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.