ACFT EQUIP PROB. WX RADAR.

Narrative

ON OCT/XX/95 I WAS ASSIGNED AS THE CAPT OPERATING PHL-DEN IN AN A320 AIRBUS; EQUIPPED WITH A BENDIX RDR-4B COLOR WX RADAR UNIT. THE WX IN THE NE UNITED STATES WAS DOMINATED BY THE REMAINS OF HURRICANE OPAL WHICH WAS PRODUCING CONTINUOUS MODERATE RAIN SHOWERS IN THE PHILADELPHIA AREA AT THE TIME OF OUR AM DEP. DURING THE TAXI OUT WITH RAIN SHOWERS POURING DOWN ON THE ACFT; I TURNED ON THE WX RADAR UNIT AND ADJUSTED THE TILT TO 15 DEGS UP AND SELECTED THE RANGE TO 40 MI IN AN ATTEMPT TO SCAN THE TERMINAL AREA FOR THE PRESENCE OF WX ECHO RETURNS. THE WX RADAR DISPLAY INDICATED WHAT IS EUPHEMISTICALLY TERMED A 'BLOOM' CONDITION. THIS CONDITION OF SEVERE RADAR ATTENUATION IS DOCUMENTED IN THE FLT MANUAL ON PAGE 6-52. THIS FLT MANUAL STATES; 'AT LOWER ALTS IN THE PRESENCE OF PRECIPITATION; THE RADAR DISPLAY MAY TURN MOSTLY OR COMPLETELY RED OR GREEN. THIS 'BLOOM' CONDITION MAY OBSCURE DEFINITION OF CELLS. THE CONDITION MAY BE ALLEVIATED BY MANUALLY ADJUSTING THE GAIN KNOB. A RECOMMENDED TECHNIQUE PRIOR TO ENTERING THE APCH ENVIRONMENT OR AT LOW ALT IS TO SELECT THE 1 O'CLOCK POS MANUALLY. SMALL ADJUSTMENTS MAY HAVE TO BE MADE TO DEFINE SPECIFIC CELLS. WHEN GAIN IS NO LONGER NEEDED; RETURN GAIN KNOB TO AUTO.' WITH THE RADAR GAIN CTL IN THE AUTO POS; THE RADAR DISPLAY INDICATED A CONTINUOUS ATTENTUATION ARC FROM THE RADOME OUT TO 10-15 MI. THIS ARC WAS 90 PERCENT RED RADAR DISPLAY AND 10 PERCENT AMBER. NO RADAR RETURNS WERE VISIBLE IN THE AREA BEYOND THIS ATTENUATION ARC. I THEN SELECTED MANUAL GAIN CTL AS RECOMMENDED; WITH THE ONLY DISPLAY CHANGE BEING THE COLOR CHANGED TO 90 PERCENT GREEN AND 10 PERCENT AMBER RETURNS IN THE ATTENUATION ARC. SINCE OTHER ACFT WERE DEPARTING THE ARPT IN FRONT OF US AND NO TSTMS WERE FORECAST IN THE AREA FOR 4 HRS; WE DEPARTED. WHILE AIRBORNE BTWN 5000 FT AND FL200 WE FLEW THROUGH AN AREA OF RAIN SHOWERS FOR 5- 10 MINS IN WHICH THE INTENSITY OF THE RAIN POUNDING ON THE WINDSHIELD WAS CONSISTENT WITH MODERATE SHOWERS. THESE SHOWERS SHOULD HAVE BEEN VISIBLE ON ANY PROPERLY FUNCTIONING AIRBORNE WX RADAR UNIT. BUT NO RADAR RETURNS WERE DISPLAYED ON OUR RADAR UNIT BEYOND THE CONTINUOUS ATTENUATION ARC. ONCE OUT OF THE LOW ALT STRUCTURE AND AT CRUISING FLT LEVEL FOR 2 HRS; THE RADAR UNIT OPERATED PROPERLY. THIS KNOWN ANOMALY IS GOING TO BE A CONTRIBUTING FACTOR TO A FUTURE WX RELATED ACCIDENT INVOLVING THIS RADAR UNIT BEING FLOWN AT LOW ALTS IN VISIBLE MOISTURE. THIS PROB NEEDS TO BE FIXED NOW!