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STBY PITOT HT FAIL caution Crew Alerting System (CAS) messages.
In addition to the CAS messages, the OFF legend in the PITOT switch
plays a role in notifying the flight crew of pitot probe heat failures. On
airplanes SN 1000 through 1143 not having Aircraft Service Change (ASC)
187, the OFF legend will illuminate if either the PITOT switch is selected
OFF or if a single or dual heating element failure occurs. On airplanes SN
1144 and subsequent and SN 1000 through 1143 having ASC 187, the
OFF legend will illuminate if either the PITOT switch is selected OFF or if a
dual heating element failure occurs.
Selection of the PITOT switch to ON supplies 28 VDC power to energize
the left and right pitot probe heat relays. Once energized, the relays
extinguish the OFF legend in the switch and route single-phase 115 VAC
power to the relay’s associated heating element. (See Controls and
Indications for specific control and operating power sources.) When the
current sensors sense normal current flow through the heating elements,
the associated L-R PITOT HT FAIL caution CAS messages are cleared.
Selection of the STBY PITOT switch to ON supplies Essential 28 VDC bus
power to energize the standby pitot probe heat relay. Once energized, the
relay extinguishes the OFF legend in the switch and routes Essential 115
VAC bus power to the heating element. When the current sensor senses
normal current flow through the heating element, the STBY PITOT HT FAIL
caution CAS message is cleared.
Airplanes having ASC 10 have a pitot heat inhibit relay installed that
energizes when the outside battery switch is on the ON position. When
energized, the relay opens the circuit between the PITOT and STBY PITOT
switches and their associated probe heat relays.
B. Angle of Attack (AOA) Probe Heat System:
The AOA probes are located on either side of the forward fuselage.
The AOA probe heat system uses both a case heater and probe heater
controlled by the AOA switch (cockpit overhead panel, ANTI ICE section)
through two probe heat relays. Two thermal switches are also incorporated:
one for the case to maintain a moisture-free environment within the case
and one for the probe to prevent overheating.
OPERATING MANUAL
PRODUCTION AIRCRAFT SYSTEMS 2A-30-00
Page 27
October 11/02
Revision 8
Selection of the AOA switch to ON supplies Right Main and Essential 28
VDC bus power through the PITOT HT circuit breakers to the No. 1 and No.
2 probe heat relays. Once energized, the relay routes Right Main and
Essential 28 VDC bus power to the case and probe heating elements. The
OFF legend in the switch is then extinguished and the AOA HEAT 1-2 FAIL
caution CAS message is cleared.
The case heater thermal switch cycles the case heating element on and off
to maintain a case temperature of 90-125° F (32-52° C). By cycling heater
power, condensation formation within the case and its resultant effect on
probe operation are avoided.
The probe thermal switch prevents overheating by inhibiting heating
element power at 280° F (138° C). When probe temperature drops below
the trip point, power to the heating element is restored.
C. Total Air Temperature (TAT) Probe Heat System:
The TAT probe is located on the lower right side of the forward fuselage
below the AOA probe.
The TAT probe heat system is a shared function of the pitot probe heat
system. When in flight, selection of the PITOT switch to ON supplies Right
Main 28 VDC bus power through the TAT probe heat relay through
nutcracker relay No. 8. Once energized, the TAT probe heat relay routes
single-phase Right Main 115 VAC bus power to the TAT probe heating
element. When the current sensor senses normal current flow through the
heating element, the TAT HT FAIL caution CAS message is cleared.
When the airplane is on the ground, selection of the SAT/TAS GND
BYPASS switch (cockpit overhead panel, ANTI ICE section) to ON (ON
legend illuminated) bypasses the nutcracker relay to allow TAT probe
heating.
To ensure accurate calibration readings, air is introduced through the TAT
probe when the airplane is on the ground. Through this process, known as
aspiration, bleed air is supplied to the probe from a shutoff valve that opens
when the nutcracker shifts to the ground mode. This shutoff valve, known
as the “total temp valve”, receives its power from the Right Main 28 VDC
bus.
3. Controls and Indications:
(See Figure 10.)
A. Power Sources:
Probe: Switch: Control Power: Operating Power:
Left Pitot PITOT ESS DC Bus ESS AC Bus φA
Right Pitot PITOT R MAIN DC Bus R MAIN AC Bus φA
TAT PITOT R MAIN DC Bus R MAIN AC Bus φB
　
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