Instruments Info

AERO1600 - 1 - Aircraft Instruments

Aircraft Instruments of 15 AERO1600 Workshop Technology Session Worksheet

AERO1600

Workshop Technology

Aircraft Instruments



INTRODUCTION................................................................................................................. 3 PRESSURE MEASURING EQUIPMENT................................................................................. 3

Altimeter ...................................................................................................................... 6 ASI (Airspeed Indicators)............................................................................................ 7 VSI (Vertical Speed Indicator) .................................................................................... 8

ENGINE INSTRUMENTS ..................................................................................................... 9 Tachometer.................................................................................................................. 9 Oil Temperature ........................................................................................................ 10 Oil Pressure............................................................................................................... 10 CHT (Cylinder Head Temperature) .......................................................................... 11

AIRCRAFT WIRING ......................................................................................................... 12 Wire Types................................................................................................................. 12 Wire Bundling ........................................................................................................... 12 Wire Stripping ........................................................................................................... 14 Terminal Crimping.................................................................................................... 14 Fuses.......................................................................................................................... 15



Introduction Aircraft instruments play a very important role in aircraft operations. With out the use of instruments a pilot can be ignorant to many important aspects, airspeed, altitude, oil temperature and pressure. The list goes on, for aircraft to operate safely and efficiently the incorporation of basic instruments must be considered.

Instruments allow aircraft to fly safely in most weather conditions and to fly literally anywhere on the globe. For example, the Instrument Landing System (ILS) allows a pilot to land at airfields with ceilings (the bottom of a cloud deck above the ground) below 200 feet and visibility less than one fourth of a mile

For the AERO1600 course we will deal with two types of instruments, pressure reading instruments and engine instruments. The first section will deal with pressure measurements, the three main instruments that we will be using are: ASI (Airspeed Indicator), VSI (Vertical Speed Indicator) and the Altimeter. The second section provides information regarding engine instruments. The final section gives some ideas on how aircraft systems are to be wired up. Pressure Measuring Equipment Pressure is the force per unit area acting on a body. Pressure can have several units of measurement, the common ones used in aviation are: inches of mercury, pounds per square inch (Psi), Pascals (Pa) or bars. Pressure is measured on aircraft from two probes: 1) Static probe which measures the atmospheric pressure. 2) Pitot pressure which measures the ram pressure. Absolute Pressure:

Pressure above a vacuum, absolute pressure is used on aircraft as a comparison to other pressures. A device called an Aneroid Barometer is used, it is a device that consists of an enclosed chamber made from thin sheet metal, and this chamber is evacuated to provide reference to measure absolute pressure. See Diagram.



Gauge Pressure: Pressure above or below ambient. Gauge pressure is determined by comparing the difference between atmospheric pressure and the pressure being measured. This type of pressure measurement is the most common in today’s light aircraft instruments. Lower pressures are compared using “bellows” see diagram, while higher pressures are compared using a bourdon tube, a bourdon tube tends to straighten when high pressure is applied, see diagram.

Jeppessen ® Sanderson A&P Technician Airframe Textbook



Differential Pressure: The difference between two pressures. This is the system used in an airspeed indicator.




Altimeter An Altimeter is a barometer that measures the absolute pressure of the air, as we know the pressure changes with altitude due to the weight/density of air decreasing as we move up through the atmosphere. The altimeter is the oldest form of aircraft instrument and one of the most critical, for the obvious reason. For most aircraft it is very important that the altimeter be very responsive and accurate to within a few feet. An important note about the pressure variation through the atmosphere is that it is not linear. This complicates the measurement process somewhat. Below are two common altimeters.

Altimeters have an adjustment that is used prior to takeoff, the barometric window is a hole in the dial where a scale is calibrated in either inches of mercury or millibars. Airport control towers and flight services provide pilots with the corrected barometric pressure for that region in relation to sea level. Hence this enables the instruments to be used when there is a variation in the pressure at a particular airport due to height or weather conditions. There are several factors that affect the correct operation of an altimeter, some of these are listed below: � Scale Error � Friction � Hysteresis � Case Leak � After Effect � Barometric Scale Error



ASI (Airspeed Indicators) The ASI is an instrument that measures differential pressure. The ASI measures the difference between Pitot (ram pressure) and static pressure. The ASI consists of an airtight case in which a thin metal capsule is mounted. Pitot pressure in taken into the capsule which expands in proportion to the difference in Pitot and static pressure which is then amplified into a reading on the dial that is graduated in the appropriate units of measurement. The position of the Pitot sensor is usually on the wing where it will relatively unaffected by disturbed air from the aircraft. Take note that the ASI does not necessarily give the pilot the speed in relation to the ground.

