Narrative:

On a flight from dfw-phl that was scheduled to arrive at XA59 local time in phl, phl WX was reported sky obscured 0 ceiling in fog, visibility was reported to be 1/16 mi in fog. While we were being vectored for the approach an air carrier B jet missed the approach in front of us. At that time the approach controller reported that the tower had checked (after his missed approach) and the ILS critical area was protected. This makes me wonder if they had checked to ensure the critical area was protected before his approach. At this time we missed the CAT ii ILS approach to runway 9R. The aircraft was coupled to the autoplt as per our company SOP. We would take over visually at 80' AGL, release the autoplt and manually land the aircraft. All cockpit indications were normal. Tower was reporting above minimums RVR on T/D, midpoint and rollout. Crew callouts were performed as per SOP. At 400' AGL autoplt began to slightly pitch up and down. We considered this normal as small deviations in autoplt flight path are normal. But at 200' AGL, still totally in instrument conditions, autoplt pitched violently up and down in less than 1 second. At the moment of downward pitch movement, the approach light system came into view. Only quick action by captain and first officer of disconnecting the autoplt rotating the aircraft at least 25 degrees nose up and adding maximum power allowed us to escape contact with the approach lights on the ground. From where I was sitting it appeared the aircraft sunk below the approach lighting system. I feel sure the miss was only in inches. We regained our composure and elected not to attempt another approach into phl. Divert was to ewr where first officer and myself inspected the aircraft and found no damage. The aircraft was not taken OTS. Since we have 2 hydraulic control channels for the autoplt, only the one we were using at the time was placarded as inoperative. I feel this was a mistake due to the fact that the problem could be in the black boxes of the autoplt and not just the autoplt hydraulic channel. Another contributing factor to this incident was that our airline no longer requires airborne CAT ii equipment checks. The only way to check the CAT ii functions of the autoplt is to use it during an approach, or at captain's discretion during normal line operations. Another contributing factor is that it is possible that the ILS components may have malfunctioned, since the aircraft ahead of us missed his approach due to localizer deflections. In the future I will encourage all the crews I fly with to practice CAT ii approachs in VFR conditions. I will also encourage all the crews that during actual CAT ii approachs, if the autoplt gives the slightest sign of malfunction, I will encourage the crew to disconnect and abort the approach. In 12 yrs of flying, I have never came this close to an accident. I will try very hard not to come this close again. Callback conversation with reporter revealed the following: revealed no further follow up information. Reporter feels deviation was caused by boat passing under G/south on river. Feels there was nothing wrong with hydraulic and maintenance just used that as sign off to get aircraft underway.

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Original NASA ASRS Text

Title: ACR LGT DIVES BELOW GLIDE SLOPE ON CAT II APCH AND NEARLY STRIKES APCH LIGHTS BEFORE RECOVERY.

