Other Autothrottle Functions; Engage/Disengage; Airspeed Limit Protection; Engine Limit Protection - Honeywell SPZ-8000 Maintenance Manual

4.
c.
(5) Other Autothrottle Functions
Several autothrottle functions are performed in addition to the
basic operating modes:
(a) Engage/Disengage
While normal engagement and disengagement is by the switch on
the throttles, other means are available to disengage the
autothrottle. As mentioned earlier, a positive disconnect
button provides a direct-to-hardware path for disengagement,
or the pilot can manually move the throttles, causing an
autothrottle disconnect.
Logic is included in the autothrottle
to detect an engine failure and immediately disengage. If a
fault
is detected in the autothrottle system or in a system
providing required inputs, the autothrottle will also
disconnect.
(b) Airspeed Limit Protection
Maximum airspeed limits are observed by the autothrottle when
it is operating in speed hold mode. These limits include
maximum operating speeds (VM~M ), as well as placards for all
'!" flap and landing gear conflgura ions. Minimum speed protection
is Drovided whenever the system is ot)eratinain the automatic
(FM~) speed selection mode: In this'mode,
set by the PZ-800 Performance Computer and
drop below Vref.
(c) Engine Limit Protection
~he speed command is
is never allowed to
The autothrottle uses the selected EPR rating as an upper limit
for control in all modes of operation. Automatic rating
selection is available, where the PZ-800 Performance Computer
chooses the engine rating based on the phase of flight. Also,
minimum engine limits are observed to avoid the nonlinear
flat-response area near idle throttle settings.
(d) Synchronization of Engine Rotor Speeds
Engine rotor speed synchronization is provided by the
autothrottle when it is engaged in speed hold mode.
Low-pressure rotor speed (Nl) or high-pressure rotor speed (N2)
can be selected by the pilot for synchronization. Normally, N1
synchronization is selected in cruise to prevent any potential
beating noise from the engines. If neither rotor speed is
selected, a default EPR equalization is in effect.
22-14-00
Page 298.184
Aug 15/91
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