Narrative:

Cleared after takeoff to climb on south course of ILS (164 degrees) to 15000', etc. Aircraft is medium large transport with FMC, autoplt, etc. Altitude on MCP of autoplt flight director system (AFDS) was set at 15000' and should give normal audio & visibility warnings approaching and also if autoplt engaged, level aircraft at set altitude. The altitude is set by manual drum knob and indicated electronically. It is part of a system. The copilot was flying and we both ascertained the altitude set at 15000'. Sometime during climb the altitude set (15000') in MCP changed to 16000'. We were detracted outside looking for traffic and copilot navigating and did not realize altitude change until approaching 15000'. As copilot flew through 15000' and trying to smoothly correct error we were cleared higher. We obviously both were surprised to see 16000' altitude selected. I later called rno departure controller (he was a supervisor) and discussed the situation with him. He did an excellent job and was very professional and capable. He said he could see the problem coming and there was no incident or anything further. I desire he & ATC not be brought into this as we had a very respectful attitude of cooperation and trust and I wouldn't want to damage that. The procedure occurred when we were busy and occupied with numerous obligations. True, we should not completely rely on an altitude warning and automatic level off, but at times, possible due to complacency, we do rely on it. Fortunately in this case, nothing serious occurred, but it could. Maintenance advised this has occurred before west/the system. A fix should be found & implemented.

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Original NASA ASRS Text

Title: ADVANCED TECHNOLOGY ALT SELECTOR SPONTANEOUSLY CHANGED 1000' CAUSING ALT DEVIATION.

Narrative: CLRED AFTER TKOF TO CLB ON S COURSE OF ILS (164 DEGS) TO 15000', ETC. ACFT IS MLG WITH FMC, AUTOPLT, ETC. ALT ON MCP OF AUTOPLT FLT DIRECTOR SYS (AFDS) WAS SET AT 15000' AND SHOULD GIVE NORMAL AUDIO & VIS WARNINGS APCHING AND ALSO IF AUTOPLT ENGAGED, LEVEL ACFT AT SET ALT. THE ALT IS SET BY MANUAL DRUM KNOB AND INDICATED ELECTRONICALLY. IT IS PART OF A SYS. THE COPLT WAS FLYING AND WE BOTH ASCERTAINED THE ALT SET AT 15000'. SOMETIME DURING CLB THE ALT SET (15000') IN MCP CHANGED TO 16000'. WE WERE DETRACTED OUTSIDE LOOKING FOR TFC AND COPLT NAVIGATING AND DID NOT REALIZE ALT CHANGE UNTIL APCHING 15000'. AS COPLT FLEW THROUGH 15000' AND TRYING TO SMOOTHLY CORRECT ERROR WE WERE CLRED HIGHER. WE OBVIOUSLY BOTH WERE SURPRISED TO SEE 16000' ALT SELECTED. I LATER CALLED RNO DEP CTLR (HE WAS A SUPVR) AND DISCUSSED THE SITUATION WITH HIM. HE DID AN EXCELLENT JOB AND WAS VERY PROFESSIONAL AND CAPABLE. HE SAID HE COULD SEE THE PROB COMING AND THERE WAS NO INCIDENT OR ANYTHING FURTHER. I DESIRE HE & ATC NOT BE BROUGHT INTO THIS AS WE HAD A VERY RESPECTFUL ATTITUDE OF COOPERATION AND TRUST AND I WOULDN'T WANT TO DAMAGE THAT. THE PROC OCCURRED WHEN WE WERE BUSY AND OCCUPIED WITH NUMEROUS OBLIGATIONS. TRUE, WE SHOULD NOT COMPLETELY RELY ON AN ALT WARNING AND AUTOMATIC LEVEL OFF, BUT AT TIMES, POSSIBLE DUE TO COMPLACENCY, WE DO RELY ON IT. FORTUNATELY IN THIS CASE, NOTHING SERIOUS OCCURRED, BUT IT COULD. MAINT ADVISED THIS HAS OCCURRED BEFORE W/THE SYS. A FIX SHOULD BE FOUND & IMPLEMENTED.

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.