AIR AMBULANCE HAS NAV SYS FAILURE ON OVERWATER; NON RADAR FLT.

Narrative

WE DEPARTED ISLIP; NY; FOR AN AIR AMBULANCE TRIP ENRTE TO AGUADILLA; PR. AFTER BEING VECTORED; WE WERE CLRED DIRECT TO CHAMP INTXN; THEN TO PROCEED FLT PLANNED RTE; WHICH WAS A300 TO PLING INTXN; THEN DIRECT AGUADILLA. WE NAVIGATED USING OUR GNS; WHICH SEEMED TO BE PERFORMING NORMALLY. THE ONLY UNUSUAL INDICATION WAS THAT THE WINDS ON THE GNS WERE HIGHER THAN THOSE FORECAST. SHORTLY AFTER XING ELWOD INTXN; WE WERE ADVISED BY COMMERCIAL RADIO TO CONTACT ZMA; WHO ADVISED US THAT WE WERE IN RADAR CONTACT 180 MI NE OF NASSAU. AT THIS POINT; WE WERE GIVEN A VECTOR TOWARDS GRAND TURK VOR AND RECLRED TO AGUADILLA. WE PROCEEDED TO THE DEST WITHOUT INCIDENT. BEFORE DEPARTING AGUADILLA; WE RE-FILED FOR AN INLAND RTE THAT WOULD KEEP US IN RADAR CONTACT. I PLACARDED THE GNS INOP IN ACCORDANCE WITH OUR MEL AND RETURNED THE PATIENT TO ISLIP WITHOUT INCIDENT. THE DIRECTOR OF MAINT FELT THAT THE MALFUNCTION MAY HAVE BEEN CAUSED BY A TEMPORARY BLOCKAGE IN ONE OF THE GNS UNIT'S STATIC SOURCES. ONE SUGGESTION FOR PREVENTING SUCH AN EVENT MIGHT BE TO REQUIRE 2 LONG RANGE NAV SYS WHENEVER OPERATING OUT OF RADAR CONTACT.