27-08-2014, 10:38 AM
Aircraft Instruments
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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.
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.
Aircraft Instruments
Aircraft Instruments Page 7 of 15
AERO1600 Workshop Technology Session Worksheet
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.
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:
Aircraft Instruments
Aircraft Instruments Page 11 of 15
AERO1600 Workshop Technology Session Worksheet
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.
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