Narrative: ON A FLT FROM DFW-PHL THAT WAS SCHEDULED TO ARRIVE AT XA59 LCL TIME IN PHL, PHL WX WAS RPTED SKY OBSCURED 0 CEILING IN FOG, VISIBILITY WAS RPTED TO BE 1/16 MI IN FOG. WHILE WE WERE BEING VECTORED FOR THE APCH AN ACR B JET MISSED THE APCH IN FRONT OF US. AT THAT TIME THE APCH CTLR RPTED THAT THE TWR HAD CHKED (AFTER HIS MISSED APCH) AND THE ILS CRITICAL AREA WAS PROTECTED. THIS MAKES ME WONDER IF THEY HAD CHKED TO ENSURE THE CRITICAL AREA WAS PROTECTED BEFORE HIS APCH. AT THIS TIME WE MISSED THE CAT II ILS APCH TO RWY 9R. THE ACFT WAS COUPLED TO THE AUTOPLT AS PER OUR COMPANY SOP. WE WOULD TAKE OVER VISUALLY AT 80' AGL, RELEASE THE AUTOPLT AND MANUALLY LAND THE ACFT. ALL COCKPIT INDICATIONS WERE NORMAL. TWR WAS RPTING ABOVE MINIMUMS RVR ON T/D, MIDPOINT AND ROLLOUT. CREW CALLOUTS WERE PERFORMED AS PER SOP. AT 400' AGL AUTOPLT BEGAN TO SLIGHTLY PITCH UP AND DOWN. WE CONSIDERED THIS NORMAL AS SMALL DEVIATIONS IN AUTOPLT FLT PATH ARE NORMAL. BUT AT 200' AGL, STILL TOTALLY IN INSTRUMENT CONDITIONS, AUTOPLT PITCHED VIOLENTLY UP AND DOWN IN LESS THAN 1 SEC. AT THE MOMENT OF DOWNWARD PITCH MOVEMENT, THE APCH LIGHT SYS CAME INTO VIEW. ONLY QUICK ACTION BY CAPT AND F/O OF DISCONNECTING THE AUTOPLT ROTATING THE ACFT AT LEAST 25 DEGS NOSE UP AND ADDING MAX PWR ALLOWED US TO ESCAPE CONTACT WITH THE APCH LIGHTS ON THE GND. FROM WHERE I WAS SITTING IT APPEARED THE ACFT SUNK BELOW THE APCH LIGHTING SYS. I FEEL SURE THE MISS WAS ONLY IN INCHES. WE REGAINED OUR COMPOSURE AND ELECTED NOT TO ATTEMPT ANOTHER APCH INTO PHL. DIVERT WAS TO EWR WHERE F/O AND MYSELF INSPECTED THE ACFT AND FOUND NO DAMAGE. THE ACFT WAS NOT TAKEN OTS. SINCE WE HAVE 2 HYD CONTROL CHANNELS FOR THE AUTOPLT, ONLY THE ONE WE WERE USING AT THE TIME WAS PLACARDED AS INOP. I FEEL THIS WAS A MISTAKE DUE TO THE FACT THAT THE PROB COULD BE IN THE BLACK BOXES OF THE AUTOPLT AND NOT JUST THE AUTOPLT HYD CHANNEL. ANOTHER CONTRIBUTING FACTOR TO THIS INCIDENT WAS THAT OUR AIRLINE NO LONGER REQUIRES AIRBORNE CAT II EQUIP CHKS. THE ONLY WAY TO CHK THE CAT II FUNCTIONS OF THE AUTOPLT IS TO USE IT DURING AN APCH, OR AT CAPT'S DISCRETION DURING NORMAL LINE OPS. ANOTHER CONTRIBUTING FACTOR IS THAT IT IS POSSIBLE THAT THE ILS COMPONENTS MAY HAVE MALFUNCTIONED, SINCE THE ACFT AHEAD OF US MISSED HIS APCH DUE TO LOC DEFLECTIONS. IN THE FUTURE I WILL ENCOURAGE ALL THE CREWS I FLY WITH TO PRACTICE CAT II APCHS IN VFR CONDITIONS. I WILL ALSO ENCOURAGE ALL THE CREWS THAT DURING ACTUAL CAT II APCHS, IF THE AUTOPLT GIVES THE SLIGHTEST SIGN OF MALFUNCTION, I WILL ENCOURAGE THE CREW TO DISCONNECT AND ABORT THE APCH. IN 12 YRS OF FLYING, I HAVE NEVER CAME THIS CLOSE TO AN ACCIDENT. I WILL TRY VERY HARD NOT TO COME THIS CLOSE AGAIN. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: REVEALED NO FURTHER FOLLOW UP INFO. RPTR FEELS DEVIATION WAS CAUSED BY BOAT PASSING UNDER G/S ON RIVER. FEELS THERE WAS NOTHING WRONG WITH HYD AND MAINT JUST USED THAT AS SIGN OFF TO GET ACFT UNDERWAY.

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.