Pitot Probe

True airspeed can be calculated but this must take into account the effects of temperature. There are instruments which do this but in most cases the pilot must simply calculate the TAS (True Airspeed)




VSI (Vertical Speed Indicator) Also know as the rate-of-climb indicator mainly serves as a backup for the altimeter. The main purpose of the VSI is to assist the pilot in achieving the correct rate of climb. The VSI is not a very responsive instrument, hence making it only useful for extended climbs/descents. The VSI consists of a bellows system similar to an altimeter but has a calibrated diffuser. Below is two VSI’, the one on the left is a conventional VSI while the right is an instantaneous VSI.




Engine Instruments The importance of engine instruments is not to be underestimated, just like the pressure instruments the engine instruments provide vital information regarding the condition of the engine. For a pilot flying without any idea of what the engine is doing is a safety hazard by any means. The common engine instruments in light aircraft today are: oil temperature and pressure, tachometer (rpm), CHT (Cylinder Head Temperature), fuel flow, EGT (Exhaust Gas Temperature), and engine hour meter (this is often incorporated into the tachometer. For this session students need to understand the three following instruments: oil pressure and temperature, tachometer and CHT. Tachometer The tachometer is particularly important for the pilot since engine speed is essential for the correct operation of an aircraft. The speed of the engine directly relates to the thrust generated by the propeller, tachometers provide the pilot with a way of determining the correct engine speed for the particular situation. Most tachometers, like the ones in cars have a red line at the higher end of the scale, this is the engine speed that should not be exceeded in normal operations. Tachometers work via either a mechanical driving unit or an electronic pulse system. The mechanical system operates by a steel cable that rotates with the engine to generate a reading on a dial, similar to older style cars. The electronic type is where a pulse is generated from the distributor/magneto/sender and is converted to a dial reading. This is type that will be used for the AERO1600 class. Below is a picture of a tachometer:



Oil Temperature The oil temperature gauge that we will be using operates through a change in resistance. There is a temperature probe located in the engine oil sump, this probe is earthed and wired to the instrument. As the temperature of the oil increases/decreases the resistance of the probe changes, this is then measured by a calibrated instrument. For the AERO1600 course we can simulate this via the use of a common potentiometer. The resistance change is sent to a Wheatstone bridge circuit shown below.

Jeppessen ® Sanderson A&P Technician Airframe Textbook Oil Pressure Oil pressure operates in a very similar way to the oil temperature, a probe is located in the engine block that as pressure changes the resistance of the probe changes, this change is displayed via the wheatstone bridge as before. Again we will simulate the operation of this instrument via the use of a variable resistor (potentiometer).



CHT (Cylinder Head Temperature) The instrument that is used to measure the temperature of the cylinders is a voltage generation system. A voltage generation system works on the principle that certain dissimilar metals will produce a very small voltage when subjected to heat. The instrument must be able to sensor very small voltages i.e. millivolts. This type of probe is known as a Thermocouple, this is a very common probe for light aircraft. Thermocouples are also used to measure the inlet temperatures of turbines and exhaust gas temperatures. Thermocouples are very sensitive, hence they need to be treated with care, it is important that the connections are very clean and properly torqued. Thermocouple probes usually come with a set length of wire, this must not be changed since it will change the resistance of the wire causing an incorrect reading.

CHT Thermocouple wiring loom.



Aircraft Wiring Aircraft wiring must be performed to a certain standard using certain wire and crimps, the loss of electrical systems can present a very serious safety hazard for pilots, hence the correct design and manufacture of circuits is essential. The following section is a basic guide for students who will perform minor wiring for the AERO1600 course. Wire Types There are many different types of wire each with its own particular purpose and application. The correct wire must be selected for the job, one must consider the load the wire must take. Below are the two main types of wire used in aircraft: stranded wire and shielded wire.

Jeppessen ® Sanderson A&P Technician Airframe Textbook Wire Bundling Wire bundling is important to ensure wires are located in the right areas and will not interfere with other equipment. Neat wiring means troubleshooting is a lot easier! Wire bundling is done two ways, with the use of “P” clips and the use of cable ties, cable ties are self-locking straps that are quick to install. See diagram below.



Wire Stripping Wire need to have terminals attached so that they may be in turn attached to bus bars and instruments, in order to attach terminals, the wire must have a small part of its insulated sheath removed, this is called stripping. Stripping must be done without damage to the conductor, a special wire-stripping tool is used to perform this task, shown below.

Terminal Crimping Terminal crimping is the attachment of a terminal to a stripped wire. Ensure the correct amount of insulation is removed from the wire, too much will leave an exposed wire while too little will create a poor contact.




Fuses Fuses are a current limiting device. Fuses protect a circuit from large currents which may result from a short-circuit or an incorrectly wired circuit. Some electrical set-ups have several fuses for various parts of the circuit depending on the sensitivity of the instruments. References:

1. Jeppessen ® Sanderson 1997 A&P Technician General Textbook, 2. National Aeroskills Trade Curriculum, February 1995, Trade Skills 1 &2 3. Jeppessen ® Sanderson 1985 Standard Aviation maintenance Handbook 4. Bruhn.E.F, Analysis and Design of Flight Vehicle Structures. June 1973 5. http://www.tpub.com/air/, Aircraft mechanics

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