TM 1-1520-238-10
Change 9
2-29
ENG START switch is placed to START, pneumatic mo-
toring of the engine starter takes place and the ignition
system is energized. Ignition cutout is automatic after the
engine starts. Following aborted starts (Chapter 9, Emer-
gency Procedures), the engine must be motored with the
ignition system disabled. This is done by placing the ENG
START switch to IGN OVRD. Chapter 5 contains the start-
ing cycle limitations.
2.30 ENGINE STARTING SYSTEM.
The engine uses an air turbine starter (fig 2-16) for engine
starting. System components for starting consist of the
engine starter, a start control valve, an external start con-
nector, check valves, controls, and ducting. Three
sources may provide air for engine starts: The shaft driv-
en compressor (SDC) (normally driven by the APU for en-
gine starts), No. 1 engine compressor bleed air, or an ex-
ternally connected ground source. In any case, the start
sequence is the same. With the ENG FUEL switch ON
and MASTER IGN switch ON, placing the ENG START
switch momentarily to START will initiate an automatic
start sequence. This will be evidenced by the illumination
of the ENG 1 or ENG 2 advisory light above the ENG
START switch on the pilot PWR lever quadrant. Com-
pressed air is then directed through the start control valve
to the air turbine starter. As the air turbine starter begins to
turn, an overrun clutch engages which causes the engine
to motor. The starter turbine wheel and gear train auto-
matically disengage from the engine when engine speed
exceeds starter input speed. At approximately 52% NG,
air to the starter shuts off, and ignition is terminated. If the
engine does not start, the PWR lever must be returned to
OFF and the ENG START switch must be placed at IGN
OVRD (which aborts the automatic engine start se-
quence) momentarily before another start is attempted. If
the engine is equipped with a DECU
, fuel flow to the
engine will be automatically shut off if TGT exceeds 900
C during the start sequence. If this occurs, the PWR lever
must be returned to OFF and the ENG START switch
must be placed in IGN OVRD (which aborts the automatic
engine start sequence).
2.30.1 Engine Start Using APU.
The APU provides
on-board power for system check by ground personnel.
The APU is capable of driving the main transmission ac-
cessories which include the ac generators, the hydraulic
pumps, and the shaft driven compressor (SDC). The APU
is normally left on during both engine starts, but it may be
shut down after one engine has reached 100% Np. When
Np has reached 100% and the APU shut down, that en-
gine may be used to drive the SDC through the transmis-
sion and accessory gear train for starting the second en-
gine. A complete description of the auxiliary power unit
appears in Section XII.
2.30.2 Engine Start Using External Source.
An exter-
nal air receptacle under the No. 1 engine nacelle provides
an attachment point for an air line to start either engine
from an external source. The assembly contains a check
valve to prevent engine bleed air or SDC pressurized air
from being vented overboard. An external air source of 40
psig and 30 pounds per minute (ppm) pressurizes the
start system up to the engine start control valves which re-
quires only that electrical power be applied for a normal
start sequence.
2.30.3 Engine Start Using Engine Bleed Air
Source.
When the No. 1 engine is operating and it is
necessary to start the No. 2 engine, bleed air may be used
from the No. 1 engine compressor under certain circum-
stances. This technique is not normally used; however, it
is fully automatic, and provides an alternative starting ca-
pability if the SDC shaft fails or if the SDC throttle valve or
surge valve clogs (Section VI, Pressurized Air System).
The starting sequence is the same as for APU starting;
only the source of air to the start motor is different. When
using this technique to ensure an adequate air pressure
and flow to the No. 2 engine starter, collective pitch must
be increased to a value that will increase the NG of the
No. 1 engine to a minimum of 95%.
