Different Types of RPM Gauges

Exhaust Gas Temperature System.      The thermometer is used in jet engine aircraft to indicate the temp of exhaust gas. The Exhaust Gas Temperature indicating system provides a visual temperature indication in the cockpit of the turbine exhaust gases as they leave the turbine unit.

A gas temperature thermocouple is mounted in a ceramic insulator and incased in metal sheath; the assembly forms a probe, which projects, into the exhaust stream. The thermocouple is made from Chromel (A Nickel/Chromium Alloy) and Alumel (a Nickel/Aluminium alloy). The hot junction protrudes into a space inside the sheath. The sheath has transfer holes in the end of it, which allow the exhaust gases to flow across the hot junction.

The hot junction senses the EGT and emf is induced in milli volts, and this voltage, after amplification is applied to the indicator which actuates the mechanism to drives the indicator pointer over the graduated dial.

Fuel Gauge System

This system is used to give a continuous indication or fuel contents in the tanks of the aircraft.  There are three types of fuel contents gauge use, i.e., Liquido meter, Capacitance type fuel contents gauge and Measurement of fuel quantity by weight.

There are various fuel quantity indicating systems used in aircraft.  They range from simple mechanical, direct indicating systems used with light aircraft to large electronic system used with heavy and modern aircrafts.

 (a) Liquido Meter, i.e Float Type Fuel Contents Gauge. Again this can be of two types, one direct indicating and other can be remote indicating by means of wiper linked with float and running over a potentiometer.

 (b) Capacitance Type Fuel Contents Gauge. This system consists of a variable capacitor located in fuel, an amplifier and indicator, the complete circuit forms an electrical bridge which is continuously being rebalanced as fuel quantity changes.

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 (c) Measurement of Fuel Quantity by Weight. This is more useful and accurate method.  This system measures the weight of fuel in tanks rather than its volume. The measuring device must be sensitive to volume and density.

Fuel System Monitoring Instruments. Several different types of fuel pressure gauges are in use for aircraft engines, each being designed to meet the requirements of the particular engine fuel system with which it is associated. Any fuel system utilizing an engine driven or electric fuel pump must have a fuel pressure gauge to ensure that the system is working properly.

If an engine is equipped with either a direct fuel injection system or a continuous flow fuel injection system, the fuel pressure is a direct indication of power output.  Since engine power is proportional to pressure, fuel pressure can be translated in engine power or fuel flow rate or both.  The fuel pressure gauge, therefore, can be calibrated in terms of percent of power.

Fuel Flow Meter. Fuel flow meters are used with fuel systems to show the amount of fuel in gallons/pounds/Kgs consumed per hour.

A Fuel flow indicator system consists of a fuel flow transmitter located in the fuel line leading from the tank to the engine and an indicator located on the instrument panel. The transmitter signal may be developed by a single movable vane mounted in the fuel flow path in such a manner that its movement will be proportional to fuel flow.  As fuel flow increases, the vane must move to allow more fuel to pass, and this movement is linked to a synchro unit, which develops the electrical signal to be sent to the indicator.

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Oil Pressure Gauges. It is used to indicate the pressure in the oil line at which system is working.  Oil Pressure Indicators can be mechanically operated or electrically powered.  A mechanically operated gauge uses an oil pressure line from the engine to the instrument to operate a bourdon tube and gear segment to position the indicator needle.  The oil line should have a restrictor at the engine to prevent rapid oil loss, if the line   should break. Some aircraft use light oil in the line between the gauge and the engine so that there will be no delay in oil pressure indication due to cold engine oil being in line.

Electric Oil Pressure sensors use a pressure sensor on the engine, which varies in resistance as the pressure changes.  As this pressure signal is generated, the pressure is indicated by one of the electrical indicating methods.

Oil Temperature Indicators. Oil Temperature Indicators can be electrical or mechanical. To operate electrically, a resistance probe is placed in the oil line where the oil enters the engine. The Oil temperature is derived by the change in probe resistance due to the temperature change.

Function, Maintenance and Inspection of Oxygen Systems.  Oxygen systems are required on airplanes that fly for extended periods at altitude substantially above 10,000 ft. Although the normal human body can survive without a special supply of oxygen at altitude of over 15,000 ft, the mental and physical capacities of a human being are reduced when the usual supply of oxygen is not available in the air. It is particularly important that the pilot and crew of an airplane should have an adequate supply of oxygen when operating an unpressurised airplane at altitude in excess of 10,000 ft.

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A lack of oxygen causes a person to experience a condition called hypoxia. This condition results in “light headedness”, headaches, dizziness, nausea, unconsciousness or death depending upon its duration and degree. When permanent physical damage results from lack of oxygen, the condition is defined as anoxia.

The importance of oxygen, especially when flying at higher altitudes, is not appreciated by many persons who fly, including pilots.  It is generally known that the human body require oxygen to sustain life, but the effects of a lack of sufficient oxygen on various functions of the body are not understood by a large percentage of the flying public.

Studies have shown that the effects of hypoxia become apparent at approximately 5,000 ft altitude in the form of Night Vision.  It is recommended, therefore, that a pilot flying above 5,000 ft altitude at night use oxygen.  Pilots flying above 10,000 ft altitude should use oxygen.

Normal Air contains approximately 21% Oxygen and this provides adequate oxygen for human body at lower altitudes.  At 34,000 ft altitude a person must be breathing 100% oxygen to absorb the same amount of Oxygen as when breathing air at sea level.  It is, therefore, apparent that the percentage of oxygen in the air that a person is breathing must be increased in keeping with altitude if the person is to receive an adequate supply of oxygen for optimum functioning of physical and mental faculties and functions.

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