AN MD80 FIRST OFFICER RPTS THE RADAR GAIN CTL SENSITIVITY KNOB IF POSITIONED AGAINST THE STOP IN AUTO MODE WILL TRANSITION TO MIN SENSITIVITY. MIN SENSITIVITY WILL NOT PAINT STORM CELLS.

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I THINK WHAT I AM GOING TO TELL YOU IS A 'RECIPE FOR DISASTER' IF IT IS NOT TAKEN CARE OF. THE 'GAIN' CTL KNOB ON THE RADAR CTL HEAD OF ACFT X IS BROKEN. BUT THERE IS A CATCH. YOU CAN ONLY DETECT IT IF YOU TURN THE KNOB ALL THE WAY INTO THE 'AUTO' POSITION. IF THE KNOB IS NOT UP AGAINST THE STOP OF ITS TRAVEL IN THE 'AUTO' POSITION YOU WILL NOT NOTICE ANY PROB AND IT WILL OPERATE NORMALLY. WHAT IS HAPPENING IS THAT WHEN YOU TAKE THE 'GAIN' CTL KNOB AND TURN IT ALL THE WAY INTO THE AUTO POSITION; UP AGAINST ITS MECHANICAL STOP; THE 'GAIN' OR SENSITIVITY OF THE RADAR WILL TRANSITION FROM THE AUTO MODE TO THE MIN SENSITIVITY MODE AND THE ONLY WAY THAT YOU WILL NOTICE THIS IS THAT ON THE RADAR SCREEN IT WILL SAY 'VAR' UNDER WHERE IT ALSO DISPLAYS THE ABBREVIATION 'WX' AND THE AMOUNT OF TILT SELECTED. I'VE NOTICED IN TIMES PAST THAT VERY FEW PLTS CATCH THE 'VAR' WHEN THE RADAR IS NOT OPERATING IN THE AUTO MODE. OF COURSE THE OTHER BIG CLUE IS THAT YOU WILL HAVE A VERY HARD TIME GETTING THE RADAR TO PAINT ANYTHING. WHAT HAPPENED TO US ON THIS PARTICULAR DAY WAS THAT THE GAIN CTL KNOB WAS ALREADY IN THIS STATE OF MIN SENSITIVITY WHEN WE TURNED THE RADAR ON ALT. WE DID NOT FIGURE OUT WHAT WAS GOING ON UNTIL WE NOTICED THAT SOME CELLS IN OUR AREA THAT SHOULD HAVE BEEN PAINTING ON THE RADAR WERE ONLY BARELY SHOWING UP AS A VERY FAINT GREEN DOT. AFTER A WHILE I FIGURED OUT WHAT WAS GOING ON AND WHAT WAS BROKEN. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THIS ANOMALY IS COMMON TO ALL MD80 ACFT. THE RPTR DESCRIBES THE ANOMALY ON THE RADAR CTL PANEL SENSITIVITY KNOB AS WHEN IN AUTO MODE AND TURNED FULL UP TO THE MECHANICAL STOP THE RADAR SENSITIVITY TRANSITIONS TO MIN SENSITIVITY. THE RPTR SAID THE ONLY WARNING IS A 'VAR' ON THE RADAR DISPLAY AND IS EASILY MISSED. THE RPTR STATED THIS IS NOT A RADAR FAILURE BUT A DESIGN FAULT THAT IS NOT EASY TO DETECT.