Narrative:

It was my leg to fly. The flight went along as usual, nothing out of the ordinary except that the WX at our destination had deteriorated more quickly than forecasted. The WX was (approximately) 2000 ft overcast, 1 mi in light rain and fog. The ATIS advised us to expect the ILS runway 27R approach to phl. The descent and approach checks and company calls were completed well ahead of our impending approach. The last vector had us intercept the localizer and shortly thereafter were 'cleared for the approach' followed by the instruction 'contact tower now on 118.5.' we complied and during the next min we were 'cleared to land on runway 27R, you're following a B737, just over the numbers.' it was at this point that the excitement/confusion began. When the approach lights started to come into sight, the captain (PNF) called, 'approach lights in sight, 12 O'clock, wait a min, what the, ahh, something's not right with this picture, we have 2 sets of approach lights!' during the entire approach, I had the localizer exactly in the center, but when he called out that he had not 1 but 2 sets of approach lights in sight (not believing what I had just heard), I had to look up and sure enough there were 2 sets of (what we thought were) approach lights, except that the lights on the right side were brighter. My first instinct was to make a small correction and head towards those lights. But upon doing so, I noticed that the localizer needle was starting to make a very small but growing deflection. It was at that instant the warning bells went off (in my head). 'Trust your instruments' I told myself, and immediately turned back to reintercept the localizer. An uneventful landing ensued followed by some lengthy conversation over coffee. What had happened was that on the right hand side of the approach lights to the ILS runway 27R approach into phl, there is an access road. Because it was both raining and foggy out, the 7 or 8 cars who happened to be right behind each other all had their headlights on and apparently were stopped on the road. And at that instant, at an altitude of approximately 1200 ft, for what seemed like 2 or 3 very long seconds, we honestly could not tell the difference between the string of headlights and the approach lights. I read about a similar incident several yrs ago in an aviation safety publication, and learned that our eyes can play tricks on us when we least expect them to and concluded that it is absolutely imperative to trust our instruments. If the visibility had been worse, a possible tragedy could have occurred. I feel that to prevent an occurrence of this nature from happening again, a note should be made on the approach plate warning that in certain conditions, this event could happen. I believe this action could prevent this potentially dangerous situation from happening to another pilot or crew in the future. This should be a fairly simple, necessary and inexpensive fix. Callback conversation with reporter revealed the following information: the reporter stated he was flying a BE1900C. He said that airplane was not at all at fault for the illusion -- any airplane would have been as bad or worse. The problem was the cars had stopped with their headlights on. This road was not an access road but a regular road that happens to parallel the approach lights. This was kind of a freak thing in that the cars were lined up and stopped with their headlights on. He believes strongly that a warning should be on the approach plate of the possible illusion and to follow the localizer and GS carefully.

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Original NASA ASRS Text

Title: RPTR EXPERIENCES FREAK OPTICAL ILLUSION SUCH THAT CARS STOPPED ON A ROAD PARALLEL TO THE APCH LIGHTS MIMIC THE REAL APCH LIGHTS AND BEING BRIGHTER CAUSE CONFUSION.

