Narrative:

'Look before you switch' this captain normally flies DC10-10 aircraft. This incident happened to be in a 10-30. O'hare WX was about 800 ft and 4 light snow. Parallel runway 27 approach in progress. Flight had just leveled at 4000 ft proceeding downwind for runway 27R. Captain was hand flying and noticed that his FD display showed altitude still armed and no capture in spite of the fact that 4000 ft was set and armed. (A previous log item was that autoplt did not capture altitude -- 'fix' -- unable to duplicate.) the captain asked his first officer is his FD had captured and first officer replied 'yes.' the flight was then given a heading at 180 degrees to turn base. While turning the captain reached up and turned what he thought was the FD both on first officer switch he either switched CADC or emergency power, but movement of the switch caused the horizon to start to tumble and switch was immediately turned back to the original position. (Fraction of a second). When wings rolled level, both compass cards had different headings neither at which jived with the locator pointing at 'taffs.' aircraft was then given a new heading to intercept. Captain turned to this new heading but locator pointer still did not jive with indicated heading. Tower noticed the aircraft passing through the localizer and repeated the intercept instructions. We then replied that we had a compass card problem and would like to be pulled off of the approach. Approach control told aircraft to turn to 330 degrees. We told approach that we had no compass and were in a right turn and to please 'stop our turn.' approach replied 'turn right to 360 degrees.' we repeated that we had no compass. They replied 'turn to 360 degrees maintain 4OOO ft. The captain then instructed the first officer to pull down and light up the wet compass. Since none of us were able to recall which way the wet compass precessed in a turn, it took a couple of wings level to set on a north heading. We then switched both compass cards to dc, caged them to north and reslaved them. After that we were vectored for a normal approach. After we were on the ground, switching the air data computer or emergency power would not duplicate the problem. Neither will this captain duplicate by not looking before he switches. Maintenance determined that it was the pitch computer that was the problem.

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Original NASA ASRS Text

Title: DURING A VECTORING APCH TO FINAL, THE CAPT OF AN WDB MISTAKENLY SWITCHED OFF THE PWR TO THE DIRECTIONAL GYROS CAUSING A DEV FROM ASSIGNED HDG.

Narrative: 'LOOK BEFORE YOU SWITCH' THIS CAPT NORMALLY FLIES DC10-10 ACFT. THIS INCIDENT HAPPENED TO BE IN A 10-30. O'HARE WX WAS ABOUT 800 FT AND 4 LIGHT SNOW. PARALLEL RWY 27 APCH IN PROGRESS. FLT HAD JUST LEVELED AT 4000 FT PROCEEDING DOWNWIND FOR RWY 27R. CAPT WAS HAND FLYING AND NOTICED THAT HIS FD DISPLAY SHOWED ALT STILL ARMED AND NO CAPTURE IN SPITE OF THE FACT THAT 4000 FT WAS SET AND ARMED. (A PREVIOUS LOG ITEM WAS THAT AUTOPLT DID NOT CAPTURE ALT -- 'FIX' -- UNABLE TO DUPLICATE.) THE CAPT ASKED HIS FO IS HIS FD HAD CAPTURED AND FO REPLIED 'YES.' THE FLT WAS THEN GIVEN A HDG AT 180 DEGS TO TURN BASE. WHILE TURNING THE CAPT REACHED UP AND TURNED WHAT HE THOUGHT WAS THE FD BOTH ON FO SWITCH HE EITHER SWITCHED CADC OR EMER PWR, BUT MOVEMENT OF THE SWITCH CAUSED THE HORIZON TO START TO TUMBLE AND SWITCH WAS IMMEDIATELY TURNED BACK TO THE ORIGINAL POS. (FRACTION OF A SECOND). WHEN WINGS ROLLED LEVEL, BOTH COMPASS CARDS HAD DIFFERENT HDGS NEITHER AT WHICH JIVED WITH THE LOCATOR POINTING AT 'TAFFS.' ACFT WAS THEN GIVEN A NEW HDG TO INTERCEPT. CAPT TURNED TO THIS NEW HDG BUT LOCATOR POINTER STILL DID NOT JIVE WITH INDICATED HDG. TWR NOTICED THE ACFT PASSING THROUGH THE LOC AND REPEATED THE INTERCEPT INSTRUCTIONS. WE THEN REPLIED THAT WE HAD A COMPASS CARD PROB AND WOULD LIKE TO BE PULLED OFF OF THE APCH. APCH CTL TOLD ACFT TO TURN TO 330 DEGS. WE TOLD APCH THAT WE HAD NO COMPASS AND WERE IN A R TURN AND TO PLEASE 'STOP OUR TURN.' APCH REPLIED 'TURN R TO 360 DEGS.' WE REPEATED THAT WE HAD NO COMPASS. THEY REPLIED 'TURN TO 360 DEGS MAINTAIN 4OOO FT. THE CAPT THEN INSTRUCTED THE FO TO PULL DOWN AND LIGHT UP THE WET COMPASS. SINCE NONE OF US WERE ABLE TO RECALL WHICH WAY THE WET COMPASS PRECESSED IN A TURN, IT TOOK A COUPLE OF WINGS LEVEL TO SET ON A N HDG. WE THEN SWITCHED BOTH COMPASS CARDS TO DC, CAGED THEM TO N AND RESLAVED THEM. AFTER THAT WE WERE VECTORED FOR A NORMAL APCH. AFTER WE WERE ON THE GND, SWITCHING THE AIR DATA COMPUTER OR EMER PWR WOULD NOT DUPLICATE THE PROB. NEITHER WILL THIS CAPT DUPLICATE BY NOT LOOKING BEFORE HE SWITCHES. MAINT DETERMINED THAT IT WAS THE PITCH COMPUTER THAT WAS THE PROB.

Data retrieved from NASA's ASRS site as of July 2007 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.