Narrative:

I was the captain and the PNF. The first officer was the PF. At FL390 we were advised by ZLA that there was turbulence ahead at FL390 and to expect descent to FL310. We requested descent to FL310 at that time. After the leveloff at FL310, I had just keyed my microphone to make a PA to have the flight attendants take their seats when ZLA told us to slow to 250 KTS and then descend to FL280. Since I had not yet said anything on the PA, I switched my communication selector switch back to the communication #1 radio and acknowledged the instructions from ZLA. The first officer entered FL280 in the altitude window, we both pointed at the '280' in the altitude window, and he started a descent at 310 KTS. I reminded the first officer that ZLA wanted 250 KTS before he started the descent. The first officer entered 250 in the airspeed window and restarted his descent. At that point I told the first officer I was 'off the air' and he acknowledged. I turned my communication selector switch to PA and made an announcement. I told the passenger that the seat belt sign was on, to please remain seated for the remainder of the flight, and then (still on the PA) asked the flight attendants to complete their cabin check and then take their seats. I then called the senior flight attendant to make sure she heard and understood my PA announcement. As I was clarifying this information with her, I was aware that the first officer was talking to ZLA. The senior flight attendant asked if I wanted the flight attendants to situation right now or to finish the cabin check and then situation down. As I was in the process of telling her I wanted her to do her cabin check first then situation down and then stay down, the first officer disconnected the autoplt and started a pull-up. I immediately terminated my conversation with the flight attendant, reselected my communication control switch to the #1 communication radio, and asked the first officer what happened. He said that ZLA wanted us at FL280. I checked our altitude and saw that we were between FL270 and FL280. The number '250' was displayed in both the altitude window and airspeed window. Since I was not sure what conversation had taken place between ZLA and the first officer, I asked him if we were cleared to FL280 or FL250. He said he thought we had been cleared to FL250. It was still unclr if he had received new instructions from ZLA while I was on the PA/intercom. ZLA could well have given him a clearance to FL250 and later, as he passed through FL280, asked him to stop the descent at FL280. Thus, it would make sense why he had disconnected the autoplt just below FL280 and was now climbing back up to FL280. But I was very uncomfortable with this. To resolve this conflict in my own mind, I asked ZLA if we were originally cleared to FL280 or FL250. ZLA responded that we were cleared to FL280. I then realized that I had not asked the right question. Of course we had originally been cleared to FL280. The question I should have asked was, were we ever cleared to FL250. But by this time it was academic. I asked ZLA if there had been a conflict and they said there had not been a conflict. Before there was time to properly resolve this nagging question, ZLA gave us a frequency change. I thanked ZLA and changed frequencys. I still was not quite sure what happened, so I asked my first officer if we were ever cleared to FL250. He said he thought so. By this time we were encountering turbulence, had changed frequencys, and were once again in a descent. We needed to get on with the approach so I let the matter drop for the time being. After we landed and got to the gate, I asked my first officer if we had ever been cleared to FL250. He told me he thought so but was not sure. He said that maybe he got confused when I told him ZLA wanted 250 KTS. I told my first officer that I thought we should both file a NASA report. He agreed. What I am sure of is that before I turned my communication switch to PA, the first officer had set and I had confirmed by physically touching the flight control panel that the number '250' was set in the airspeed window and the number '280' was set in the altitude window. Later, when the first officer disconnected the autoplt, which immediately got me back in the loop, we were between FL270 and FL280 and the number '250' was set in both the airspeed window and the altitude window. I can see 2POSSIBLE scenarios as to what may have happened. The first scenario is that ZLA cleared us to FL250 while I was on PA/interphone and then asked us to level at FL280. If this is the case there was no altitude bust and my concern is unfounded. But what I fear happened is that the first officer remembered me telling him he had to slow to 250 KTS before he started the descent. The number '250' was running around in his head and the more he thought about it, the more he thought we were cleared to FL250, so he put the number '250' in the altitude window. Hopefully, I will never have to find out which of these scenarios was correct.

