Narrative:

This incident began just prior to descent into lax area on a nonstop lft from bos. We were cleared pgs J-128 to lax and shortly after passing over the colorado river were cleared to descended to FL240 with pilot's discretion out of FL310. Descent was begun and then stopped at FL310. Shortly thereafter the controller cleared us to FL220. Since we had previously been cleared pilot's discretion and since this was an unusually low altitude for our position, I asked the controller for confirmation that the clearance was also pilot's discretion. The controller's reply was rushed and unintelligible, so I asked a second time if clearance was pilot's discretion to make certain there was no misunderstanding. His reply was simply, 'yes.' discussion with the other 2 crew members in the cockpit confirmed an agreement on what each of us heard the controller say and that we were in fact cleared to FL220 pilot's discretion. We remained at FL310. 2 or 3 mins later the controller called us, his voice tinged with panic, saying: 'you are still at 310, I cleared you 10 mi back to descended to 220, descended immediately to 220 and slow to 250!!!' I acknowledged the clearance and began descent from FL310. It is common in this area to be vectored, slowed, asked to speed up, etc. The vol of traffic into the lax approach system is apparently just too much for the system to handle on the handoff from the center to approach control in lax. The controller in this time frame was very busy and the tone of his voice indicated he was overloaded and barely able to handle the vol of traffic he had. Communication, when either the controller or pilot is under unusual stress, tends to lose its formality and therefore as in this instance, clarity is lost. Although I asked twice for a clarification of the clearance and all 3 cockpit crewmembers understood that we had it, I should have insisted on a full clearance readback from the controller. I believe the root cause of the incident is the overloaded handoff system into lax coupled with the giving and accepting of less than formal working in a clearance during a stressful time. Callback conversation with reporter revealed the following: reporter says that one center controller gave the first descent clearance with pilot's discretion out of FL310, and then they were changed to another center controller frequency. This second ZLA controller must have noted conflicting traffic at FL310 and tried to get reporter to descended to FL220. Reporter questioned controller because they were too far out to go that low. All flight crew members thought controller said ok for pilot's discretion descent. Controller was extremely busy and they thought he had more traffic than he could handle. Realized later that controller wanted them to descended immediately. I counseled reporter that he had levelled off improperly at FL310 when he had been told, 'pilot discretion out of FL310.' explained to him that he should have continued descent to next lower altitude (FL290) before applying pilot's discretion.

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Original NASA ASRS Text

Title: ACR FLT CREW LEVELED OFF AT INCORRECT ALT. SET UP POTENTIAL CONFLICT. REPORTER STATES ATC FAC HAS MORE TRAFFIC THAN IT CAN SAFELY HANDLE.

Narrative: THIS INCIDENT BEGAN JUST PRIOR TO DSCNT INTO LAX AREA ON A NONSTOP LFT FROM BOS. WE WERE CLRED PGS J-128 TO LAX AND SHORTLY AFTER PASSING OVER THE COLORADO RIVER WERE CLRED TO DESCENDED TO FL240 WITH PLT'S DISCRETION OUT OF FL310. DSCNT WAS BEGUN AND THEN STOPPED AT FL310. SHORTLY THEREAFTER THE CTLR CLRED US TO FL220. SINCE WE HAD PREVIOUSLY BEEN CLRED PLT'S DISCRETION AND SINCE THIS WAS AN UNUSUALLY LOW ALT FOR OUR POS, I ASKED THE CTLR FOR CONFIRMATION THAT THE CLRNC WAS ALSO PLT'S DISCRETION. THE CTLR'S REPLY WAS RUSHED AND UNINTELLIGIBLE, SO I ASKED A SECOND TIME IF CLRNC WAS PLT'S DISCRETION TO MAKE CERTAIN THERE WAS NO MISUNDERSTANDING. HIS REPLY WAS SIMPLY, 'YES.' DISCUSSION WITH THE OTHER 2 CREW MEMBERS IN THE COCKPIT CONFIRMED AN AGREEMENT ON WHAT EACH OF US HEARD THE CTLR SAY AND THAT WE WERE IN FACT CLRED TO FL220 PLT'S DISCRETION. WE REMAINED AT FL310. 2 OR 3 MINS LATER THE CTLR CALLED US, HIS VOICE TINGED WITH PANIC, SAYING: 'YOU ARE STILL AT 310, I CLRED YOU 10 MI BACK TO DESCENDED TO 220, DESCENDED IMMEDIATELY TO 220 AND SLOW TO 250!!!' I ACKNOWLEDGED THE CLRNC AND BEGAN DSCNT FROM FL310. IT IS COMMON IN THIS AREA TO BE VECTORED, SLOWED, ASKED TO SPD UP, ETC. THE VOL OF TFC INTO THE LAX APCH SYS IS APPARENTLY JUST TOO MUCH FOR THE SYS TO HANDLE ON THE HDOF FROM THE CENTER TO APCH CTL IN LAX. THE CTLR IN THIS TIME FRAME WAS VERY BUSY AND THE TONE OF HIS VOICE INDICATED HE WAS OVERLOADED AND BARELY ABLE TO HANDLE THE VOL OF TFC HE HAD. COM, WHEN EITHER THE CTLR OR PLT IS UNDER UNUSUAL STRESS, TENDS TO LOSE ITS FORMALITY AND THEREFORE AS IN THIS INSTANCE, CLARITY IS LOST. ALTHOUGH I ASKED TWICE FOR A CLARIFICATION OF THE CLRNC AND ALL 3 COCKPIT CREWMEMBERS UNDERSTOOD THAT WE HAD IT, I SHOULD HAVE INSISTED ON A FULL CLRNC READBACK FROM THE CTLR. I BELIEVE THE ROOT CAUSE OF THE INCIDENT IS THE OVERLOADED HDOF SYS INTO LAX COUPLED WITH THE GIVING AND ACCEPTING OF LESS THAN FORMAL WORKING IN A CLRNC DURING A STRESSFUL TIME. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR SAYS THAT ONE CENTER CTLR GAVE THE FIRST DSCNT CLRNC WITH PLT'S DISCRETION OUT OF FL310, AND THEN THEY WERE CHANGED TO ANOTHER CENTER CTLR FREQ. THIS SECOND ZLA CTLR MUST HAVE NOTED CONFLICTING TFC AT FL310 AND TRIED TO GET RPTR TO DESCENDED TO FL220. RPTR QUESTIONED CTLR BECAUSE THEY WERE TOO FAR OUT TO GO THAT LOW. ALL FLT CREW MEMBERS THOUGHT CTLR SAID OK FOR PLT'S DISCRETION DSCNT. CTLR WAS EXTREMELY BUSY AND THEY THOUGHT HE HAD MORE TFC THAN HE COULD HANDLE. REALIZED LATER THAT CTLR WANTED THEM TO DESCENDED IMMEDIATELY. I COUNSELED RPTR THAT HE HAD LEVELLED OFF IMPROPERLY AT FL310 WHEN HE HAD BEEN TOLD, 'PLT DISCRETION OUT OF FL310.' EXPLAINED TO HIM THAT HE SHOULD HAVE CONTINUED DSCNT TO NEXT LOWER ALT (FL290) BEFORE APPLYING PLT'S DISCRETION.

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.