For specific procedures, you should refer to the
first be checked to determine if the instrument or
MIM.
the installation is at fault. Usually, instrument
problems fall into three groups--trouble in the
1. Disconnect all altimeters, airspeed, and
power supply, trouble in the unit, or trouble in
rate-of-climb indicators, and any other systems
the connections to units, either electrical or
receiving information from the pitot-static system.
mechanical. If the installation is faulty, line
Disconnect the lines from pitot or pitot-static
maintenance can correct the problem. If the
tubes.
instrument is the fault, you can remove and
replace it with a serviceable unit. The defective
2. Remove all drain caps in the system.
unit goes to a qualified instrument overhaul depot
for detailed inspection, overhaul, and repair.
3. Circulate a stream of clean, dry, filtered
air at medium pressure through the complete
Only authorized instrument shops can open
system. Be careful n o t to include the cabin
instrument cases and make repairs or adjustments.
pressurization system static vent. Be certain that
air is flowing from the exit end of each line.
When making ground tests of electrical
instruments, you should connect an external
4. Inspect all static vents and
the pitot tube
power supply to the aircraft. Do not u s e
water removal drain holes for
damage and
the battery when conducting ground tests of
evidence of foreign matter and
obstructions.
equipment.
Check all low points in the lines for
possible cracks
due to icing in the lines.
When performing ground testing, you should
u s e portable field test sets, such as the
5. Replace and secure all system drain caps.
TTU-205C/E pressure-temperature tester and the
TTU-27/E tachometer tester. Test sets are
6. Reconnect all instruments. Tighten con-
discussed in chapter 2 of this TRAMAN.
nections properly; do not kink or bend the lines.
7. Using a field test set or other approved
Always use a precision voltmeter to check
tester, thoroughly check the system for proper
instrument power. You can check most electrical
operation and leaks.
instruments with a test indicator to determine
where the trouble lies. For example, you can check
The maintenance of the pitot-static system is
synchro indicators using a synchro test indicator.
relatively simple when compared to more complex
systems. However, its maintenance is not a minor
task.
TROUBLESHOOTING
The manuals you use for instrument mainte-
INSTRUMENT TESTING
nance contain troubleshooting charts. These
charts help you determine what is wrong with an
Operation of most aircraft instruments is
instrument system. When you know what the
entirely automatic. Once installed, the units
problem of a particular instrument or system
require no further maintenance or servicing other
is, refer to the troubleshooting chart for the
than routine and periodic inspections. If a
particular instrument. The chart gives a listing of
system or instrument malfunctions, you must
the common troubles, probable causes, and
first localize the source of trouble. Develop a
remedies.
systematic troubleshooting procedure. The
Efficient troubleshooting calls for an orderly,
procedure should include the possible service
troubles and their remedies for each type of
systematic plan of attack and a good under-
instrument. You will find most of this information
standing of the theory of equipment operation.
in applicable aeronautic publications, such as
A visual inspection should be made first; this will
the MIM for specific aircraft and the service
frequently pinpoint the trouble. When making
instruction manual for specific instruments.
this inspection, look for discolored or burned
wires and terminal boards and corroded switch
An instrument that doesn't function properly
contacts. Also look for broken or frayed wires,
or that is suspected of being unserviceable must
loose connector plugs, and loose mechanical
