Narrative:

Approaching den from the east on J80 the captain (PF) asked copilot (PNF) to request FL390 due to building thunderstorms over the rocky mountains. I (copilot) put FL390 in the right FMS computer to check aircraft capability for FL390. After entering and executing FL390 in 1 left on FMS, I verified that the altitude window on MCP was at 35000' and that the autothrottles did not add power for the climb. At this pint, the MCP altitude window was holding the aircraft at current cruise altitude of 35000'. This has been an accepted procedure in this situation. After checking altitude capability in the FMC, I mentioned to the captain that we could make FL390 and would save approximately 1% of fuel with the climb. This whole check took probably less than 20-30 seconds. I then called den ATC and was advised to expect FL390 in approximately 2 mins due to traffic. Anticipating the higher altitude, I left FL390 in the FMC active cruise page, once again checking to make sure the window read 35000'. I continued to prepare the ACARS position report to be transmitted over den. We were approximately 3 mins east of den. I remember checking the ETA for slc and entering the fuel over den as 22.5. Since I was preparing the position report I changed from the cruise page on the FMC to the progress page, but the captain still had the cruise page in view with the FL390 cruise active page on it. During the minute or minute and a half of preparing the ACARS position report and waiting for the ATC clearance to FL390 the captain (PF) changed the MCP altitude window to 39000', anticipating the climb. Of course, the FMC not being constrained at 35000' any longer started a slow climb to FL390. The captain also began a passenger announcement to the passenger about den and the turbulence, and that we expected a climb to a higher altitude shortly. The center called, 'maintain FL350.' west/O even hesitating, I responded 'roger, maintain 350.' by this time the captain (PF) had already started a push-over. The aircraft had reached an altitude of approximately FL357. After the aircraft was returned to FL350, I checked the MCP altitude window and was surprised to see 39000'. We returned it to 35000', our cleared altitude. Within a few mins, center cleared to FL390. Crew coordination and lack of communication may have contributed to the altitude excursion and conflict. The MCP altitude window is, in my judgement, the last step in the altitude change process, to be changed after clearance has been received. The autoflt system will not depart the MCP altitude, even if the FMC is programmed for a different altitude.

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Original NASA ASRS Text

Title: ALT DEVIATION DUE TO MISUSE OF FMC.

Narrative: APCHING DEN FROM THE E ON J80 THE CAPT (PF) ASKED COPLT (PNF) TO REQUEST FL390 DUE TO BUILDING TSTMS OVER THE ROCKY MOUNTAINS. I (COPLT) PUT FL390 IN THE RIGHT FMS COMPUTER TO CHK ACFT CAPABILITY FOR FL390. AFTER ENTERING AND EXECUTING FL390 IN 1 L ON FMS, I VERIFIED THAT THE ALT WINDOW ON MCP WAS AT 35000' AND THAT THE AUTOTHROTTLES DID NOT ADD PWR FOR THE CLB. AT THIS PINT, THE MCP ALT WINDOW WAS HOLDING THE ACFT AT CURRENT CRUISE ALT OF 35000'. THIS HAS BEEN AN ACCEPTED PROC IN THIS SITUATION. AFTER CHKING ALT CAPABILITY IN THE FMC, I MENTIONED TO THE CAPT THAT WE COULD MAKE FL390 AND WOULD SAVE APPROX 1% OF FUEL WITH THE CLB. THIS WHOLE CHK TOOK PROBABLY LESS THAN 20-30 SECS. I THEN CALLED DEN ATC AND WAS ADVISED TO EXPECT FL390 IN APPROX 2 MINS DUE TO TFC. ANTICIPATING THE HIGHER ALT, I LEFT FL390 IN THE FMC ACTIVE CRUISE PAGE, ONCE AGAIN CHKING TO MAKE SURE THE WINDOW READ 35000'. I CONTINUED TO PREPARE THE ACARS POS RPT TO BE XMITTED OVER DEN. WE WERE APPROX 3 MINS E OF DEN. I REMEMBER CHKING THE ETA FOR SLC AND ENTERING THE FUEL OVER DEN AS 22.5. SINCE I WAS PREPARING THE POS RPT I CHANGED FROM THE CRUISE PAGE ON THE FMC TO THE PROGRESS PAGE, BUT THE CAPT STILL HAD THE CRUISE PAGE IN VIEW WITH THE FL390 CRUISE ACTIVE PAGE ON IT. DURING THE MINUTE OR MINUTE AND A HALF OF PREPARING THE ACARS POS RPT AND WAITING FOR THE ATC CLRNC TO FL390 THE CAPT (PF) CHANGED THE MCP ALT WINDOW TO 39000', ANTICIPATING THE CLB. OF COURSE, THE FMC NOT BEING CONSTRAINED AT 35000' ANY LONGER STARTED A SLOW CLB TO FL390. THE CAPT ALSO BEGAN A PAX ANNOUNCEMENT TO THE PAX ABOUT DEN AND THE TURB, AND THAT WE EXPECTED A CLB TO A HIGHER ALT SHORTLY. THE CENTER CALLED, 'MAINTAIN FL350.' W/O EVEN HESITATING, I RESPONDED 'ROGER, MAINTAIN 350.' BY THIS TIME THE CAPT (PF) HAD ALREADY STARTED A PUSH-OVER. THE ACFT HAD REACHED AN ALT OF APPROX FL357. AFTER THE ACFT WAS RETURNED TO FL350, I CHKED THE MCP ALT WINDOW AND WAS SURPRISED TO SEE 39000'. WE RETURNED IT TO 35000', OUR CLRED ALT. WITHIN A FEW MINS, CENTER CLRED TO FL390. CREW COORD AND LACK OF COM MAY HAVE CONTRIBUTED TO THE ALT EXCURSION AND CONFLICT. THE MCP ALT WINDOW IS, IN MY JUDGEMENT, THE LAST STEP IN THE ALT CHANGE PROCESS, TO BE CHANGED AFTER CLRNC HAS BEEN RECEIVED. THE AUTOFLT SYS WILL NOT DEPART THE MCP ALT, EVEN IF THE FMC IS PROGRAMMED FOR A DIFFERENT ALT.

Data retrieved from NASA's ASRS site as of August 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.