Basic Electrical Engineering - Aviation Instruments

Importance of Flight Instruments

1.         With the evolution of flying machines, there is a growing challenge to flying bigger aircraft and navigating long distance day and night under all weather conditions, controlling and operating multiple engines, etc.

2.            The pilot now requires to be assisted by several instrumentation and systems so as to fly and navigate most complex machine like an aircraft. Various instruments are provided in the aircraft cockpit to assist the pilot for correct perception of aircraft altitude, speed, attitude, bank, rate of ascend or descent and course of heading, fuel content in tanks and cabin pressure. The Instruments like engine rpm indicator, engine oil pressure and temperature gauge, engine exhaust gas temperature indicator, etc. are provided to monitor aircraft performance. Various systems like autopilot, Flight Data Recorder, Oxygen, instrument landing system (ILS), critical altitude recovery systems etc are provided to ease the pilot from routine flying burdens.

Aircraft Instruments

3.            Introduction.            Aircraft instruments are the means of providing information required by the pilot at any moment.  They provide reliable, accurate & continuous information to the pilot by means of direct reading on dial with pointer or with the help of counters. Aircraft instruments are as important as the eyes in our body. It also provides certain information/warning by means of audio signal or visually by glowing lamp making it simple.

Fig 1 Pitot Static Tubes

Note:   Pitot Static tube is installed on the aircraft to pick up pitot and static pressures. It is provided with a heating element to avoid icing effect.

4.         Elements of Instrument.   An instrument as being made up of the following four principle elements:

(a)          Detecting Elements.    Which detect changes in value of physical quantity or condition presented to it.

(b)          Measuring Element.     Which actually measures the value of the physical quantity or condition in terms of small translation or angular displacements.

(c)          Coupling Element.        By which displacement are magnified and transmitted.

(d)          Indicating Element.     Which exhibits the value of the measured quantity transmitted by coupling element by the relative position of a pointer, or index, and a scale.

5.         Instrument Panels.      All instruments essential for the effective monitoring and controlling of an aircraft are accommodated on special panels, the number and distribution of which vary in accordance with the number of instruments, the size of aircraft and the cockpit layout.

6.         Basically, there are six flight instruments whose indications are so coordinated so as to create a picture of aircraft’s flight condition. They are Altimeter, Rate of climb indicator, Air speed indicator, Turn and Bank indicator, Direction indicator, Gyro horizon Indicator. These instruments are grouped to maintain coordination and to assist the pilot in observing them with the minimum effort. The first real attempt to establish a standard method of grouping was the “Blind flying panel”.

Fig 2 Elements of an Instrument

7.         Different Types of Flight Instruments.      These instruments can be grouped under the following headings:

(a)       Flight Performance Instruments.           These instruments are required to monitor the speed of aircraft, altitude of aircraft at which it is flying and rate of ascend/descend.

(i)         Altimeter.

(ii)        Air Speed Indicator (ASI).

(iii)       Mach Meter.

(iv)       Rate of Climb indicator (ROCI)

(b)       Engine Performance Instruments.               These are also known as power plant instruments used for monitoring the performance of engine:
                        (i)         RPM gauge.

                        (ii)        Exhaust Gas or Tail Pipe Temp Gauge.

                        (iii)       Fuel Content Gauge.

                        (iv)       Oil Temperature Gauge

                        (v)        Oil Pressure gauge

(c)        Navigational Instruments.            These instruments are required to indicate the direction/ heading of aircraft as Magnetic compass.

(d)       Gyroscopic Instruments.       These instruments utilizes gyroscope to indicate the attitude, rate of turn and pitch/bank angle of aircraft. So the exact global position of the aircraft can be known:

                        (i)         Artificial Horizon Indicator.

                        (ii)        Turn Indicator.

                        (iii)       Direction Indicator.

8.         In addition to above categories of instruments, various other instrument systems Gyro Gun Sight are provided to facilitate accurate weapon aiming & delivery of weapon. Auto pilot is provided for reducing physical and metal fatigue of controlling the aircraft. Flight Data Recorder is provided for system health monitoring.

