FLC OF ACR WDB ACFT DEVIATED FROM ASSIGNED OCEANIC TRACK DUE ERRONEOUS INDICATIONS FROM THE ACFT WX RADAR.

Narrative

WE WERE CRUISING AT FL330; ON NAT V AT APPROX 45W LONGITUDE WHEN WHAT APPEARED TO BE TSTMS APPEARED ON OUR WX RADAR. BY ADJUSTING THE TILT OF THE RADAR ANTENNA; IT APPEARED THAT THE STORM CELLS WERE AT OR ABOVE FL330. A SLIGHT DEV TO THE N OF NAT V WAS MADE; AND WE PARALLELED NAT V WITH A 5-6 NM OFFSET TO THE L UNTIL PASSING 35W LONGITUDE. THIS OFFSET WAS TO AVOID CELLS WHICH APPEARED TO BE ON OR JUST TO THE S OF NAT V. AFTER PASSING 35 DEG W WE RETURNED TO THE CTRLINE OF NAT V AND THE REST OF THE FLT WAS UNEVENTFUL. APCHING THE COAST OF IRELAND; IT BECAME APPARENT FROM THE RADAR RETURNS OF THE COASTLINE; THAT THE TILT CTL ON OUR RADAR WAS NOT OPERATING PROPERLY. WITH 3.5 DEGS OF TILT ABOVE THE HORIZON; GND RETURNS WERE STILL VISIBLE. ORDINARILY; GND RETURNS DISAPPEARED WITH THE TILT 1 DEG ABOVE HORIZON. BECAUSE OF THE TILT PROBLEM; WE HAD BEEN DEVIATING AROUND CELLS THAT IN FACT HAD BEEN WELL BELOW OUR ALT. HOWEVER; AT THE TIME; WE THOUGHT WE WERE DEALING WITH MUCH HIGHER CELLS; BECAUSE WE WERE IN AND OUT OF THE CLOUD TOPS; WITH LIGHT TO OCCASIONAL MODERATE TURB. THE ENRTE WX DEPICTION CHART PROVIDED BY OUR COMPANY FORECAST SCATTERED TSTMS IN THE AREA WITH TOPS TO FL350. NIGHTTIME LIGHTING CONDITIONS DID NOT ALLOW US TO VISUALLY ASSESS CELL HEIGHTS. WE REQUESTED FL350 FROM GANDER OCEANIC CTL TO GET ON TOP OF THE CLOUDS; BUT WE WERE TOLD THAT FL350 WAS NOT AVAILABLE. THEREFORE; A SLIGHT DEV FROM CTRLINE WAS MADE TO CLR WHAT WE THOUGHT WAS; BASED UPON ALL AVAILABLE INFO; HAZARDOUS WX.