Failure of the GAPCU in the FAILSAFE Mode

TASK 24-41-00-810-832-A
Failure of the GAPCU in the FAILSAFE Mode

1. Possible Causes

GAPCU [24XG]

2. Job Set-up Information
A. Referenced Information

REFERENCE	DESIGNATION
AMM 24-41-00-740-002	Operational Check of the GAPCU via the CFDS
AMM 24-41-00-861-002	Energize the Aircraft Electrical Circuits from the External Power
AMM 24-41-00-862-002	De-energize the Aircraft Electrical Circuits Supplied from the External Power
AMM 24-41-34-000-001	Removal of the Ground Power Control Unit (GPCU)
AMM 24-41-34-400-001	Installation of the Ground Power Control Unit (GPCU)

3. Fault Confirmation
Subtask 24-41-00-740-067-A ** ON A/C NOT FOR ALL

A. Test
(1) Remove all AC and DC power from the GAPCU (24XG):

de-energize the aircraft electrical circuits from the APU AMM 24-41-00-862-002
de-energize the aircraft electrical circuits from the external power AMM 24-41-00-862-002
disconnect the batteries or open the GAPCU circuit breaker (17XG) to remove the 28VDC backup power-supply from the GAPCU.

(2) Apply power to the GAPCU:

energize the aircraft electrical circuits from the APU AMM 24-41-00-861-002
energize the aircraft electrical circuits from the external power AMM 24-41-00-861-002.

(3) Do the BITE test of the GAPCU AMM 24-41-00-740-002.
(a) If the BITE test gives the message TEST FAILED:

push the line key adjacent to the TEST FAILED message.

1 If the maintenance message GAPCU (24XG) comes into view:

do the trouble shooting procedure given in the Para 4.A.

(b) If the BITE test gives the message TEST PASSED:

stop the trouble shooting.

4. Fault Isolation
Subtask 24-41-00-810-093-A ** ON A/C NOT FOR ALL

A. If the BITE test gives the maintenance message GAPCU (24XG) or if the APU generator or the external power does not energize the aircraft electrical circuits:

replace the GAPCU [24XG] AMM 24-41-34-000-001 and AMM 24-41-34-400-001.

Subtask 24-41-00-860-072-A ** ON A/C NOT FOR ALL

B. Make sure that the aircraft electrical circuits operate correctly in APU configuration and in external power configuration.
(1) Energize the aircraft electrical circuits from the APU AMM 24-41-00-861-002.
(2) Let the APU generator operate for five minutes.
(3) Energize the Aircraft Electrical Circuits from the external power AMM 24-41-00-861-002.
(4) De-energize the aircraft electrical circuits from the APU AMM 24-41-00-862-002.
(5) De-energize the aircraft electrical circuits from the external power AMM 24-41-00-862-002.

[Rev.8 from Aug 2018] 2026.04.04 04:26:55 UTC