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A 737-300/400/500MAINTENANCE MANUAL
P5
TO INSTRUMENT ACCESSORY UNITCREW CALL HORN ON GROUND
EFIS Cooling System Circuit Figure 3
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6.
7.
8.
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737-300/400/500MAINTENANCE MANUAL
C. Actuation of EFIS indication control relay sends a discrete signal to the EFIS system to downgrade the picture enhancement (Ref 34-22-00).
D. On the ground with either IRU powered, loss of supply cooling will
actuate the crew call horn in the nose wheel well.Bleed Orifice
_____________
A. A fixed bleed orifice, venting to the open area on the right side of the nose gear wheel well area, is installed in the supply ducts just aft of the low flow sensor.
Ducts and E3-5 Shelf
____________________
A. A combination of metal, fiberglass and flexible hose ducts provide passage for cooling air to the EFIS modules in the Captain's and First Officer's instrument panels and the E3-5 shelf in the E/E compartment. Air passes through holes in the E3-5 shelf through the IRU's for cooling.
Operation
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A. The operation of the equipment cooling supply system is constant once electrical power is applied to the airplane. With the fan selector switch (PS) positioned to NORMAL the normal fan will operate unless the BUS No.1 is de-energized. If the normal fan fails to operate, the fan selector switch may be positioned to ALTERNATE and the alternate fan will then operate.
B. For maintenance activities requiring main electrical power on for more than 20 minutes at outside ambient temperature greater than 100° F (37.8°C), it is recommended that conditioned air be supplied to the cabin (Ref 21-00-00 M/P). This will improve the thermal environment for the electrical equipment.
C. When electrical power is connected to the airplane, circuit breaker panel P18-3 is energized and the fan and the low flow sensor are energized.
D. Cooling air exhausts from the EFIS instruments through the control cabin floor into the area forward of the nose wheel well. Cooling air exhausts from the IRU's into the E/E compartment and exhaust from the airplane through the forward or aft outflow valves.
E. Normal airflow past the low flow sensor probe stabilizes the current required to heat the probe. This current is sensed by the electronic circuit in the sensor. Loss of air flow causes a change in the current to heat the probe triggering a transistor to supply a ground to the EFIS indication control relay (R401).
F. Actuation of the low flow sensor by loss of airflow sends a signal to the EFIS symbol generators which removes the raster display from the EADI's and EHSI's. In addition, WXR DSPLY and EXCESS DATA will be annunciated on the EHSI's. These annunciations will remain until supply cooling is restored.
NOTE: Airplanes with symbol generator P/N 622-8000-004 will also
____ display the EXCESS DATA message on the left and right EHSI's.
G. Following a nineteen second delay, if one or both of the IRUs are powered and the airplane is on the ground, actuation of the low flow sensor will operate the crew call horn in the nose wheel well alerting the ground crew of an impending overheat condition in the IRU.
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A
737-300/400/500MAINTENANCE MANUAL
H. Actuation of the low flow sensor illuminates an EQUIP COOLING SUPPLY OFF light (P5) and the MASTER CAUTION light (Pilots glareshield).
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