necessary, clean the barrel and shell threads with
a die. Visually inspect the exposed ceramic
section. Any cracks are cause for rejection.
The functional testing of the capacitor
discharge ignition system is a simple operation.
WARNING
You should exercise caution while per-
forming this test. Do not come into contact
with the igniter plugs or leads while the
power is on in the ignition system. Some
systems have voltages up to 28,000 volts
or more. These high voltages could cause
serious injury or death. Prevent fuel or fuel
fumes from gathering under the engine
while the igniter plugs are being ground
tested.
Perform a dry run of the engine (operate the
ignition system). Listen at the tailpipe. You can
determine if the unit is working by listening for
the spark. Another way is to remove the spark
igniters, leave them hanging on the high-tension
leads, and operate the ignition system. The spark
can be seen at the plug if the unit is working. The
spark should be brilliant and accompanied by a
sharp report.
STARTING SYSTEMS
Starting a jet engine requires rotating the
Figure 6-8.-Air turbine starting system.
compressor fast enough to begin the engine
combustion cycle. Starting systems must be
capable of providing both high starting torque and
Compressed air, supplied to the scroll inlet,
high speed. High starting torque is required to
is sent to the turbine wheel by the nozzle in the
overcome the large amount of weight of the engine
scroll assembly. The reduction gear system
rotor. High speed is required to increase rotor rpm
transforms the high speed and low torque of the
until the rotor is self-sustaining. There are several
turbine wheel to low speed and high torque at the
ways to accomplish these objectives. The follow-
output shaft. An overspeed switch mechanism is
ing paragraphs describe four methods. They are
used to limit maximum rotational speed. When
the air turbine starter, the direct turbine impinge-
the desired starter rotational speed is reached, the
ment starter, the electrical starter, or the hydraulic
fly weights in the governor assembly will open the
starter.
limit switch. This section sends a signal that shuts
off the supply of air. At a higher, predetermined
Air Turbine Starter
rotational speed, the overrunning clutch assembly
disengages the output shaft from the rotating
The air turbine starter is a lightweight unit for
assembly.
starting engines with compressed air. The starter
is a turbine air motor equipped with a radial
Turbine Impingement Starter
inward-flow turbine wheel assembly, reduction
gearing, splined output shaft, and a quick-
Some naval aircraft are started by means of
detaching coupling assembly. See figure 6-8.
low-pressure air directed onto the turbine or
6-11