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Original NASA ASRS Text

Title: B757 FLC DSNDED BELOW CLRED ALT ON CIVET ARR INTO LAX.

Narrative: I WAS THE CAPT AND THE PNF. THE FO WAS THE PF. AT FL390 WE WERE ADVISED BY ZLA THAT THERE WAS TURB AHEAD AT FL390 AND TO EXPECT DSCNT TO FL310. WE REQUESTED DSCNT TO FL310 AT THAT TIME. AFTER THE LEVELOFF AT FL310, I HAD JUST KEYED MY MIKE TO MAKE A PA TO HAVE THE FLT ATTENDANTS TAKE THEIR SEATS WHEN ZLA TOLD US TO SLOW TO 250 KTS AND THEN DSND TO FL280. SINCE I HAD NOT YET SAID ANYTHING ON THE PA, I SWITCHED MY COM SELECTOR SWITCH BACK TO THE COM #1 RADIO AND ACKNOWLEDGED THE INSTRUCTIONS FROM ZLA. THE FO ENTERED FL280 IN THE ALT WINDOW, WE BOTH POINTED AT THE '280' IN THE ALT WINDOW, AND HE STARTED A DSCNT AT 310 KTS. I REMINDED THE FO THAT ZLA WANTED 250 KTS BEFORE HE STARTED THE DSCNT. THE FO ENTERED 250 IN THE AIRSPD WINDOW AND RESTARTED HIS DSCNT. AT THAT POINT I TOLD THE FO I WAS 'OFF THE AIR' AND HE ACKNOWLEDGED. I TURNED MY COM SELECTOR SWITCH TO PA AND MADE AN ANNOUNCEMENT. I TOLD THE PAX THAT THE SEAT BELT SIGN WAS ON, TO PLEASE REMAIN SEATED FOR THE REMAINDER OF THE FLT, AND THEN (STILL ON THE PA) ASKED THE FLT ATTENDANTS TO COMPLETE THEIR CABIN CHK AND THEN TAKE THEIR SEATS. I THEN CALLED THE SENIOR FLT ATTENDANT TO MAKE SURE SHE HEARD AND UNDERSTOOD MY PA ANNOUNCEMENT. AS I WAS CLARIFYING THIS INFO WITH HER, I WAS AWARE THAT THE FO WAS TALKING TO ZLA. THE SENIOR FLT ATTENDANT ASKED IF I WANTED THE FLT ATTENDANTS TO SIT RIGHT NOW OR TO FINISH THE CABIN CHK AND THEN SIT DOWN. AS I WAS IN THE PROCESS OF TELLING HER I WANTED HER TO DO HER CABIN CHK FIRST THEN SIT DOWN AND THEN STAY DOWN, THE FO DISCONNECTED THE AUTOPLT AND STARTED A PULL-UP. I IMMEDIATELY TERMINATED MY CONVERSATION WITH THE FLT ATTENDANT, RESELECTED MY COM CTL SWITCH TO THE #1 COM RADIO, AND ASKED THE FO WHAT HAPPENED. HE SAID THAT ZLA WANTED US AT FL280. I CHKED OUR ALT AND SAW THAT WE WERE BTWN FL270 AND FL280. THE NUMBER '250' WAS DISPLAYED IN BOTH THE ALT WINDOW AND AIRSPD WINDOW. SINCE I WAS NOT SURE WHAT CONVERSATION HAD TAKEN PLACE BTWN ZLA AND THE FO, I ASKED HIM IF WE WERE CLRED TO FL280 OR FL250. HE SAID HE THOUGHT WE HAD BEEN CLRED TO FL250. IT WAS STILL UNCLR IF HE HAD RECEIVED NEW INSTRUCTIONS FROM ZLA WHILE I WAS ON THE PA/INTERCOM. ZLA COULD WELL HAVE GIVEN HIM A CLRNC TO FL250 AND LATER, AS HE PASSED THROUGH FL280, ASKED