An MD80 first officer reports the radar gain control sensitivity knob if positioned against the stop...

Attributes
ACN	585441
Time
Date	200306
Day	Fri
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : zzz.airport
State Reference	US
Altitude	msl single value : 31000
Environment
Weather Elements	Thunderstorm
Aircraft 1
Controlling Facilities	artcc : zzz.artcc
Operator	common carrier : air carrier
Make Model Name	MD-80 Super 80
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : multi engine pilot : instrument
ASRS Report	585441
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	aircraft equipment problem : critical other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : radar ctl panel other flight crewa other flight crewb
Resolutory Action	none taken : detected after the fact
Consequence	other Other
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft

Narrative:

I think what I am going to tell you is a 'recipe for disaster' if it is not taken care of. The 'gain' control knob on the radar control head of aircraft X is broken. But there is a catch. You can only detect it if you turn the knob all the way into the 'automatic' position. If the knob is not up against the stop of its travel in the 'automatic' position you will not notice any problem and it will operate normally. What is happening is that when you take the 'gain' control knob and turn it all the way into the automatic position, up against its mechanical stop, the 'gain' or sensitivity of the radar will transition from the automatic 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 pilots catch the 'var' when the radar is not operating in the automatic 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 control knob was already in this state of min sensitivity when we turned the radar on altitude. 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 reporter revealed the following information: the reporter stated this anomaly is common to all MD80 aircraft. The reporter describes the anomaly on the radar control panel sensitivity knob as when in automatic mode and turned full up to the mechanical stop the radar sensitivity transitions to min sensitivity. The reporter said the only warning is a 'var' on the radar display and is easily missed. The reporter stated this is not a radar failure but a design fault that is not easy to detect.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

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.

An MD80 first officer reports the radar gain control sensitivity knob if positioned against the stop in automatic mode will transition to min sensitivity. Min sensitivity will not paint storm cells.