Narrative:

Air carrier X had just leveled off at an intermediate altitude of FL270, approximately 15 to 20 mi east of elx VOR, when chicago center called to inform us that our cleared altitude was FL260 and not FL270. We were then instructed to descend back to FL260. Almost immediately after leveling off, we were reclred to continue our climb to our final cruise altitude of FL310. During the climb, chicago center informed us that we were earlier involved in a possible altitude conflict with another aircraft cruising at FL270. After landing at bdl, we immediately called our company. It was determined from the tapes that I (the PNF) had read back FL260. We had indeed made the mistake, somehow, of setting the wrong altitude. At the time of the incident, both the first officer and myself were certain that someone else had made the mistake. When we were told of the results of the tape reading, we were both stunned. Our cockpit discipline was good and our procedures were standard, but I had accepted FL260 as the clearance and set FL270 in the altitude preselect window west/O either of us catching the mistake. Fortunately, the ATC system worked as designed and a much more serious consequence was avoided. We allegedly missed mlt Y by 400' and 1/2 mi. After much soul searching and discussion with other medium large transport pilots, I believe I know what happened. Relying on the altitude preselect readout as a memory tool can be a mistake that leads to this type of error. Dialing in frequency, heading, altitude, and altimeter changes while simultaneously trying to read them back, can cause a pilot to rush things too much. The medium large transport altitude preselect knob must be pulled to arm the autoplt/flight director capture mode. While pulling on the knob, if one drags a finger or twists it just slightly, the wrong altitude can be inadvertently set. If this window readout is then used as the only recording of a clearance, it is unlikely that either pilot will catch the mistake. Several other pilots have had this happen but caught it. We didn't! It's no excuse, but I believe this to be a reason which could have led to this mishap. Supplemental information from acn 148396. Just after level off at FL270, the center queried us as to our altitude. They had in fact cleared us to FL260. Center turned us and gave us descent to FL260 but not before a conflict occurred with another mlt Y. I was concentrating on flying and probably not as attentive to the radio as I could have been. Had I been on autoplt, I may have heard the climb clearance and prevented the incident. The altitude preselect on the fls, once set, becomes the gospel for the assigned altitude. The preselect knob must be pulled to arm the altitude. In so doing, it is very easy to turn the knob enough to cause a 1000' error in the altitude readout. I believe this is what happened today. Supplemental information from acn 148401. Air carrier X departed ord requesting 330 checked on frequency and was cleared to FL260 for his traffic. Mlt Y, at FL270. Air carrier X read back climbing to FL260 and proceeded to climb through FL260 to FL270. Upon noticing this, air carrier X was asked if he was level at FL260. Air carrier X responded climbing to FL260. Air carrier X was descended to FL260 and placed on a heading. Pilots paying more attention to the frequency could have prevented this.

Related reports:

Google
 

Original NASA ASRS Text

Title: ACR X CLIMBED TO OCCUPIED ALT AND HAD LESS THAN STANDARD SEPARATION FROM MLT Y. PLTDEV. SITUATION. EQUIPMENT DESIGN ALT PRE-SELECT CAN CHANGE WHEN PULLED TO ARM.

