Mooney 20 -pilot descended below instrument approach crossing fix due to misreading his actual location on GPS depiction. He noticed his error at the time approach controller was alerting him of his altitude and position.

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Attributes
ACN	520595
Time
Date	200108
Day	Sun
Local Time Of Day	1201 To 1800
Place
Locale Reference	intersection : nazar
State Reference	PA
Altitude	msl bound lower : 2100 msl bound upper : 3000
Environment
Flight Conditions	Marginal
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : abe.tracon
Operator	general aviation : personal
Make Model Name	M-20 Scotsman
Operating Under FAR Part	Part 91
Navigation In Use	other
Flight Phase	descent : approach
Route In Use	approach : instrument precision arrival : on vectors arrival other
Flight Plan	IFR
Person 1
Affiliation	other
Function	flight crew : single pilot
Qualification	pilot : commercial pilot : instrument
Experience	flight time last 90 days : 45 flight time total : 1100 flight time type : 200
ASRS Report	520595
Person 2
Affiliation	government : faa
Function	controller : approach
Events
Anomaly	altitude deviation : crossing restriction not met altitude deviation : excursion from assigned altitude inflight encounter : weather non adherence : clearance non adherence : published procedure other spatial deviation
Independent Detector	other controllera other flight crewa
Resolutory Action	flight crew : took precautionary avoidance action flight crew : became reoriented flight crew : exited adverse environment other
Supplementary
Problem Areas	Flight Crew Human Performance Weather
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

I was flying into lehigh valley international airport (abe) under IFR flight plan. I had been vectored onto final approach course and 'cleared' for the GPS-24 approach. At the time clearance was given, I was about 10 mi from the descent point (wiley) but had lost situational awareness, I was on the approach course at 3000 ft, but did not see the descent point (nazar) on my GPS. I was approaching wiley (shown as next waypoint) on my GPS and assumed I had passed nazar (a mistake). Since I had been cleared for the approach, I started my descent to 1900 ft (the altitude for wiley). As I was descending (at about 2100 ft) nazar came onto the GPS screen and I realized I was 900 ft low. At the same time, abe approach called and noted that I was at nazar and 2100 ft. There was nothing to do but be embarrassed, and continue my descent to 1900 ft. Even though in MVFR, I had clear sight of the terrain, so at no time was this flight in jeopardy. But clearly it could have been. On reflection, I should have noticed/read the GPS distance from wiley. My approach plate shows nazar 5.1 mi from wiley. Since I was on course, the DME would have showed me my position relative to nazar, and I could have planned accordingly. This problem will not happen again.

Original NASA ASRS Text

Title: MOONEY 20 -PLT DSNDED BELOW INST APCH XING FIX DUE TO MISREADING HIS ACTUAL LOCATION ON GPS DEPICTION. HE NOTICED HIS ERROR AT THE TIME APCH CTLR WAS ALERTING HIM OF HIS ALT AND POS.

Narrative: I WAS FLYING INTO LEHIGH VALLEY INTL ARPT (ABE) UNDER IFR FLT PLAN. I HAD BEEN VECTORED ONTO FINAL APCH COURSE AND 'CLRED' FOR THE GPS-24 APCH. AT THE TIME CLRNC WAS GIVEN, I WAS ABOUT 10 MI FROM THE DSCNT POINT (WILEY) BUT HAD LOST SITUATIONAL AWARENESS, I WAS ON THE APCH COURSE AT 3000 FT, BUT DID NOT SEE THE DSCNT POINT (NAZAR) ON MY GPS. I WAS APCHING WILEY (SHOWN AS NEXT WAYPOINT) ON MY GPS AND ASSUMED I HAD PASSED NAZAR (A MISTAKE). SINCE I HAD BEEN CLRED FOR THE APCH, I STARTED MY DSCNT TO 1900 FT (THE ALT FOR WILEY). AS I WAS DSNDING (AT ABOUT 2100 FT) NAZAR CAME ONTO THE GPS SCREEN AND I REALIZED I WAS 900 FT LOW. AT THE SAME TIME, ABE APCH CALLED AND NOTED THAT I WAS AT NAZAR AND 2100 FT. THERE WAS NOTHING TO DO BUT BE EMBARRASSED, AND CONTINUE MY DSCNT TO 1900 FT. EVEN THOUGH IN MVFR, I HAD CLR SIGHT OF THE TERRAIN, SO AT NO TIME WAS THIS FLT IN JEOPARDY. BUT CLRLY IT COULD HAVE BEEN. ON REFLECTION, I SHOULD HAVE NOTICED/READ THE GPS DISTANCE FROM WILEY. MY APCH PLATE SHOWS NAZAR 5.1 MI FROM WILEY. SINCE I WAS ON COURSE, THE DME WOULD HAVE SHOWED ME MY POS RELATIVE TO NAZAR, AND I COULD HAVE PLANNED ACCORDINGLY. THIS PROB WILL NOT HAPPEN AGAIN.

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.