Purpose of Aircraft Instruments

9.       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.

Fig 3 Sensitive Altimeter

Fig 4 Cut Views of Altimeter

10.       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 in side 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.

Fig 5 Typical Rate of Climb Indicator

Fig 6 Typical Rate of Climb Indicator Mechanism

11.       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.

12.       Air speed indicator (ASI) consists primarily of a sensitive metal capsule. pitot pressure is given inside the capsule and static pressure is given out side 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).(Fig 7).

Fig 7(a)  Air Speed Indicater

Fig 7(b)  A Cut View of Air Speed Indicator

13.       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

14.       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.

15.       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.

Fig 8 Different Types of RPM Gauges

16.       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. 

17.       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.

18.       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.

Fig 9 Thermocouple Temperature Indicators

19.       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.

20.       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.

(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.

21.       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.

22.       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.

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

24.       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.

25.       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.

26.       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.

27.       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.

28.       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.

29.       A lack of oxygen causes a person to experience a condition called hypoxia. This condition results in “light headedness”, head aches, 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.

30.       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.

31.       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.

32.       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.

33.       Requirements of Typical Oxygen Installation.           The requirement which to be have satisfied to unable an oxygen installation to function efficiently is as follows:

(a)       A means of storing oxygen in the aircraft.

(b)       A means regulating the flow to the crew member only as he breathes in and an indication that oxygen is flowing to his position.

(c)        A means of regulating the oxygen to the user remembering that the higher he files, the more oxygen, he will require.

(d)       The means of coupling various components together and switching them on and off as required.
34.       Component which meet requirements are the aircraft cylinders, regulators flow indicators ,economizers, masks charging and line valves, high medium and low pressure pipe lines, filters, non returns valves connecting pieces, transport cylinders, charging pipe lines and charging regulators.

35.       Service and Maintenance of Oxygen System.            The service and maintenance of oxygen systems should be performed only by technicians who are qualified through training and experience. Oxygen is not an explosive and is not flammable in its pure state, but it supports combustion. Sometimes violently, when it contact or mixed with other materials. It is, therefore, important that certain precautions be exercised when working with oxygen system. The following are essential:

(a)       Smoking, open flames, or items that may cause spark, must not be permitted near aircraft when maintenance is being performed on the oxygen system.

(b)       All electrical power must be performed on the oxygen system must be grounded.

(c)        Oxygen must not be permitted to come in contact with oils, greases, or solvents. Such contacts can cause spontaneous explosions.

36.       Oxygen cylinders are inspected for damage such as nicks, dents, corroded fittings, hydrostatic test date, DOT designation and leakage.  Cylinders that do not meet requirements must be completely disassembled and inspected in an approved facility.  DOT numbers, serial numbers, and dates of hydrostatic testing are stamped on the shoulder or neck of each cylinder.

37.       Whenever a component of a high pressure oxygen system has been removed and replaced or whenever the system has been disassembled in any way, the system must be tested for leaks and purged.  The leak detector solution (soapy water) is completely removed after each test and wherever leaks are found, fitting must be repaired or replaced.

38.       Purging the Oxygen system involves fully charging the system in accordance with service instructions and then releasing oxygen from the system. The airplane should be out door if possible; otherwise it should be isolated in a well ventilated building with no smoking or open flame permitted in the area. No grease or other lubricants should be near enough to come in contact with the Oxygen. The doors and windows of the airplane must be open.

39.       After the system is fully charged with oxygen, all oxygen marks are plugged into their outlets and the oxygen is allowed to flow for about 10 minutes. When the Oxygen is flowing from the masks in odorless, the purging is complete and the oxygen is shut off. The masks are removed from their outlets and the system is recharged. Recharging is usually accomplished through a filling value and fitting mounted conveniently on the lower part of the airplane or accessible through the baggage area. The cap is removed from the fitting and the refill hose is connected oxygen is allowed to flow into the system until it is automatically or manually shut off when the required pressure is registered on the high pressure oxygen gauge. Do not assume that because a cylinder is coloured green, it contains breathing oxygen. Cylinders containing other gases are sometimes coloured green.

