Something affecting the normal flow of
never use part replacement as a method of
troubleshooting. Each year, thousands of dollars
compressed gases is classified as a pneumatic
worth of engine parts are returned to overhaul
malfunction. This type of malfunction stems from
activities with labels stating that they are faulty.
the same general sources as hydraulic malfunc-
Upon completion of test and check by the
tions mentioned in the previous paragraph.
overhaul activity, many parts show no defect. This
Many units that operate hydraulically or
practice is wasteful and very expensive. Another
problem occurs when mechanics replace a major
pneumatically use mechanical linkage. If a
assembly. The problem could have been corrected
discrepancy in the linkage exists, it will affect the
system's operation. Many mechanical discrepancies
by replacing a subassembly. The replacement of
are found during visual inspections. Common
an entire unit when only a small part is at fault
mechanical problems are worn linkages, broken
reflects poor maintenance practices. When
linkages, improperly adjusted linkages, or
troubleshooting, remember you are working with
improperly installed linkages.
equipment that is both expensive and scarce.
Replace equipment only after it is thoroughly
You must be able to determine if the electrical
tested and determined to be faulty.
system is functioning normally. Common elec-
trical problems are complete power failure,
6. Conduct a final operational check. The
circuit failure, or component failure. Electrical
affected system must have a thorough check to
components have circuit breakers and switches to
determine proper operation. Checking the engine
operate or control them. Check the obvious things
for proper operation consists primarily of reading
such as tripped circuit breakers or switches in the
engine instruments. Compare the readings with
w r o n g position. Although AEs normally
those given by the manufacturer for specific
engine conditions, atmospheric pressures, and
troubleshoot electrical problems, sometimes you
will work together on a problem. Learn to read
temperatures.
electrical schematics and use a multimeter so you
c a n trace electrical power requirements
Early model engine test procedures used rpm
as the engine operating parameter to establish
throughout the affected system.
thrust. Today's engine test procedures use engine
4. Isolate the trouble. This step calls for sound
pressure ratio (EPR) as the primary thrust in-
reasoning. A full and complete knowledge of
dicator. EPR is the ratio of the total pressure at
engine theory, as well as a complete under-
the front of the compressor to the total pressure
standing of the affected system is necessary.
at the rear of the turbine.
During this step you must use knowledge and
Using EPR as the thrust indicator means that
known facts to determine where the malfunction
on a hot day it is quite possible for the engine rpm
exists in the system. Usually the trouble can
be pinned down to one or two areas. Eliminate
to exceed 100 percent. If you use rpm on a hot
day instead of EPR, you would actually have a
those units that could not cause the known
symptoms and those that can be shown to operate
lower amount of thrust at 100 percent, Just the
normally.
opposite is true on a cold day; desired thrust
ratings may be reached at something less than 100
Accomplish the isolating and correcting
percent using EPR. Other variables, such as a
dirty or damaged compressor, will also affect
of engine discrepancies by comparing symptoms
thrust. These conditions reduce thrust for a given
with probable cause. Begin with the most
rpm. Exhaust gas temperature (EGT) is NEVER
obvious and proceed to the less likely causes.
used for setting thrust. Monitor EGT carefully to
A combination of several small maladjustments
prevent excessive temperature readings.
or malfunctions may contribute to make a
complex discrepancy.
5. Correct the trouble. Accomplish this step
TROUBLESHOOTING ERRORS
only after the trouble has been definitely located.
Malfunctions are corrected by servicing, adjust-
There are certain errors that you should avoid
ments, and part replacement. Part replacement
if you are to become an effective troubleshooter.
is often needed to correct a malfunction, but you
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