Narrative:

We were cruising at FL330, on nat V at approximately 45W longitude when what appeared to be thunderstorms 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 deviation to the north of nat V was made, and we paralleled nat V with a 5-6 NM offset to the left until passing 35W longitude. This offset was to avoid cells which appeared to be on or just to the south of nat V. After passing 35 degree west we returned to the centerline of nat V and the rest of the flight was uneventful. Approaching the coast of ireland, it became apparent from the radar returns of the coastline, that the tilt control on our radar was not operating properly. With 3.5 degrees of tilt above the horizon, ground returns were still visible. Ordinarily, ground returns disappeared with the tilt 1 degree above horizon. Because of the tilt problem, we had been deviating around cells that in fact had been well below our altitude. 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 turbulence. The en route WX depiction chart provided by our company forecast scattered thunderstorms 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 control to get on top of the clouds, but we were told that FL350 was not available. Therefore, a slight deviation from centerline was made to clear what we thought was, based upon all available information, hazardous WX.

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Original NASA ASRS Text

Title: 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.

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.