Narrative: IT WAS MY LEG TO FLY. THE FLT WENT ALONG AS USUAL, NOTHING OUT OF THE ORDINARY EXCEPT THAT THE WX AT OUR DEST HAD DETERIORATED MORE QUICKLY THAN FORECASTED. THE WX WAS (APPROX) 2000 FT OVCST, 1 MI IN LIGHT RAIN AND FOG. THE ATIS ADVISED US TO EXPECT THE ILS RWY 27R APCH TO PHL. THE DSCNT AND APCH CHKS AND COMPANY CALLS WERE COMPLETED WELL AHEAD OF OUR IMPENDING APCH. THE LAST VECTOR HAD US INTERCEPT THE LOC AND SHORTLY THEREAFTER WERE 'CLRED FOR THE APCH' FOLLOWED BY THE INSTRUCTION 'CONTACT TWR NOW ON 118.5.' WE COMPLIED AND DURING THE NEXT MIN WE WERE 'CLRED TO LAND ON RWY 27R, YOU'RE FOLLOWING A B737, JUST OVER THE NUMBERS.' IT WAS AT THIS POINT THAT THE EXCITEMENT/CONFUSION BEGAN. WHEN THE APCH LIGHTS STARTED TO COME INTO SIGHT, THE CAPT (PNF) CALLED, 'APCH LIGHTS IN SIGHT, 12 O'CLOCK, WAIT A MIN, WHAT THE, AHH, SOMETHING'S NOT RIGHT WITH THIS PICTURE, WE HAVE 2 SETS OF APCH LIGHTS!' DURING THE ENTIRE APCH, I HAD THE LOC EXACTLY IN THE CTR, BUT WHEN HE CALLED OUT THAT HE HAD NOT 1 BUT 2 SETS OF APCH LIGHTS IN SIGHT (NOT BELIEVING WHAT I HAD JUST HEARD), I HAD TO LOOK UP AND SURE ENOUGH THERE WERE 2 SETS OF (WHAT WE THOUGHT WERE) APCH LIGHTS, EXCEPT THAT THE LIGHTS ON THE R SIDE WERE BRIGHTER. MY FIRST INSTINCT WAS TO MAKE A SMALL CORRECTION AND HEAD TOWARDS THOSE LIGHTS. BUT UPON DOING SO, I NOTICED THAT THE LOC NEEDLE WAS STARTING TO MAKE A VERY SMALL BUT GROWING DEFLECTION. IT WAS AT THAT INSTANT THE WARNING BELLS WENT OFF (IN MY HEAD). 'TRUST YOUR INSTS' I TOLD MYSELF, AND IMMEDIATELY TURNED BACK TO REINTERCEPT THE LOC. AN UNEVENTFUL LNDG ENSUED FOLLOWED BY SOME LENGTHY CONVERSATION OVER COFFEE. WHAT HAD HAPPENED WAS THAT ON THE R HAND SIDE OF THE APCH LIGHTS TO THE ILS RWY 27R APCH INTO PHL, THERE IS AN ACCESS ROAD. BECAUSE IT WAS BOTH RAINING AND FOGGY OUT, THE 7 OR 8 CARS WHO HAPPENED TO BE RIGHT BEHIND EACH OTHER ALL HAD THEIR HEADLIGHTS ON AND APPARENTLY WERE STOPPED ON THE ROAD. AND AT THAT INSTANT, AT AN ALT OF APPROX 1200 FT, FOR WHAT SEEMED LIKE 2 OR 3 VERY LONG SECONDS, WE HONESTLY COULD NOT TELL THE DIFFERENCE BTWN THE STRING OF HEADLIGHTS AND THE APCH LIGHTS. I READ ABOUT A SIMILAR INCIDENT SEVERAL YRS AGO IN AN AVIATION SAFETY PUB, AND LEARNED THAT OUR EYES CAN PLAY TRICKS ON US WHEN WE LEAST EXPECT THEM TO AND CONCLUDED THAT IT IS ABSOLUTELY IMPERATIVE TO TRUST OUR INSTS. IF THE VISIBILITY HAD BEEN WORSE, A POSSIBLE TRAGEDY COULD HAVE OCCURRED. I FEEL THAT TO PREVENT AN OCCURRENCE OF THIS NATURE FROM HAPPENING AGAIN, A NOTE SHOULD BE MADE ON THE APCH PLATE WARNING THAT IN CERTAIN CONDITIONS, THIS EVENT COULD HAPPEN. I BELIEVE THIS ACTION COULD PREVENT THIS POTENTIALLY DANGEROUS SIT FROM HAPPENING TO ANOTHER PLT OR CREW IN THE FUTURE. THIS SHOULD BE A FAIRLY SIMPLE, NECESSARY AND INEXPENSIVE FIX. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED HE WAS FLYING A BE1900C. HE SAID THAT AIRPLANE WAS NOT AT ALL AT FAULT FOR THE ILLUSION -- ANY AIRPLANE WOULD HAVE BEEN AS BAD OR WORSE. THE PROB WAS THE CARS HAD STOPPED WITH THEIR HEADLIGHTS ON. THIS ROAD WAS NOT AN ACCESS ROAD BUT A REGULAR ROAD THAT HAPPENS TO PARALLEL THE APCH LIGHTS. THIS WAS KIND OF A FREAK THING IN THAT THE CARS WERE LINED UP AND STOPPED WITH THEIR HEADLIGHTS ON. HE BELIEVES STRONGLY THAT A WARNING SHOULD BE ON THE APCH PLATE OF THE POSSIBLE ILLUSION AND TO FOLLOW THE LOC AND GS CAREFULLY.

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.