Construction of Various Types of Direct Reading Compass

40.       Direct Reading Compass     The purpose of liquid compass is to indicate in which direction the magnetic north pole lies, to enable and ac position to be determine with respect to that pole. They are classified under two headings.

(a)       Pilot Type.    It is used to indicate the magnetic heading of the aircraft.

(b)       Observer Type.       It is used with adjustable sights to enable bearing to be taken of distant object (ac)

41.       Pilot Type.    The magnetic compass is a simple, self-contained instrument, which operates on the principle of magnetic attraction. It is designed to indicate the direction in which aircraft is headed. The magnetic compass is an independently operating instrument described here as mechanical because it requires no power from any aircraft system
Fig 10 Magnetic Compass

42.       If a bar magnet is mounted on a pivot to be free to rotate in horizontal plane, it will assume a position with one of its ends pointing towards earths north magnetic pole. The magnetic compass consists of a liquid filled bowl containing a pivoted float element to which one or more bar magnets are fastened. The liquid in the bowl dampens oscillation of the float, decreases the friction of pivot and lubricants the pivot bearing. A diaphragm and vent provide for expansion and contraction of the liquid that results from change in temperature and altitude. A circular compass card, usually graduated in increments of 5/10 deg is attached to the float element of the compass. A fixed reference marker, called a lubber line, is attached to the compass bowl. The lubber line and graduation on the card is visible through a glass window. The compass card assembly is mounted on a jeweled pivot bearing on the jewel port assembly.

43.       The compass provides an indication of magnetic direction rather than true direction. The bar magnet in the compass aligns itself with the magnetic lines of force in the earth’s magnetic field.

44.       Magnetic compass is subjected to directional inaccuracy, which is caused by other stray magnetic influence other than earth’s magnetic field. These are also caused by the magnetic influence developed in side the aeroplane due to the proximity of magnetic material, such as iron and steel and by the current flowing through near by electrical circuit. The compass is adjustable, so as to correct for the NORTH-SOUTH deviation and EAST-WEST deviation. A compensating device containing small permanent magnet is incorporated in the compass to correct for deviation of the compass. Two screws on the face of instruments are used to move the magnets and thus counter balance the local magnetic influences acting on the main compass magnet. Two set screws are labeled as N-S and E-W.

45.       Vertical Dial Compass.     The vertical dial direction indicator is actually a direct reading compass, instead of the direction is being read from swinging compass card, however, the reading is taken from a vertical dial.

46.       The reference index on the vertical dial can be set any desired heading by turning the knob at the bottom of the dial, and it is necessary only to match the indicator needle with the reference pointer to hold a course. The design of the dial provides easy reading of direction and also quick indication of deviation from a selected heading. The compass liquid in the instrument is contained in a separate chamber and the dial, instead of floating in the liquid as in other types of magnetic compasses, is completely dry. This makes the indicator easier to read and eliminates fluid leakage around the dial. The float assembly, which contains the direction magnet, is located in fluid filled bowl.

Fig 11 Magnetic Direction Indicator

47.       Occasion of Compass Swinging.          The process of “compensating for errors” is known as Compass Swinging.  Compass compensation procedure varies, depending on the type of the aircraft. There are following few Occasions, when compass swinging should be carried out.

(a)       Any change of compass or corrector box.

(b)       On receipt of ac in IAF.

(c)       After any change of major components of aircraft such as tail plane, oleo leg, engine, radio equipment or any other equipment that could have an effect on the compass.

(d)       After the ac has been standing on any one heading for more than four weeks.

(e)       Periodical swinging (every three months).

(f)        The direction of flight commander.

(g)          At any time when its accuracy is doubted.

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