Narrative:

While cruising at FL330, the #1 flight attendant came to the cockpit. She was concerned with passengers who were worried about connecting flts at ord because we were behind schedule. I was flying the aircraft, but was interested in how the captain would handle this situation since I'm getting close to captain upgrade. While we were discussing this matter, ord ARTCC gave us a descent clearance to FL290. The captain acknowledged on the radio and I set and armed 290 in the digital altitude window. I then began a slow (1000 FPM) descent using the vertical speed mode of the autoplt. The conversation continued. Passing through FL300, the captain started to say, '1000 ft to go', but he noticed 280 was set in the altitude window and that it was not armed. So he said, '2000 ft to go, but the altitude is not armed.' I simply reached up and armed the altitude with 280 set in the window. More conversation about the passenger connections. 'Passing 290 for 280 and it's armed' was interjected by the captain. I was slowly changing the vertical speed 100 ft at a time for a smooth level off, and passing FL284 was descending at 300 FPM. Suddenly, it struck me that FL280 was the wrong altitude for our direction of flight. I was trying to trace the sequence of events backward in my mind and started to level off thinking, 'we should be at FL290', when ATC called and asked our altitude. The captain said, 'passing 283 for 280.' ATC replied, 'your assigned altitude was 290, however it's not a problem and you can continue your descent to FL270.' the captain acknowledged, 'descending to 270.' when given a frequency change by this controller, the captain asked if there was any problem created by our altitude deviation. This controller said, 'no problem.' after replaying the above scenario numerous times, I'm sure I now know exactly what happened. I had set and armed FL290 in the altitude window. I was turned to my left with my left arm behind my seat back so I could see the flight attendant as she talked. I was using heading select on the autoplt to fly the VOR radial inbound. 2 or 3 times during this descent, I remember changing heading a couple degrees to make minor course corrections. On at least 1 occasion, I reached for the heading knob with my right hand and I can recall very lightly brushing something as I retracted it from the heading knob. I'm sure that I must have just ticked the altitude knob and that caused it to change from 290 to 280 and disarm. I have been a cockpit crew member with scheduled passenger carriers for over 17 yrs now. Prior to 1991, I had filed only 1 NASA report (on a near miss). During 1991, this is now my third NASA report. These last 3 reports have all been on the medium large transport and all pertain to altitude deviations. I have heard that this medium large transport is by far the worst offender concerning altitude excursions. By no stretch of the imagination can this altitude alert/autoplt level off system be considered fool proof. In fact, it is a very mixed up bag of tricks waiting to bite the unwary. It needs to be fixed, now! I can only hope it doesn't take a midair collision and hundreds off fatalities to get a correction for this system.

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Original NASA ASRS Text

Title: ALT DEV ALT OVERSHOT IN DSCNT.

