Use in aircraft Altimeter

schematic of drum-type aircraft altimeter, showing small kollsman windows @ bottom left , bottom right of face.

in aircraft, aneroid barometer measures atmospheric pressure static port outside aircraft. air pressure decreases increase of altitude—approximately 100 hectopascals per 800 meters or 1 inch of mercury per 1000 feet near sea level.

the aneroid altimeter calibrated show pressure directly altitude above mean sea level, in accordance mathematical model atmosphere defined international standard atmosphere (isa). older aircraft used simple aneroid barometer needle made less 1 revolution around face 0 full scale. design evolved altimeters primary needle , 1 or more secondary needles show number of revolutions, similar clock face. in other words, each needle points different digit of current altitude measurement. design has fallen out of favor due risk of misreading in stressful situations. design evolved further drum-type altimeters, final step in analogue instrumentation, each revolution of single needle accounted 1,000 feet, thousand foot increments recorded on numerical odometer-type drum. determine altitude, pilot had first read drum determine thousands of feet, @ needle hundreds of feet. modern analogue altimeters in transport aircraft typically drum-type. latest development in clarity electronic flight instrument system integrated digital altimeter displays. technology has trickled down airliners , military planes until standard in many general aviation aircraft.

modern aircraft use sensitive altimeter . on sensitive altimeter, sea-level reference pressure can adjusted setting knob. reference pressure, in inches of mercury in canada , united states, , hectopascals (previously millibars) elsewhere, displayed in small kollsman window, on face of aircraft altimeter. necessary, since sea level reference atmospheric pressure @ given location varies on time temperature , movement of pressure systems in atmosphere.

diagram showing internal components of sensitive aircraft altimeter.

in aviation terminology, regional or local air pressure @ mean sea level (msl) called qnh or altimeter setting , , pressure calibrate altimeter show height above ground @ given airfield called qfe of field. altimeter cannot, however, adjusted variations in air temperature. differences in temperature isa model accordingly cause errors in indicated altitude.

in aerospace, mechanical stand-alone altimeters based on diaphragm bellows replaced integrated measurement systems called air data computers (adc). module measures altitude, speed of flight , outside temperature provide more precise output data allowing automatic flight control , flight level division. multiple altimeters can used design pressure reference system provide information airplane s position angles further support inertial navigation system calculations.