Narrative:

We were inbound to phl on the bunts one arrival. Due to ATC congestion for arrs into phl; we were instructed to enter holding at bunts intersection at 10000 ft MSL. I was flying at the time; and the first officer programmed the FMC to enter holding. Meanwhile; I sent dispatch an ACARS message advising them that we entered holding at bunts. We entered holding with the autoplt/LNAV engaged and completed the first turn uneventfully. After we turned outbound; I gave control of the aircraft to the first officer so I could better manage the aircraft (brief passenger on status; communicate with dispatch; and monitor our fuel). As we approached bunts for the second time; ATC cleared us to 9000 ft. At this time; the first officer went to the cruise page of the FMC and entered 9000 ft. We then got an FMC message stating; 'unable hold airspace.' since we had already completed one turn; we chose to clear the message and execute the altitude change. The problem occurred on the next outbound leg of the holding pattern where the commanded bank angle was only 10 degrees left bank. This seemed odd because we had overshooting winds (210 degrees/20 KTS approximately) and a steeper bank angle was required to remain within the holding airspace. I then crosschecked the navigation display and we were right on our published course (solid magenta line). I was fairly certain that altitude hold and LNAV were selected on the MCP. I thought about manually overriding the LNAV mode by going to either heading select or cws. Then I remembered other times where I felt LNAV holding was in error; but when crosschecked with navaids; we were right in position. About 30 seconds after we completed the outbound turn; approach control saw that we had deviated outside holding airspace. They immediately turned us to a heading of 230 degrees to reenter holding airspace. I then went to the 'legs' page of the FMC and saw that we were approximately 8 mi from bunts (the published holding pattern depicts 4 mi legs). Once we were back in holding airspace; approach then cleared us direct to bunts and to depart bunts on an easterly heading. We continued for the ILS to runway 27R and landed uneventfully. As of this report; I do not know why we deviated outside the published holding pattern in the LNAV mode. For future reference; I will better monitor the FMC and pay more attention to its messages. Second; after I returned home; I reviewed the holding section in LNAV study guide from about 3 yrs ago and could not find our error. As of this date; I've been in contact with the training department to further investigate our lateral deviation.

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Original NASA ASRS Text

Title: A B737-700 FLT CREW DEVIATED FROM HOLDING PATTERN AIRSPACE AFTER THE FMC WARNED 'UNABLE HOLD AIRSPACE.'

Narrative: WE WERE INBOUND TO PHL ON THE BUNTS ONE ARR. DUE TO ATC CONGESTION FOR ARRS INTO PHL; WE WERE INSTRUCTED TO ENTER HOLDING AT BUNTS INTXN AT 10000 FT MSL. I WAS FLYING AT THE TIME; AND THE FO PROGRAMMED THE FMC TO ENTER HOLDING. MEANWHILE; I SENT DISPATCH AN ACARS MESSAGE ADVISING THEM THAT WE ENTERED HOLDING AT BUNTS. WE ENTERED HOLDING WITH THE AUTOPLT/LNAV ENGAGED AND COMPLETED THE FIRST TURN UNEVENTFULLY. AFTER WE TURNED OUTBOUND; I GAVE CTL OF THE ACFT TO THE FO SO I COULD BETTER MANAGE THE ACFT (BRIEF PAX ON STATUS; COMMUNICATE WITH DISPATCH; AND MONITOR OUR FUEL). AS WE APCHED BUNTS FOR THE SECOND TIME; ATC CLRED US TO 9000 FT. AT THIS TIME; THE FO WENT TO THE CRUISE PAGE OF THE FMC AND ENTERED 9000 FT. WE THEN GOT AN FMC MESSAGE STATING; 'UNABLE HOLD AIRSPACE.' SINCE WE HAD ALREADY COMPLETED ONE TURN; WE CHOSE TO CLR THE MESSAGE AND EXECUTE THE ALT CHANGE. THE PROB OCCURRED ON THE NEXT OUTBOUND LEG OF THE HOLDING PATTERN WHERE THE COMMANDED BANK ANGLE WAS ONLY 10 DEGS L BANK. THIS SEEMED ODD BECAUSE WE HAD OVERSHOOTING WINDS (210 DEGS/20 KTS APPROX) AND A STEEPER BANK ANGLE WAS REQUIRED TO REMAIN WITHIN THE HOLDING AIRSPACE. I THEN XCHKED THE NAV DISPLAY AND WE WERE RIGHT ON OUR PUBLISHED COURSE (SOLID MAGENTA LINE). I WAS FAIRLY CERTAIN THAT ALT HOLD AND LNAV WERE SELECTED ON THE MCP. I THOUGHT ABOUT MANUALLY OVERRIDING THE LNAV MODE BY GOING TO EITHER HDG SELECT OR CWS. THEN I REMEMBERED OTHER TIMES WHERE I FELT LNAV HOLDING WAS IN ERROR; BUT WHEN XCHKED WITH NAVAIDS; WE WERE RIGHT IN POS. ABOUT 30 SECONDS AFTER WE COMPLETED THE OUTBOUND TURN; APCH CTL SAW THAT WE HAD DEVIATED OUTSIDE HOLDING AIRSPACE. THEY IMMEDIATELY TURNED US TO A HDG OF 230 DEGS TO REENTER HOLDING AIRSPACE. I THEN WENT TO THE 'LEGS' PAGE OF THE FMC AND SAW THAT WE WERE APPROX 8 MI FROM BUNTS (THE PUBLISHED HOLDING PATTERN DEPICTS 4 MI LEGS). ONCE WE WERE BACK IN HOLDING AIRSPACE; APCH THEN CLRED US DIRECT TO BUNTS AND TO DEPART BUNTS ON AN EASTERLY HDG. WE CONTINUED FOR THE ILS TO RWY 27R AND LANDED UNEVENTFULLY. AS OF THIS RPT; I DO NOT KNOW WHY WE DEVIATED OUTSIDE THE PUBLISHED HOLDING PATTERN IN THE LNAV MODE. FOR FUTURE REF; I WILL BETTER MONITOR THE FMC AND PAY MORE ATTN TO ITS MESSAGES. SECOND; AFTER I RETURNED HOME; I REVIEWED THE HOLDING SECTION IN LNAV STUDY GUIDE FROM ABOUT 3 YRS AGO AND COULD NOT FIND OUR ERROR. AS OF THIS DATE; I'VE BEEN IN CONTACT WITH THE TRAINING DEPT TO FURTHER INVESTIGATE OUR LATERAL DEV.

Data retrieved from NASA's ASRS site as of January 2009 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.