325 feet. Thus altimeter measures the height of the airplane above a given pressure level.
Servo assisted altimeter:
The servo assisted altimeter is designed to relieve the capsule of the work...
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