For the 767 only.
General
The purpose of the power isolation logic is to cause the power
sources to separate during a triple channel approach.
The standby electrical system can be used as an independant power
source to supply the center channel during a triple channel autoland.
During autoland, the center dc bus receives power directly from the
hot battery bus and the center ac bus is supplied from the standby
inverter. The BTBs are locked in their current state as at bus
isolation.
.
The following will cause the power isolation logic to be set:
• Triple channel arm - all three channels either engaged or armed.
• None of the three channels generating NO LAND 3 logic.
• Approach mode armed.
• Airplane above alert height (200 feet RA).
.
Any of the following will cause the power isolation logic to reset:
• Total autopilot disconnect.
• Any channel generating NO LAND 3 logic above alert height.
• Go-around initiated.
.
Single power source failure above alert height after LAND 3 annunciated:
Above alert height, a single power source failure will lead to the
respective autopilot disengaging. The autoland status annunciator will
display LAND 2/NO LAND 3. As a consequence of NO LAND 3
condition, the autopilot system will drop the request for bus isolation.
If the APU generator is available, the failed electrical bus will
automatically be transferred to the APU generator.
.
Single power source failure below alert height after LAND 3 annunciated:
Below alert height, a single power source failure will not affect
LAND 3 status. The power isolation logic will not reset until the
autopilots are disengaged or go-around is initiated. The affected FCC
will disengage due to the failed power source.
.
NOTE:
NO LAND 3 status will cause power isolation logic to be
reset but the re-establishment of fail-operational capability
will not re-establish power isolation unless the approach
mode or triple arm is deactivated and re-established.
Through the power isolation circuits, NO LAND 3 logic
will be generated if isolation does not occur within 4
seconds of being requested by the FCC and isolation will
be reset and not requested again.
Keep the q's coming. keeps the brain working!