Fuel Quantity Probes, Left Wing Tank Double Probe Failure

TASK 28-42-00-810-854-A
Fuel Quantity Probes, Left Wing Tank Double Probe Failure

WARNING: OBEY THE FUEL SAFETY PROCEDURES.

1. Possible Causes

Left Wing Tank CLASS 3 probe failures (will be more than one)

2. Fault Confirmation
Subtask 28-42-00-865-085-A ** ON A/C NOT FOR ALL

A. Table of the circuit breakers used in this procedure:

PANEL	DESIGNATION	FIN	LOCATION
** ON A/C NOT FOR ALL
49VU	FUEL/FQI/CHAN/1	1QT	A13
121VU	FUEL/FQI/CHAN/2	2QT	M27

Subtask 28-42-00-710-100-A ** ON A/C NOT FOR ALL

B. To keep the number of computer removals to a minimum do the Power On BITE test of the FQI system as follows:
(1) Apply electrical power to the FQI system.

OR (if the FQIS is already powered).

Reset the FQIC. (Open, then after 10 seconds, close the two CBs 1QT and 2QT).
(2) When the BITE test is complete (XX shown on the FQI for 35 seconds and the display has no dashes for 60 seconds) set the MCDU to:
(a) CFDS
(b) SYSTEM REPORT/TEST
(c) FUEL
(d) FQIS STATUS.
(3) Examine the FQIS STATUS page to see if it agrees with the initial defect.
(4) Maintenance steps are not necessary if the three conditions that follow are all correct:

the L DOUBLE PROBE FAILURE message is not shown
the Engine/Warning Display FOB reads correctly (dashes or XXs not shown)
the System Display Left Wing fuel tank FQI reads correctly (dashes or XXs not shown).

NOTE: The CLASS 1 double probe failure message is given/shown when certain pairs of probes fail which individually are CLASS 3.
If a pair of these related probes fail there is a cockpit effect, either degraded or failed.
More data reference Double Probe Failure can be got from P. Block 301.

3. Fault Isolation
Subtask 28-42-00-810-100-A ** ON A/C NOT FOR ALL

A. If the L DOUBLE PROBE FAILURE message is shown in Para. 3.B.(3):
(1) Examine the FQIS STATUS page for Left Wing Tank CLASS 3 probe failures (will be more than one).
(2) Make a note of the Left Wing Tank CLASS 3 probe failures shown.
(3) Go to and find the CFDS fault, and troubleshoot the identified probes, as identified in Step (2).
(4) Troubleshoot the identified probes.

[Rev.8 from Aug 2018] 2026.04.04 04:29:08 UTC