Narrative:

We were cleared direct to subic bay VOR, expecting the VOR approach. The clearance came at a point between sarsi and cabin attendant VOR. Although we were in radar contact and radar idented, this descent would be during night IMC conditions into mountainous terrain. Additionally, subic bay is notamed for possible map shifts, therefore I took special care to brief for the descent and approach. After checking our direct course to the VOR, I 'hardwired' the outbound course on the first officer's side and the inbound course on my VOR in the FMS. Both of us were selected to map for the arrival and monitoring the VOR course on each of our maps and the VOR course and DME readouts at the bottom of each of our nd's. At 40 DME we were cleared to descend to 8000 ft and asked to report sba 10 DME. The autoplt was flying the descent on schedule and we arrived at 10 DME at 245 KTS. At about 8500 ft the controller then cleared us to descend 6000 ft, to fly the VOR runway 7 approach and to report procedure turn inbound. We were descending at about 2000 FPM when between 7000 ft and 7500 ft we heard a conflict alert warning system (CAWS) warning of '1000 ft.' I immediately checked my map with the current barometer altitude and it showed a good safe altitude. I then checked for any RA readouts and saw none. Since I experienced false warnings of this same type at cruise altitudes, I assumed the 1000 ft call was a false one and told the first officer my reasoning. During our discussions, the CAWS called out '500 ft.' I took the call as real and felt ground impact was imminent since I could not understand both calls being false and I could not take the chance that it wasn't. We both pushed the throttles to firewall power through the fadec bar and rotated the aircraft past 15 degrees and I called 'spoilers are up.' I was looking at the altitude and pitch attitude waiting for stick shaker. From what the instruments indicated, the aircraft climbed and then descended with the right wing rolling over to one side more than 45 degrees. I put full left aileron in and we got our second '500 ft' call. Again I felt ground impact was imminent, but the aircraft wasn't climbing so I put the nose down and the first officer put 28 degree flaps out and the aircraft responded well and climbed out. We leveled at 9500 ft and found ourselves outbound almost over the sba VOR. We reported turning outbound, received clearance to land and landed uneventfully. On the ground we called subic approach and they confirmed we were on the direct track to the VOR, which means there were no mountains that we passed over when we got the 1000 ft and 500 ft calls. On the next leg to sin with the same aircraft, a jumpseater that could access the third mcdu discovered that the gpwc sensor readout was reading 6233 ft too low. On our way to sin at FL390 the gpwc sensor read 32767 ft. I have a hard copy of this readout. On descent through FL240 the sensor corrected itself and was ok the rest of the flight. I believe: 1) the gpwc sensors were reading too low and gave us false CAWS warnings. 2) the stick shaker sensors and system did not work. 3) presently, there is no way to know that these conditions exist and that the MD11 should have ways for the pilots to know if these system are inoperative or not reading correctly.

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Original NASA ASRS Text

Title: AN ACR MD11 CREW RESPONDED PROPERLY TO A FALSE GPWS WARNING. ALTDEV. FOREIGN AIRSPACE. NIGHT OP. DESIGNATED MOUNTAINOUS TERRAIN.

