Narrative:

During the taxi out flight control check; I discovered that the rudder unrestricted light went out when I moved the rudder to the right. I asked the first officer to call operations and let them know we needed to return to the gate and have maintenance come out. I then had to explain to the first officer the significance of the light failing to remaining illuminated during the test. It was no reflection on him that he wasn't aware of this -- I haven't found an first officer yet that was. It's not in the operations manuals. There is no requirement to observe the operation of the light as you move the rudder pedals during the flight control check. In a recurrent systems class I took sometime back; the instructor wasn't aware of this possible malfunction; although I recall one other captain in the class knew about this problem. When I've talked to the techs they are well aware of this malfunction. Basically; what I've been able to glean from the number of times this malfunction has occurred is that; as you move the rudder out of the neutral position toward the stops; the electrical signal runs through multiple sensors. If a sensor is bad; or if grime has found its way into a canon plug and caused an open on one of the pins; or if a mechanical target is out of alignment; the light will go out. Sometimes the light will flicker while doing the test. In that case you can slowly move the rudder until you find the dead spot at which time the rudder unrestricted light will go out. The worst case I've found is when the light extinguished when moving the rudder out of neutral in either direction. I asked the mechanic later what he found wrong with the system and he gave me the specific degrees out of calibration. Basically the rudder had no limiting capacity anywhere close to the 2.5% - 22% travel restrictions. The MD80's are fast becoming old airplanes. Line pilots are beginning to see mechanical problems that were not very prevalent in the past. The rudder unrestricted system is fairly complicated. The most recent rudder problem was caused by a white corrosion found on one of the electrical connections to a sensor. My suggestion is that additional information be added in the MD80 flight control section under 'rudder throw limiter;' explaining the function of the rudder unrestricted light during the flight control check; and a note be added to OM taxi/takeoff under 'flight controls;' 'during the rudder travel check ensure that the rudder unrestricted light remains illuminated at all times.' it is my belief that most pilots do not check this light during the flight control check. They have never been told the need to. Malfunctions of this system are pernicious in nature. Although large rudder inputs are not used in-flight except in emergent occasions; when they are used; if the rudder restricting system does not work properly one emergent occasion can lead to a far worse one.callback conversation with reporter revealed the following information: the rudder system is a mechanical; cable operated system. The 'rudder unrestricted' light indicates that full rudder travel is available. When airspeed increases to approximately 175 KTS; rudder travel is reduced. The pre-takeoff rudder check presently calls for full rudder pedal deflection to check for freedom of movement; but there is no emphasis placed on monitoring the 'rudder unrestricted' light during this check. The reporter stated that he has experienced the light indication extinguishing on 4 occasions when performing the preflight rudder check. There is a requirement to check the light on final approach to confirm full rudder authority. If the light is not illuminated; the aircraft crosswind limitation is reduced.

Google
 

Original NASA ASRS Text

Title: AN MD80 CAPT NOTES THAT A PRE-TAKEOFF RUDDER CHECK. DOES NOT REQUIRE OBSERVATION OF THE 'FULL RUDDER AUTHORITY' LIGHT; AND THAT A RUDDER CONTROL MALFUNCTION MIGHT THUS GO UNDETECTED.

