Tower controller intervened to the flight crew of a B757 to a low altitude alert while on a VOR approach. The first officer had started descending early due to the navigation map display showed them at the fix to start down. The captain then noticed that the 'raw data' showed them a mi short of the final fix or VORTAC.

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Attributes
ACN	404473
Time
Date	199806
Day	Tue
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : cri airport : jfk
State Reference	NY
Altitude	msl bound lower : 1300 msl bound upper : 1500
Environment
Light	Daylight
Aircraft 1
Controlling Facilities	tower : jfk tower : ord
Operator	common carrier : air carrier
Make Model Name	B757 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	descent : approach
Route In Use	arrival other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
ASRS Report	404473
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : commercial pilot : instrument
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : published procedure non adherence : clearance other anomaly other other spatial deviation
Independent Detector	other controllera
Resolutory Action	flight crew : returned to intended course or assigned course other
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

On approach to jfk VOR runway 13L in use. First officer started to leave 1500 ft over cri VOR using map mode. Raw data showed .9 NM to go to cri, map display had us over cri. We had a .9 NM map shift and should have remained at 1500 ft until cri. Got as low as 1300 ft and received a low altitude alert from tower. Corrected and continued approach. Just goes to show you the importance of raw data which was available at all times.

Original NASA ASRS Text

Title: TWR CTLR INTERVENED TO THE FLC OF A B757 TO A LOW ALT ALERT WHILE ON A VOR APCH. THE FO HAD STARTED DSNDING EARLY DUE TO THE NAV MAP DISPLAY SHOWED THEM AT THE FIX TO START DOWN. THE CAPT THEN NOTICED THAT THE 'RAW DATA' SHOWED THEM A MI SHORT OF THE FINAL FIX OR VORTAC.

Narrative: ON APCH TO JFK VOR RWY 13L IN USE. FO STARTED TO LEAVE 1500 FT OVER CRI VOR USING MAP MODE. RAW DATA SHOWED .9 NM TO GO TO CRI, MAP DISPLAY HAD US OVER CRI. WE HAD A .9 NM MAP SHIFT AND SHOULD HAVE REMAINED AT 1500 FT UNTIL CRI. GOT AS LOW AS 1300 FT AND RECEIVED A LOW ALT ALERT FROM TWR. CORRECTED AND CONTINUED APCH. JUST GOES TO SHOW YOU THE IMPORTANCE OF RAW DATA WHICH WAS AVAILABLE AT ALL TIMES.

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.