Narrative: WE WERE CLRED DIRECT TO SUBIC BAY VOR, EXPECTING THE VOR APCH. THE CLRNC CAME AT A POINT BTWN SARSI AND CAB VOR. ALTHOUGH WE WERE IN RADAR CONTACT AND RADAR IDENTED, THIS DSCNT WOULD BE DURING NIGHT IMC CONDITIONS INTO MOUNTAINOUS TERRAIN. ADDITIONALLY, SUBIC BAY IS NOTAMED FOR POSSIBLE MAP SHIFTS, THEREFORE I TOOK SPECIAL CARE TO BRIEF FOR THE DSCNT AND APCH. AFTER CHKING OUR DIRECT COURSE TO THE VOR, I 'HARDWIRED' THE OUTBOUND COURSE ON THE FO'S SIDE AND THE INBOUND COURSE ON MY VOR IN THE FMS. BOTH OF US WERE SELECTED TO MAP FOR THE ARR AND MONITORING THE VOR COURSE ON EACH OF OUR MAPS AND THE VOR COURSE AND DME READOUTS AT THE BOTTOM OF EACH OF OUR ND'S. AT 40 DME WE WERE CLRED TO DSND TO 8000 FT AND ASKED TO RPT SBA 10 DME. THE AUTOPLT WAS FLYING THE DSCNT ON SCHEDULE AND WE ARRIVED AT 10 DME AT 245 KTS. AT ABOUT 8500 FT THE CTLR THEN CLRED US TO DSND 6000 FT, TO FLY THE VOR RWY 7 APCH AND TO RPT PROC TURN INBOUND. WE WERE DSNDING AT ABOUT 2000 FPM WHEN BTWN 7000 FT AND 7500 FT WE HEARD A CONFLICT ALERT WARNING SYS (CAWS) WARNING OF '1000 FT.' I IMMEDIATELY CHKED MY MAP WITH THE CURRENT BAROMETER ALT AND IT SHOWED A GOOD SAFE ALT. I THEN CHKED FOR ANY RA READOUTS AND SAW NONE. SINCE I EXPERIENCED FALSE WARNINGS OF THIS SAME TYPE AT CRUISE ALTS, I ASSUMED THE 1000 FT CALL WAS A FALSE ONE AND TOLD THE FO MY REASONING. DURING OUR DISCUSSIONS, THE CAWS CALLED OUT '500 FT.' I TOOK THE CALL AS REAL AND FELT GND IMPACT WAS IMMINENT SINCE I COULD NOT UNDERSTAND BOTH CALLS BEING FALSE AND I COULD NOT TAKE THE CHANCE THAT IT WASN'T. WE BOTH PUSHED THE THROTTLES TO FIREWALL PWR THROUGH THE FADEC BAR AND ROTATED THE ACFT PAST 15 DEGS AND I CALLED 'SPOILERS ARE UP.' I WAS LOOKING AT THE ALT AND PITCH ATTITUDE WAITING FOR STICK SHAKER. FROM WHAT THE INSTS INDICATED, THE ACFT CLBED AND THEN DSNDED WITH THE R WING ROLLING OVER TO ONE SIDE MORE THAN 45 DEGS. I PUT FULL L AILERON IN AND WE GOT OUR SECOND '500 FT' CALL. AGAIN I FELT GND IMPACT WAS IMMINENT, BUT THE ACFT WASN'T CLBING SO I PUT THE NOSE DOWN AND THE FO PUT 28 DEG FLAPS OUT AND THE ACFT RESPONDED WELL AND CLBED OUT. WE LEVELED AT 9500 FT AND FOUND OURSELVES OUTBOUND ALMOST OVER THE SBA VOR. WE RPTED TURNING OUTBOUND, RECEIVED CLRNC TO LAND AND LANDED UNEVENTFULLY. ON THE GND WE CALLED SUBIC APCH AND THEY CONFIRMED WE WERE ON THE DIRECT TRACK TO THE VOR, WHICH MEANS THERE WERE NO MOUNTAINS THAT WE PASSED OVER WHEN WE GOT THE 1000 FT AND 500 FT CALLS. ON THE NEXT LEG TO SIN WITH THE SAME ACFT, A JUMPSEATER THAT COULD ACCESS THE THIRD MCDU DISCOVERED THAT THE GPWC SENSOR READOUT WAS READING 6233 FT TOO LOW. ON OUR WAY TO SIN AT FL390 THE GPWC SENSOR READ 32767 FT. I HAVE A HARD COPY OF THIS READOUT. ON DSCNT THROUGH FL240 THE SENSOR CORRECTED ITSELF AND WAS OK THE REST OF THE FLT. I BELIEVE: 1) THE GPWC SENSORS WERE READING TOO LOW AND GAVE US FALSE CAWS WARNINGS. 2) THE STICK SHAKER SENSORS AND SYS DID NOT WORK. 3) PRESENTLY, THERE IS NO WAY TO KNOW THAT THESE CONDITIONS EXIST AND THAT THE MD11 SHOULD HAVE WAYS FOR THE PLTS TO KNOW IF THESE SYS ARE INOP OR NOT READING CORRECTLY.

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.