Compressor Bleed Air Valves
then flows through the ignition relay contacts,
energizing the fuel pump paralleling solenoid, the
The fifth and tenth bleed air valves release air
Power also goes to the fuel control shutoff valve,
from the compressor to reduce the compressor
load during engine starts. During starting, the
allowing fuel to enter the engine fuel manifold.
bleed valves are open up to 94 percent RPM. At
Extra fuel for fuel enrichment (if used) flows to
94 percent RPM, the speed-sensitive valve ports
the temperature datum valve. The temperature
compressor-discharge air to close the bleed
datum relay remains energized by a holding circuit
consisting of the lower temperature datum relay
valves.
contacts and the fuel manifold pressure switch.
ENGINE START CYCLE OPERATION
As engine speed increases, the fuel pressure
increases. When fuel manifold pressure reaches
50 PSIG, the fuel manifold pressure switch opens,
The following sequence of events is typical of
de-energizing the temperature datum relay,
a normal engine start cycle. While reading
stopping fuel enrichment. When the secondary
this section, you should assume that external
fuel-pump pressure exceeds 150 PSIG, a parallel-
compressed air and electrical power are being
applied to the aircraft. Also, assume that all other
ing light illuminates showing parallel operation
system switches are in the proper position for an
of the fuel pumps to the operator.
At 65 percent RPM, the speed-sensitive
engine start. Refer to figure 5-8 and table 5-1
control 65 percent switches open to de-energize
throughout this discussion.
The operator places the ENGINE START
the ignition relay. The ignition relay removes
SELECTOR switch to the engine number 1
power from the ignition exciter, the fuel pump
position. Position FUEL and IGNITION switch,
paralleling solenoid, and the drip valve solenoid.
The fuel pumps now operate in series, and fuel
engine 1 to the ON position, de-energizing the fuel
pressure now holds the drip valves closed. The
control relay. This allows power to pass through
the contacts of the fuel control relay, the 16
starter control valve and the engine start switch
percent speed-sensitive control switch, and the
lose their common ground, and current flow
fuel manifold pressure switch to energize the
ceases through those circuits. The starter control
valve closes, stopping airflow to the air turbine
temperature datum relay.
starter, and the engine start switch opens. The air
When the operator depresses the ENGINE
valve position switch opens, causing the starter
START switch, the current flows through the
engine start switch, engine start selector switch,
control valve light to go out. This completes the
starter control valve, and speed-sensitive control
engine start cycle.
When engine speed increases above 94 percent,
65 percent switch to ground. Current also flows
contacts in the speed-sensitive control (circuit not
through the engine start switch holding coil to
ground through the same 65 percent switch in the
shown) de-energize the temperature datum valve
speed-sensitive control.
take solenoid. This reduces the fuel take capability
With power applied to the starter control
from 50 percent to 20 percent. The fifth and tenth
stage bleed air valves also close now. When the
valve, the valve opens. This allows compressed
power lever advances above 66 degrees of
air to flow to the air turbine starter. It also closes
coordinator travel, the temperature datum system
the contacts of the air valve position switch. The
yellow starter valve lights illuminate to show the
switches from temperature limiting range to
temperature controlling.
operator the starter control valve is open. The air
turbine starter now causes engine rotation.
If fuel enrichment is needed, depress the
PRIMER switch and hold in the ON position until
REVIEW SUBSET NUMBER 4
the engine reaches 16 percent. Power then goes
through the primer relay contacts and the
Q1. During a P-3 engine start, at what percent
temperature datum relay contacts, energizing the
RPM does the start control switch re-
primer valve solenoid.
energize?
When the engine reaches 16 percent RPM, the
speed-sensitive control mechanically actuates the
16 percent switch from 16 percent to 65 percent.
Q2. What are the three switches in the speed-
Power then goes through the 16 percent switch
sensitive control?
contacts, energizing the ignition relay. Power