Narrative: ACR X HAD JUST LEVELED OFF AT AN INTERMEDIATE ALT OF FL270, APPROX 15 TO 20 MI E OF ELX VOR, WHEN CHICAGO CTR CALLED TO INFORM US THAT OUR CLRED ALT WAS FL260 AND NOT FL270. WE WERE THEN INSTRUCTED TO DSND BACK TO FL260. ALMOST IMMEDIATELY AFTER LEVELING OFF, WE WERE RECLRED TO CONTINUE OUR CLB TO OUR FINAL CRUISE ALT OF FL310. DURING THE CLB, CHICAGO CTR INFORMED US THAT WE WERE EARLIER INVOLVED IN A POSSIBLE ALT CONFLICT WITH ANOTHER ACFT CRUISING AT FL270. AFTER LNDG AT BDL, WE IMMEDIATELY CALLED OUR COMPANY. IT WAS DETERMINED FROM THE TAPES THAT I (THE PNF) HAD READ BACK FL260. WE HAD INDEED MADE THE MISTAKE, SOMEHOW, OF SETTING THE WRONG ALT. AT THE TIME OF THE INCIDENT, BOTH THE F/O AND MYSELF WERE CERTAIN THAT SOMEONE ELSE HAD MADE THE MISTAKE. WHEN WE WERE TOLD OF THE RESULTS OF THE TAPE READING, WE WERE BOTH STUNNED. OUR COCKPIT DISCIPLINE WAS GOOD AND OUR PROCS WERE STANDARD, BUT I HAD ACCEPTED FL260 AS THE CLRNC AND SET FL270 IN THE ALT PRESELECT WINDOW W/O EITHER OF US CATCHING THE MISTAKE. FORTUNATELY, THE ATC SYS WORKED AS DESIGNED AND A MUCH MORE SERIOUS CONSEQUENCE WAS AVOIDED. WE ALLEGEDLY MISSED MLT Y BY 400' AND 1/2 MI. AFTER MUCH SOUL SEARCHING AND DISCUSSION WITH OTHER MLG PLTS, I BELIEVE I KNOW WHAT HAPPENED. RELYING ON THE ALT PRESELECT READOUT AS A MEMORY TOOL CAN BE A MISTAKE THAT LEADS TO THIS TYPE OF ERROR. DIALING IN FREQ, HDG, ALT, AND ALTIMETER CHANGES WHILE SIMULTANEOUSLY TRYING TO READ THEM BACK, CAN CAUSE A PLT TO RUSH THINGS TOO MUCH. THE MLG ALT PRESELECT KNOB MUST BE PULLED TO ARM THE AUTOPLT/FLT DIRECTOR CAPTURE MODE. WHILE PULLING ON THE KNOB, IF ONE DRAGS A FINGER OR TWISTS IT JUST SLIGHTLY, THE WRONG ALT CAN BE INADVERTENTLY SET. IF THIS WINDOW READOUT IS THEN USED AS THE ONLY RECORDING OF A CLRNC, IT IS UNLIKELY THAT EITHER PLT WILL CATCH THE MISTAKE. SEVERAL OTHER PLTS HAVE HAD THIS HAPPEN BUT CAUGHT IT. WE DIDN'T! IT'S NO EXCUSE, BUT I BELIEVE THIS TO BE A REASON WHICH COULD HAVE LED TO THIS MISHAP. SUPPLEMENTAL INFO FROM ACN 148396. JUST AFTER LEVEL OFF AT FL270, THE CTR QUERIED US AS TO OUR ALT. THEY HAD IN FACT CLRED US TO FL260. CTR TURNED US AND GAVE US DSNT TO FL260 BUT NOT BEFORE A CONFLICT OCCURRED WITH ANOTHER MLT Y. I WAS CONCENTRATING ON FLYING AND PROBABLY NOT AS ATTENTIVE TO THE RADIO AS I COULD HAVE BEEN. HAD I BEEN ON AUTOPLT, I MAY HAVE HEARD THE CLB CLRNC AND PREVENTED THE INCIDENT. THE ALT PRESELECT ON THE FLS, ONCE SET, BECOMES THE GOSPEL FOR THE ASSIGNED ALT. THE PRESELECT KNOB MUST BE PULLED TO ARM THE ALT. IN SO DOING, IT IS VERY EASY TO TURN THE KNOB ENOUGH TO CAUSE A 1000' ERROR IN THE ALT READOUT. I BELIEVE THIS IS WHAT HAPPENED TODAY. SUPPLEMENTAL INFO FROM ACN 148401. ACR X DEPARTED ORD REQUESTING 330 CHKED ON FREQ AND WAS CLRED TO FL260 FOR HIS TFC. MLT Y, AT FL270. ACR X READ BACK CLBING TO FL260 AND PROCEEDED TO CLB THROUGH FL260 TO FL270. UPON NOTICING THIS, ACR X WAS ASKED IF HE WAS LEVEL AT FL260. ACR X RESPONDED CLBING TO FL260. ACR X WAS DSNDED TO FL260 AND PLACED ON A HDG. PLTS PAYING MORE ATTN TO THE FREQ COULD HAVE PREVENTED THIS.

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.