Purpose of Aircraft Instruments

In aircraft there are millions of parts and all of them play a vital and specific role to perform for the aircraft to fly safe and land safely. Some of them are not required to be monitored during the flight hours as they are maintained by the servicing crews available in ground. But some of the components require a continuous attention during flight hours by the air crew in order to keep the aircraft fly safe and those aviation instruments keep indicating the flight details so that the pilot keep monitoring it and know the real time situation inside cockpit.

Altimeter. The purpose of altimeter is to indicate the pilot, the height above mean sea level at which aircraft is flying. It works on the principle of aneroid barometer (which means no wet). The basic mechanism of altimeter is pressure-responsive elements (capsule), which expand, or contract with the pressure changes at different flight levels.  The expansion or contraction of the aneroid with pressure changes actuates the linkage, and the indicating hands show altitude. A barometer scale is located on the right of the instrument face. A knob located at the lower left of the instrument case can set this scale. The barometer scale indicates barometric pressure in inches of mercury. If this barometer is set to read pressure prevailing at aerodrome, then altimeter will indicate the height of ac from aerodrome.

Rate of Climb Indicator. The rate of climb indicator (also known as vertical speed Indicator) is used to indicate the rate at which aircraft is ascending or descending. This is a sensitive differential pressure gauge. The rate of climb indicator is connected to the static system and senses the rate of change of static pressure. The same static pressure is given inside the capsule and also out the capsule, but at one place with some lag. Whenever there is no change in altitude, inside and out pressure is equal and zero reading is shown, when ac is climbing or diving there will be a pressure difference and capsule will expand or contract to show ascend or descend.

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Air Speed Indicator. The Air Speed Indicator (ASI) is used to indicate the speed at which ac is traveling relative to surrounding air. ASI is a sensitive, differential pressure gauge, which measures the difference between the pitot (dynamic) pressure and static pressures directed by pitot-static tube. It is not indicating the ac speed in relation to any fixed point on the ground.

Air speed indicator (ASI) consists primarily of a sensitive metal capsule. pitot pressure is given inside the capsule and static pressure is given outside the capsule. Any movements, resulting from the slightest difference in dynamic and static air Pressure, are multiplied by means of a link, a rocking shaft, a sector with hair spring and pinion, and a tapered shaft to impart rotary motion to the pointer, which indicates the Aircraft speed on the dial face in terms of knots or km/hr. this speed is called Indicated Air Speed (IAS).

Engine Performance Instrument. The engine performance monitoring instruments are provided to indicate the performance and condition of the engine. It basically comprised of following systems:

 (a)          RPM Gauge or Tacho meter indicator system

 (b)          Exhaust Gas Temperature Indicator

 (c)          Fuel content Gauge

 (d)          Oil pressure indicator

 (e)          Oil temperature indicator

Tacho Indicator System or RPM Gauge. The tachometer indicator or rpm gauge is an instrument for indicating the speed of the crankshaft of a reciprocating engine or turbine wheel of jet engine. A small generator, called the Tacho Generator is connected to the shaft of the engine which will generate voltage propositional to speed of rotor. This generated voltage by the transmitter is fed to the indicator, which is a voltmeter. This voltmeter is calibrated to indicate the rpm depending on the voltage generated by the generator, which is proportional to the rpm.

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The dials of tachometer indicators used with reciprocating engines are calibrated in rpm; those used with turbine engines are calibrated in percentage of RPM being used, based on the take off rpm. This is because the actual turbine engine rpm’s are very high.