Narrative: WHILE CRUISING AT FL330, THE #1 FLT ATTENDANT CAME TO THE COCKPIT. SHE WAS CONCERNED WITH PASSENGERS WHO WERE WORRIED ABOUT CONNECTING FLTS AT ORD BECAUSE WE WERE BEHIND SCHEDULE. I WAS FLYING THE ACFT, BUT WAS INTERESTED IN HOW THE CAPT WOULD HANDLE THIS SITUATION SINCE I'M GETTING CLOSE TO CAPT UPGRADE. WHILE WE WERE DISCUSSING THIS MATTER, ORD ARTCC GAVE US A DSCNT CLRNC TO FL290. THE CAPT ACKNOWLEDGED ON THE RADIO AND I SET AND ARMED 290 IN THE DIGITAL ALT WINDOW. I THEN BEGAN A SLOW (1000 FPM) DSCNT USING THE VERT SPD MODE OF THE AUTOPLT. THE CONVERSATION CONTINUED. PASSING THROUGH FL300, THE CAPT STARTED TO SAY, '1000 FT TO GO', BUT HE NOTICED 280 WAS SET IN THE ALT WINDOW AND THAT IT WAS NOT ARMED. SO HE SAID, '2000 FT TO GO, BUT THE ALT IS NOT ARMED.' I SIMPLY REACHED UP AND ARMED THE ALT WITH 280 SET IN THE WINDOW. MORE CONVERSATION ABOUT THE PASSENGER CONNECTIONS. 'PASSING 290 FOR 280 AND IT'S ARMED' WAS INTERJECTED BY THE CAPT. I WAS SLOWLY CHANGING THE VERT SPD 100 FT AT A TIME FOR A SMOOTH LEVEL OFF, AND PASSING FL284 WAS DSNDING AT 300 FPM. SUDDENLY, IT STRUCK ME THAT FL280 WAS THE WRONG ALT FOR OUR DIRECTION OF FLT. I WAS TRYING TO TRACE THE SEQUENCE OF EVENTS BACKWARD IN MY MIND AND STARTED TO LEVEL OFF THINKING, 'WE SHOULD BE AT FL290', WHEN ATC CALLED AND ASKED OUR ALT. THE CAPT SAID, 'PASSING 283 FOR 280.' ATC REPLIED, 'YOUR ASSIGNED ALT WAS 290, HOWEVER IT'S NOT A PROBLEM AND YOU CAN CONTINUE YOUR DSCNT TO FL270.' THE CAPT ACKNOWLEDGED, 'DSNDING TO 270.' WHEN GIVEN A FREQ CHANGE BY THIS CTLR, THE CAPT ASKED IF THERE WAS ANY PROBLEM CREATED BY OUR ALT DEV. THIS CTLR SAID, 'NO PROBLEM.' AFTER REPLAYING THE ABOVE SCENARIO NUMEROUS TIMES, I'M SURE I NOW KNOW EXACTLY WHAT HAPPENED. I HAD SET AND ARMED FL290 IN THE ALT WINDOW. I WAS TURNED TO MY L WITH MY L ARM BEHIND MY SEAT BACK SO I COULD SEE THE FLT ATTENDANT AS SHE TALKED. I WAS USING HDG SELECT ON THE AUTOPLT TO FLY THE VOR RADIAL INBOUND. 2 OR 3 TIMES DURING THIS DSCNT, I REMEMBER CHANGING HDG A COUPLE DEGS TO MAKE MINOR COURSE CORRECTIONS. ON AT LEAST 1 OCCASION, I REACHED FOR THE HDG KNOB WITH MY R HAND AND I CAN RECALL VERY LIGHTLY BRUSHING SOMETHING AS I RETRACTED IT FROM THE HDG KNOB. I'M SURE THAT I MUST HAVE JUST TICKED THE ALT KNOB AND THAT CAUSED IT TO CHANGE FROM 290 TO 280 AND DISARM. I HAVE BEEN A COCKPIT CREW MEMBER WITH SCHEDULED PASSENGER CARRIERS FOR OVER 17 YRS NOW. PRIOR TO 1991, I HAD FILED ONLY 1 NASA RPT (ON A NEAR MISS). DURING 1991, THIS IS NOW MY THIRD NASA RPT. THESE LAST 3 RPTS HAVE ALL BEEN ON THE MLG AND ALL PERTAIN TO ALT DEVS. I HAVE HEARD THAT THIS MLG IS BY FAR THE WORST OFFENDER CONCERNING ALT EXCURSIONS. BY NO STRETCH OF THE IMAGINATION CAN THIS ALT ALERT/AUTOPLT LEVEL OFF SYS BE CONSIDERED FOOL PROOF. IN FACT, IT IS A VERY MIXED UP BAG OF TRICKS WAITING TO BITE THE UNWARY. IT NEEDS TO BE FIXED, NOW! I CAN ONLY HOPE IT DOESN'T TAKE A MIDAIR COLLISION AND HUNDREDS OFF FATALITIES TO GET A CORRECTION FOR THIS SYS.

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.