sensing element. The switches sense still air
temperature. The outboard leading edge overheat
warning switches open at approximately 205°F
and close at approximately 220°F. The other wing
and fuselage overheat warning switches open at
approximately 175°F and close at approximately
High temperature within the leading edge is
generally caused by bleed-air leakage or
malfunctioning modulator valves. You can detect
the portion of the leading edge that has the
overtemperature by placing the rotary selector
switch, located on the ice control protection panel,
to the different sensor positions: INBD, CTR, and
OUTBD. (See fig. 3-4.) The temperature at the
selected sensor is then read at the indicator
adjacent to the rotary switch. An excessive
temperature reading on the indicator denotes a
malfunction within the area being tested.
Figure 3-4, the ice control protection panel,
shows a basic diagram of the wing deice system.
Each engine is labeled by an engine number.
Directly below each engine block (in the diagram)
is an OPEN light that illuminates when the bleed-
air valve is open 2 degrees or more. The cross-
ship manifold from the bleed-air valves goes to
each modulating valve and the fuselage shutoff
valves. The fuselage bleed-air shutoff valves are
normally in the CLOSE position during normal
deicing operation. The bleed-air pressure gauge
reads cross-ship manifold pressure when one or
both switches are opened.
A ground air-conditioning switch is located
directly under the bleed-air manifold pressure
gauge. Located above the switch is an annunciator
light, which indicates VALVE OPEN when the
ground air-conditioning valve is open. Either one
or both fuselage bleed-air shutoff valves must be
open to direct air to the ground air-conditioning
A leak test switch is mounted on the upper
right-hand side of the panel. This switch is used
to determine if the leakage of the system is
Three modulating valve control switches are
located on the left side of the wing and empennage
ice panel. The outboard switch controls the
outboard modulating valve on the left and right
wing, the center switch controls the two center
modulating valves, and the inboard switch
controls the two inboard modulating valves.
During normal operation of the deicing
system, all four engine bleed-air valves are open
to supply bleed air to the cross-ship manifold, and
both fuselage bleed-air shutoff valves are closed.
Figure 3-4.-Ice control protection panel.