(b)
If the torque switch opens from the throttle levers being driven into the aft stop, the torque switch may remain open, even after the clutch disengages. When the A/T is ready to drive the throttle levers away from the aft stop, the torque switch must be bypassed to permit clutch engagement (if it is open).
(4)
Torque Switch Bypass
(a) When the torque switch is open aft of 6° (thrust lever angle), and the A/T mode and command is such that clutch engagement is required, a pulsed torque switch bypass ground is transmitted to the A/T servo actuator. The ground sets a 0.75 second pulse generator that closes S2 and energizes the clutch. The pulse generator is reset as soon as the torque switch closes.
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737-300/400/500MAINTENANCE MANUAL
22. A/T Annunciators________________
A. Mode Annunciation (Fig. 15)
(1)
Eight different mode annunciations are provided to indicate A/T operation.
(a) AIRPLANES WITHOUT EFIS; The captain's and first officer's annunciators are driven in parallel by logic grounds from the A/T computer. Annunciator test is addressed in the Flight Mode Annunciator description.
(2)
AIRPLANES WITHOUT EFIS; With the A/T disengaged, all mode switches are open and the blank face of the annunciators are displayed.
(3)
When the A/T is engaged, and no modes are selected, the ARM annunciation is displayed.
(4)
During operation when the A/T is responding to an N1 limit or target provided by the FMC, the annunciation is N1. Logic for the N1 mode is defined in a preceding table.
(5)
When the A/T is maintaining a target airspeed, the annunciation is either FMC SPEED or MCP SPEED depending on the source of the speed target. The FMC generated target is used when the autopilot mode is VNAV.
(6)
THR HOLD is annunciated during takeoff after CAS reaches 64 knots (or 84 knots for airplanes with A/T computers P/N 10-62017-25 and subsequent). THR HOLD terminates 17.5 seconds after liftoff.
(7)
If the selected N1 limit from the FMC is invalid (reversion mode), or the autothrottle is engaged and actual N1 is invalid, the throttle limit becomes a throttle lever position limit established by the A/T. The annunciation is A/T LIMIT.
(8)
GA is annunciated during reduced go-around operation, as N1 limit is established by the A/T. During maximum go-around, when the A/T is following the go-around N1 limit established by the FMC, the annunciation is N1.
B. AIRPLANES WITHOUT EFIS;
ARINC 429 and Fast/Slow Outputs (Fig. 16)
(1) The Autothrottle related data is provided by way of an ARINC 429 digital bus to the Flight Control Computers, the Flight Management Computer, and the Electronic Flight Instrument System symbol generators (if installed). The data words are tabulated in the illustration.
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