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MAINTENANCE MANUAL
(6)
Monitor Servo Loop
(a)
This process monitors the servo loop which consists of the servo motor and the tachometer feed back. The autothrottle servo (ATS) monitors the integrity of the servo loop. If a failure is detected, the ATS monitor will disconnect the servo excitation, disable the servo amplifier output command and disconnect all engaged modes.
(b)
The ATS monitors are designed so that the software ATS monitor can detect failures in the hardware ATS monitors. If the software ATS monitor detects a failure in the the servo loop without receiving conformation from the ATS hardware monitor, a fail state will be sent to BITE (indicating a failure in the hardware ATS monitor) and the autothrottle is disengaged.
(7)
Stopper test
(a)
When the airplane is on the ground and the takeoff mode is selected the master FMC will automatically perform the stopper test. This test, if passed, will ensure that the throttles will not retard during the critical takeoff phase. This critical phase is when the airplane is on the ground with a ground speed greater than 65 knots and when the airplane is off the ground and below 400 feet AGL.
(b)
This test consists of seven subtests in which one of the subtests is independent and performed in a different location. This independent test is known as the secondary throttle hold test.
(c)
The secondary throttle hold test controls one of the two series switches on the servo card. It performs two tests as follows: 1) Verifies that the servo excitation voltage is greater than
80 volts ac on the excitation windings.
2) Checks the ability of the independent logic to remove the excitation voltage. Since the switch provides a positive removal of the excitation voltage, if there is 5 volts ac or greater, then excitation is present, else excitation is not present.
3) In conducting these tests, a check is performed to ensure all tests have been performed and executed in the right order. These checks will be used in the first throttle hold test conducted by the main autothrottle controller.
4) The main autothrottle controller controls the second series switch on the servo card. This test consists of seven steps as follows: a) Test 1 - This test simply waits for the results from
the secondary throttle hold test for a certain amount of time. If the check sum and a valid reporting log has not been received during this time, then the secondary throttle hold test has failed.
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b) Test 2 - This test begins the main autothrottle controller checks. The first check determines weather there is excitation voltage to the servo through the servo excitation voltage monitor. If more than 80 volts ac is present then excitation is considered present, else excitation is zero.
c) Test 3 - This test ensures that the servo excitation is zero when there is no throttle rate command. If excitation voltage is greater than 5 volts ac, the test fails.
d) Test 4 - This test ensures that the tachometer has a zero value while the servo switch is open during the previous test.
e) Test 5 - This test ensures that the servo excitation is zero (less than 5 volts ac) while the throttle rate command is set to +10 deg/sec.
f) Test 6 - This test ensures that the tachometer returns to zero for a 10 deg/sec throttle rate command while the servo excitation is open.
g) Test 7 - The servo switch is now closed. This test ensures that excitation does and will exist while the switches are closed.
(8)
Annunciation
(a)
Two types of annunciations are displayed through the autothrottle control function. They are the current operating modes, displayed on the PFD, and the caution annunciation, displayed on the main EICAS displays.
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