Narrative:

Checking the logbook before flight; I had the following write-up that was deferred by MEL 34-19: 'the captain's inboard display is showing three unusual symbols near the aircraft symbols. Moving display source switch to 'all on 1' or 'all on 2' had no effect. However the display source annunciator did not go out when the switch was returned to auto. All other indications are normal.' maintenance sign-off entry: 'placarded display instrument display source switch inoperative [inop] per MEL 34-19. Entered [information] in logbook deferral sheet'. Maintenance had placed an 'inop' sticker next to the display source switch and placed the switch to 'all on 2'. Checking the MEL it states that: '(O) may be inoperative provided: a) associated instruments are operative from isolated sources; and b) inoperative switches are not moved during flight. (O) procedures: a). Check that associated instruments function properly from isolated sources.' on take-off; the pitch bar was biased out of view until engaging VNAV. On approach into our destination; we were doing a coupled ILS approach to the runway. At 1;500 AGL captain's command steering bars biased out of view. The first officer and I talked about it after landing and the next day and our first thought was that maintenance had correctly complied with the MEL; but that aircraft should also have been downgraded from CAT ii and CAT III capable.now; in reviewing the MEL once again and keying in on the part that states: 'associated instruments are operative from isolated sources'; I believe that maintenance used the wrong MEL procedure for this problem as the issue was with the captain's displays and not the transfer switch. I also believe that the initial write-up by the previous pilot was due to the 'position' switch on the EFIS panel being selected; as that would give the three position symbols near the aircraft symbol on nav display. I; my first officer; and apparently maintenance found this MEL confusing. If; in fact; it was the transfer switch that was the problem; it appears that the switch should remain in 'auto'; not 'both on 1' or 'both on 2' since the system would not now; have isolated sources.

Related reports:

Google
 

Original NASA ASRS Text

Title: A Captain describes why Maintenance may have used the wrong MEL for a B737-800 Logbook write-up; when they previously deferred; placarded and considered inoperative an Instrument Display Source Transfer switch under MEL 34-19; when the problem seemed to be the Captain's display; not the Transfer switch.

Narrative: Checking the Logbook before flight; I had the following write-up that was deferred by MEL 34-19: 'The Captain's inboard Display is showing three unusual symbols near the aircraft symbols. Moving Display Source switch to 'All on 1' or 'All on 2' had no effect. However the Display Source Annunciator did not go out when the switch was returned to auto. All other indications are normal.' Maintenance sign-off entry: 'Placarded display instrument display source switch inoperative [inop] per MEL 34-19. Entered [information] in Logbook Deferral sheet'. Maintenance had placed an 'Inop' sticker next to the display source switch and placed the switch to 'All on 2'. Checking the MEL it states that: '(O) May be inoperative provided: a) Associated instruments are operative from isolated sources; and b) Inoperative switches are not moved during flight. (O) PROCEDURES: A). Check that associated instruments function properly from isolated sources.' On take-off; the Pitch Bar was biased out of view until engaging VNAV. On approach into our destination; we were doing a coupled ILS approach to the runway. At 1;500 AGL Captain's command steering bars biased out of view. The First Officer and I talked about it after landing and the next day and our first thought was that Maintenance had correctly complied with the MEL; but that aircraft should also have been downgraded from CAT II and CAT III capable.Now; in reviewing the MEL once again and keying in on the part that states: 'associated instruments are operative from isolated sources'; I believe that Maintenance used the wrong MEL procedure for this problem as the issue was with the Captain's displays and not the transfer switch. I also believe that the initial write-up by the previous pilot was due to the 'POS' switch on the EFIS panel being selected; as that would give the three position symbols near the aircraft symbol on Nav display. I; my First Officer; and apparently Maintenance found this MEL confusing. If; in fact; it was the Transfer switch that was the problem; it appears that the switch should remain in 'auto'; not 'both on 1' or 'both on 2' since the system would not now; have isolated sources.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.