HIM TO STOP THE DSCNT AT FL280. THUS, IT WOULD MAKE SENSE WHY HE HAD DISCONNECTED THE AUTOPLT JUST BELOW FL280 AND WAS NOW CLBING BACK UP TO FL280. BUT I WAS VERY UNCOMFORTABLE WITH THIS. TO RESOLVE THIS CONFLICT IN MY OWN MIND, I ASKED ZLA IF WE WERE ORIGINALLY CLRED TO FL280 OR FL250. ZLA RESPONDED THAT WE WERE CLRED TO FL280. I THEN REALIZED THAT I HAD NOT ASKED THE RIGHT QUESTION. OF COURSE WE HAD ORIGINALLY BEEN CLRED TO FL280. THE QUESTION I SHOULD HAVE ASKED WAS, WERE WE EVER CLRED TO FL250. BUT BY THIS TIME IT WAS ACADEMIC. I ASKED ZLA IF THERE HAD BEEN A CONFLICT AND THEY SAID THERE HAD NOT BEEN A CONFLICT. BEFORE THERE WAS TIME TO PROPERLY RESOLVE THIS NAGGING QUESTION, ZLA GAVE US A FREQ CHANGE. I THANKED ZLA AND CHANGED FREQS. I STILL WAS NOT QUITE SURE WHAT HAPPENED, SO I ASKED MY FO IF WE WERE EVER CLRED TO FL250. HE SAID HE THOUGHT SO. BY THIS TIME WE WERE ENCOUNTERING TURB, HAD CHANGED FREQS, AND WERE ONCE AGAIN IN A DSCNT. WE NEEDED TO GET ON WITH THE APCH SO I LET THE MATTER DROP FOR THE TIME BEING. AFTER WE LANDED AND GOT TO THE GATE, I ASKED MY FO IF WE HAD EVER BEEN CLRED TO FL250. HE TOLD ME HE THOUGHT SO BUT WAS NOT SURE. HE SAID THAT MAYBE HE GOT CONFUSED WHEN I TOLD HIM ZLA WANTED 250 KTS. I TOLD MY FO THAT I THOUGHT WE SHOULD BOTH FILE A NASA RPT. HE AGREED. WHAT I AM SURE OF IS THAT BEFORE I TURNED MY COM SWITCH TO PA, THE FO HAD SET AND I HAD CONFIRMED BY PHYSICALLY TOUCHING THE FLT CTL PANEL THAT THE NUMBER '250' WAS SET IN THE AIRSPD WINDOW AND THE NUMBER '280' WAS SET IN THE ALT WINDOW. LATER, WHEN THE FO DISCONNECTED THE AUTOPLT, WHICH IMMEDIATELY GOT ME BACK IN THE LOOP, WE WERE BTWN FL270 AND FL280 AND THE NUMBER '250' WAS SET IN BOTH THE AIRSPD WINDOW AND THE ALT WINDOW. I CAN SEE 2POSSIBLE SCENARIOS AS TO WHAT MAY HAVE HAPPENED. THE FIRST SCENARIO IS THAT ZLA CLRED US TO FL250 WHILE I WAS ON PA/INTERPHONE AND THEN ASKED US TO LEVEL AT FL280. IF THIS IS THE CASE THERE WAS NO ALT BUST AND MY CONCERN IS UNFOUNDED. BUT WHAT I FEAR HAPPENED IS THAT THE FO REMEMBERED ME TELLING HIM HE HAD TO SLOW TO 250 KTS BEFORE HE STARTED THE DSCNT. THE NUMBER '250' WAS RUNNING AROUND IN HIS HEAD AND THE MORE HE THOUGHT ABOUT IT, THE MORE HE THOUGHT WE WERE CLRED TO FL250, SO HE PUT THE NUMBER '250' IN THE ALT WINDOW. HOPEFULLY, I WILL NEVER HAVE TO FIND OUT WHICH OF THESE SCENARIOS WAS CORRECT.

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.