Piper Cheyenne IIIA Airplane Maintenance Manual page 168

PIPER AIRCRAFT
PA-42-720 (Advanced Trainer)
MAINTENANCE MANUAL
A. Distribution
Bleed air is ducted from the nacelle firewall to the ECU in the aircraft nose. The cooling air vent in the
wing root fairing provides a flow path overboard when a bleed coupling O-ring fails. The bleed mixing
capability of the bleed air control valve assumes a lower bleed air temperature in the ducting system
and eliminates the need for bleed ducting cooling flow.
After bleed air lines penetrate the fuselage on either side, they join at a Y-duct downstream from a
check valve installed in each leg of the Y. These check valves prevent reverse flow into a non-
operating engine or non-pressurized duct system and prevent loss of cabin pressurization. The duct
from the Y-duct forward to the forward pressure bulkhead, fuselage station 57.00, is shrouded and
vented overboard at the bottom of the fuselage, with the inlet in the right hand equipment compartment
at fuselage station 57.00 bulkhead.
A ram air-bleed air intercooler is installed at the firewall to cool the bleed air supply auxiliary systems
below 250°F, prior to entering the fuel tank area aft of the firewall. When the ram air inlet to the
intercooler is blocked, a bleed air overtemperature switch is installed in the bleed air duct downstream
of the intercooler. This switch is set to close at 425°F ±5
0
F, which will illuminate the BLEED AIR HOT
light on the pilot annunciator panel. If this occurs, the following steps must be taken:
1. Shut off the ECU bleed air from that engine by using the ON-OFF switch on the overhead engine
switch panel.
2. If the light remains on after shutting off the ECU bleed air, the 425°F switch could have caused the
light to illuminate. Since this air cannot be shut off, power to that engine must be reduced.
3. If the light remains on after completing steps 1 and 2, make a precautionary landing as soon as
practical, using minimum engine power on the affected side.
4. Inspect engine bleed air duct system and make the necessary repairs.
The cabin air distribution system uses bleed air from the engines and air entering the ECU from
outside the airplane to operate the ECS. The ECS in turn produces cabin heating or cooling and
air to defrost the windshield and side windows. Hot bleed air is used to supply heat, when
required, through a bypass valve on the ECU. The bypass valve controls the amount of hot air
mixed with cold air prior to the air entering the cabin.
Air is distributed through out the cabin through a system of ducts, blower motors, louvers and
outlets. The air boxes, located forward of the spar boxes, house blower motors which help
distribute the proper volume of air to each outlet. A diverter valve, located below the junction of
the windshield and side window defroster lines, is installed to operate the distribution of defroster
air for these windows.
The ECS cabin selector panel, located at the lower portion of the copilot's instrument panel, allows
for operation of cabin air distribution with the use of electrically operated temperature and pressure
controls.
ABIN AIR
CABIN COMFORT
ECS SELECTOR
HEAT
HEAT
ECS SELECTORL
ECS Cabin Selector Panel
Figure 1
EFFECTIVITY:
040-999
21 2000
Jan
PAGE
1
21-20-00
Jan 10/90