Narrative: DURING THE TAXI OUT FLT CTL CHK; I DISCOVERED THAT THE RUDDER UNRESTRICTED LIGHT WENT OUT WHEN I MOVED THE RUDDER TO THE RIGHT. I ASKED THE FO TO CALL OPS AND LET THEM KNOW WE NEEDED TO RETURN TO THE GATE AND HAVE MAINT COME OUT. I THEN HAD TO EXPLAIN TO THE FO THE SIGNIFICANCE OF THE LIGHT FAILING TO REMAINING ILLUMINATED DURING THE TEST. IT WAS NO REFLECTION ON HIM THAT HE WASN'T AWARE OF THIS -- I HAVEN'T FOUND AN FO YET THAT WAS. IT'S NOT IN THE OPERATIONS MANUALS. THERE IS NO REQUIREMENT TO OBSERVE THE OPERATION OF THE LIGHT AS YOU MOVE THE RUDDER PEDALS DURING THE FLT CTL CHK. IN A RECURRENT SYSTEMS CLASS I TOOK SOMETIME BACK; THE INSTRUCTOR WASN'T AWARE OF THIS POSSIBLE MALFUNCTION; ALTHOUGH I RECALL ONE OTHER CAPT IN THE CLASS KNEW ABOUT THIS PROB. WHEN I'VE TALKED TO THE TECHS THEY ARE WELL AWARE OF THIS MALFUNCTION. BASICALLY; WHAT I'VE BEEN ABLE TO GLEAN FROM THE NUMBER OF TIMES THIS MALFUNCTION HAS OCCURRED IS THAT; AS YOU MOVE THE RUDDER OUT OF THE NEUTRAL POSITION TOWARD THE STOPS; THE ELECTRICAL SIGNAL RUNS THROUGH MULTIPLE SENSORS. IF A SENSOR IS BAD; OR IF GRIME HAS FOUND ITS WAY INTO A CANON PLUG AND CAUSED AN OPEN ON ONE OF THE PINS; OR IF A MECHANICAL TARGET IS OUT OF ALIGNMENT; THE LIGHT WILL GO OUT. SOMETIMES THE LIGHT WILL FLICKER WHILE DOING THE TEST. IN THAT CASE YOU CAN SLOWLY MOVE THE RUDDER UNTIL YOU FIND THE DEAD SPOT AT WHICH TIME THE RUDDER UNRESTRICTED LIGHT WILL GO OUT. THE WORST CASE I'VE FOUND IS WHEN THE LIGHT EXTINGUISHED WHEN MOVING THE RUDDER OUT OF NEUTRAL IN EITHER DIRECTION. I ASKED THE MECHANIC LATER WHAT HE FOUND WRONG WITH THE SYSTEM AND HE GAVE ME THE SPECIFIC DEGREES OUT OF CALIBRATION. BASICALLY THE RUDDER HAD NO LIMITING CAPACITY ANYWHERE CLOSE TO THE 2.5% - 22% TRAVEL RESTRICTIONS. THE MD80'S ARE FAST BECOMING OLD AIRPLANES. LINE PLTS ARE BEGINNING TO SEE MECHANICAL PROBLEMS THAT WERE NOT VERY PREVALENT IN THE PAST. THE RUDDER UNRESTRICTED SYSTEM IS FAIRLY COMPLICATED. THE MOST RECENT RUDDER PROB WAS CAUSED BY A WHITE CORROSION FOUND ON ONE OF THE ELECTRICAL CONNECTIONS TO A SENSOR. MY SUGGESTION IS THAT ADDITIONAL INFO BE ADDED IN THE MD80 FLT CTL SECTION UNDER 'RUDDER THROW LIMITER;' EXPLAINING THE FUNCTION OF THE RUDDER UNRESTRICTED LIGHT DURING THE FLT CTL CHK; AND A NOTE BE ADDED TO OM TAXI/TAKEOFF UNDER 'FLT CONTROLS;' 'DURING THE RUDDER TRAVEL CHK ENSURE THAT THE RUDDER UNRESTRICTED LIGHT REMAINS ILLUMINATED AT ALL TIMES.' IT IS MY BELIEF THAT MOST PLTS DO NOT CHK THIS LIGHT DURING THE FLT CTL CHK. THEY HAVE NEVER BEEN TOLD THE NEED TO. MALFUNCTIONS OF THIS SYSTEM ARE PERNICIOUS IN NATURE. ALTHOUGH LARGE RUDDER INPUTS ARE NOT USED INFLT EXCEPT IN EMERGENT OCCASIONS; WHEN THEY ARE USED; IF THE RUDDER RESTRICTING SYSTEM DOES NOT WORK PROPERLY ONE EMERGENT OCCASION CAN LEAD TO A FAR WORSE ONE.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RUDDER SYSTEM IS A MECHANICAL; CABLE OPERATED SYSTEM. THE 'RUDDER UNRESTRICTED' LIGHT INDICATES THAT FULL RUDDER TRAVEL IS AVAILABLE. WHEN AIRSPEED INCREASES TO APPROXIMATELY 175 KTS; RUDDER TRAVEL IS REDUCED. THE PRE-TKOF RUDDER CHECK PRESENTLY CALLS FOR FULL RUDDER PEDAL DEFLECTION TO CHECK FOR FREEDOM OF MOVEMENT; BUT THERE IS NO EMPHASIS PLACED ON MONITORING THE 'RUDDER UNRESTRICTED' LIGHT DURING THIS CHECK. THE REPORTER STATED THAT HE HAS EXPERIENCED THE LIGHT INDICATION EXTINGUISHING ON 4 OCCASIONS WHEN PERFORMING THE PREFLIGHT RUDDER CHECK. THERE IS A REQUIREMENT TO CHECK THE LIGHT ON FINAL APCH TO CONFIRM FULL RUDDER AUTHORITY. IF THE LIGHT IS NOT ILLUMINATED; THE ACFT CROSSWIND LIMITATION IS REDUCED.

Data retrieved from NASA's ASRS site as of May 2009 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.