As compressed air enters the starter inlet port
Q3. The air turbine starter converts energy from
through the pressure regulating valve, the turbine
what type of air to shaft power?
wheel rotates, transmitting torque to the drive jaw
through reduction gearing. This torque transmits
to the drive shaft through the splined drive shaft.
When aircraft engine speed exceeds the starter
Q4. The external stop switch de-energizes the air
drive shaft speed, the speed of the starter output
turbine start control holding relay by
shaft (directly connected to the engine drive) also
exceeds the drive jaw speed. When this condition
occurs, the pawls begin to ratchet. When the
output shaft (driven by the aircraft engine) attains
enough speed, centrifugal force releases the pawls
completely from the drive jaw. This action releases
IGNITION SYSTEMS
the starter from the aircraft engine. When the
starter reaches cutoff speed, the internal overspeed
Learning Objective: Recognize operating
control actuates, breaking the electrical circuit to
parameters and characteristics of aircraft
the holding relay. Then, the pressure regulating
engine ignition systems.
valve butterfly closes and prevents compressed air
from entering the starter.
Three things are necessary to cause a fire--a
combustible material (such as aircraft fuel),
NOTE: If engine shaft speed fails to exceed
oxygen, and heat. A fire will not start without all
starter driving mechanism speed before the starter
three, and removing any one of the three puts the
reaches cutoff speed, the cutout switch actuates,
fire out. All internal combustion engines use fire
shutting down the starter.
to produce mechanical energy, and the piston-type
A starting operation begins by momentarily
engine uses the higher degree of fire--an
closing the start switch to energize the pressure
explosion.
regulating valve circuit. Sequence of the regulating
The gas turbine (jet) engine also produces its
valve and start operation becomes automatic. The
energy through the use of fire. However, its
start continues until engine light-off occurs,
operation is considerably different from the piston
engine. Rather than a series of independent
and the engine overspeeds the starter driving
mechanism or until output shaft speed reaches the
explosions, a jet engine produces a continuous
calibrated cutoff point. In either condition,
burning fire. Ignition is necessary only during the
disengagement of the starter from the engine or
start cycle to ignite the fire.
interruption of the supply air to the starter is
Electronic ignition systems provide internal
combustion for turboprop, turbofan, and turbojet
automatic. Starter operation also stops when the
engines. Unlike reciprocating engine systems,
stop switch momentarily opens, which closes the
pressure regulating valve and interrupts the supply
timing is not a factor in turbine-power ignition
air to the starter.
systems. All that is needed is a series of sparks
with enough intensity to cause combustion.
NOTE: During operation of the starter, you
The exciter develops voltage of sufficient
should stand clear of the plane of rotation of the
amplitude to produce a spark. The exciter unit
high-speed rotating turbine wheel. Only qualified
contains a capacitor or capacitors to develop the
ADs should install and service the unit and its
voltage and current necessary to supply a spark
pressure regulating valve.
plug (called an igniter). The resultant spark is of
high heat intensity, capable not only of igniting
abnormal fuel mixtures but also of burning away
any foreign deposits on the plug electrodes. The
REVIEW SUBSET NUMBER 1
exciter is a dual unit and produces sparks at each
of two igniter plugs.
Q1. List the three modes of operation of the A-6
The igniter plugs are, in general, similar to the
aircraft constant speed drive/starter unit.
spark plugs on reciprocating engines. The main
differences were changes necessary to operate at
higher energies, voltages, and temperatures of jet
engines. In general, the igniter plug is larger, more
Q2. In the air turbine starter, what unit limits
rotational speed?
open in construction, and the gap is much wider
