A FOKKER 100 ON APCH IN RAIN HAD THE CAPT'S RADIO ALTIMETER FLUCTUATE 700-1000 FT WITH GPWS. WHEN CLR OF RAIN BOTH FO'S AND CAPT'S RADIO ALTIMETERS NORMAL.

Narrative

DSNDING THROUGH 11500 FT IN LIGHT TO MODERATE RAIN ON JEN.J3N5 ARR; THE CAPT'S RADIO ALTIMETER BEGAN TO FLUCTUATE AND READ 700-1000 FT. WE ALSO RECEIVED SEVERAL GPWS TERRAIN WARNINGS. WE CLBED PER PROC. WE ADVISED ATC. ALL BAROMETRIC ALTIMETERS WERE READING 12000 FT. THE FO 'S RADIO ALTIMETER WAS NORMAL AND HAD NO READING. WE STAYED AT 12000 FT TILL WE RESOLVED THE DISCREPANCY BTWN THE ALTIMETERS THEN CONTINUED A NORMAL DSCNT AND LNDG. WE HAD TO PUSH THE GPWS SWITCH TO STOP THE TERRAIN WARNINGS WHILE FLYING OVER RAIN SHOWERS. ONCE CLR OF THE PRECIP; THE RADIO ALTIMETERS WERE NORMAL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE CAPT'S RADIO ALTIMETER FLUCTUATION WHEN PASSING THROUGH COLD RAIN AND ICE IS A CHARACTERISTIC OF THE FOKKER 100. THE RPTR SAID AVIONICS ENGINEERING HAS SEEN THESE RPTS AND BELIEVES THE RETURN IS REFLECTING OFF THE AIRPLANE SURFACE. THE RPTR SAID THE FO'S RADIO ALTIMETER WAS UNAFFECTED AND OPERATED NORMALLY. THE RPTR STATED HE WAS NOT CERTAIN IF THE GPWS WAS THE ENHANCED SYS. CALLBACK CONVERSATION WITH RPTR ACN 490989 REVEALED THE FOLLOWING INFO: THE RPTR STATED THE GPWS WAS NOT THE ENHANCED SYS.