A PA44 PLT CONFUSES A GPS WAYPOINT DISTANCE WITH VOR/DME RAW DATA ON A VOR APCH AND BEGINS A DSCNT TO MINIMUM OUTSIDE THE FAF.

Narrative

WHILE ON FINAL VECTORING FOR VOR/DME APCH; VOR INDICATIONS BECAME ERRATIC; WITH LOSS OF SIGNAL OCCURRING SEVERAL TIMES. AN INQUIRY WAS MADE TO ATC FACILITY AS TO THE STATUS OF THE VOR. NO PROBS WERE RPTED AND SOON THEREAFTER; INSTRUMENT INDICATIONS RETURNED TO NORMAL. DME INFO FOR THE APCH WAS OBTAINED SOLELY FROM THE GARMIN 430 RADIO. I DECIDED THAT IT WOULD BE HELPFUL TO MONITOR THE COURSE ALIGNMENT USING THE GPS OVERLAY FOR THIS APCH IN ADDITION TO THE STANDARD VOR INDICATIONS. I DO NOT USUALLY DO THIS SINCE THE OVERLAY CAN NOT BE USED IN LIEU OF THE VOR FOR THIS APCH. HOWEVER; SINCE THE VOR INDICATIONS HAD BEEN ERRATIC; I THOUGHT THE ADDED INFO WOULD MAKE FOR A SAFER APCH. SHORTLY AFTER LOADING THE OVERLAY APCH; I WAS GIVEN THE CLRNC FOR THE APCH. BASED ON THE TIMING OF THE CLRNC AND THE DISTANCE READ OFF OF THE GPS I THOUGHT I WAS JUST A FEW MILES OUTSIDE THE FAF. I COMMENCED WITH A DSCNT TO THE INITIAL APCH ALT AND WHEN I THOUGHT I HAD ARRIVED AT THE FAF; AS INDICATED BY THE GPS DISTANCE; I CONTINUED A DSCNT TOWARDS THE NEXT STEP-DOWN FIX. WHAT I DID NOT REALIZE WAS THE DISTANCE INFO WAS NO LONGER TO THE VOR BUT TO THE FAF AS SHOWN ON THE OVERLAY FOR THAT APCH. THIS RESULTED IN AN EARLY DSCNT OUTSIDE FAF. LUCKILY; ANOTHER APCH TO AN UNCTLED ARPT LIES DIRECTLY UNDER THIS APCH AND OBSTACLE CLRNC WAS NOT AN ISSUE. WHEN I CAUGHT MY ERROR; I IMMEDIATELY COMMENCED A CLB BACK TO THE INITIAL APCH ALT AND RPTED MY ACTIONS TO THE APCH CTLR. ATC ACKNOWLEDGED MY RPT AND NO FURTHER ACTION WAS TAKEN. NO ACFT SEPARATION ISSUES CAME UP AND THE REST OF THE ARR TO THE ARPT PROCEEDED WITHOUT INCIDENT. THE MOST NOTEWORTHY ASPECT OF THIS INCIDENT DEALS WITH THE USE OF GPS SYSTEMS FOR MONITORING OF CONVENTIONAL NAVAID APCHES. SINCE IT IS COMMON TO USE THE GPS DISTANCES IN LIEU OF DME; IT IS IMPORTANT TO REALIZE THAT AN OVERLAY APCH; WHEN LOADED; WILL REPLACE PREVIOUSLY LOADED DISTANCE INFO WITH THE DISTANCES TO WAYPOINTS OF THE OVERLAY APCH. DISTANCES ARE THEN FROM ONE WAYPOINT TO ANOTHER; NOT TO THE VOR THAT DESIGNATES THE APCH COURSE. IT IS ALSO NOTEWORTHY THAT MY SAFETY PLT ALSO MISSED THE FACT THAT I HAD STARTED DOWN EARLY. WE BOTH NOTICED THE ERROR AT APPROX THE SAME TIME! A SECOND PAIR OF EYES DOES NOT ALWAYS PROVIDE THE SAFETY BUFFER THAT ONE MIGHT LIKE. ALSO; I HAVE FLOWN THIS PARTICULAR APCH HUNDREDS OF TIMES AND THIS IS THE FIRST TIME I HAVE EVER MADE SUCH AN ERROR. IT WAS ALSO THE FIRST TIME THAT I USED THE GPS EQUIP IN THIS MANNER TO SUPPORT SITUATIONAL AWARENESS ON AN INSTRUMENT APCH. ON THAT NOTE; I RECOMMEND THAT WHEN TRYING SOMETHING NEW WITH A GPS UNIT; THAT THE PLT HAVE AN EXPERIENCED INSTRUCTOR ON BOARD TO MONITOR HOW YOU SET THE UNIT UP; AND TO BETTER CATCH MISTAKES LIKE THE ONE I'VE DESCRIBED IN THIS RPT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR IS A HUMAN FACTORS RESEARCHER AT A MAJOR UNIVERSITY; BUT IS NOT CURRENTLY FLT INSTRUCTING. HE FLEW 15 YRS WITH RAW DATA ONLY AND HIS TRANSITION TO THE GPS SYSTEM INSTALLED IN THE SCHOOL'S PA44 WAS PROMPTED BY THE ATTITUDE 'YOU'VE GOT IT INSTALLED NOW LEARN HOW TO USE IT.' THE SCHOOL'S GPS BASED SYSTEM ALLOWS REMOVING THE NAV SYSTEM FROM THE ACFT AND USING IT AS A CLASSROOM BASED SIMULATOR. THE RPTR USED THE TRAINER AS WELL AS THE ACCOMPANYING DVD INSTRUCTIONS. HOWEVER; HE HAS NEVER THOUGHT THE TRAINING WAS WHOLLY ADEQUATE. HE BELIEVES THE SYSTEM'S FUNCTIONALITY IS NOT APPARENT TO THE USER; AND THEREFORE; MISTAKES ARE NOT EASILY IDENTIFIED AS WAS DEMONSTRATED BY THIS RPT WITH TWO EXPERIENCED PLTS ONBOARD. THIS PARTICULAR SYSTEM IS BETTER THAN ONE OTHER COMMERCIALLY AVAILABLE INSTALLATION. HE SEES THE PROBS AS THIS SYSTEM COMPLEXITY IS ONE STEP AWAY FROM AN FMS SYSTEM; REQUIRES NO FORMAL TRAINING TO OPERATE; AND NO REMEDIAL TRAINING IS AVAILABLE. THE NEW GLASS ACFT DISPLAYS ARE COMPELLING; NATURALLY DRAWING PLTS TO THE PRESENTATION; AND AS A RESULT ENCOURAGING PLTS TO FLY HEADS DOWN.