Narrative:

We were at cruise altitude and were told to go direct to hpw. We did; then without any cockpit indications the left seat FMS and right seat FMS dumped the argal 4 arrival and went from hpw direct to rdu. When the aircraft passed hpw is when we think the FMS's dumped the arrival. Then ATC called us and said you are off course and to turn 30 degrees left. We did. Neither myself; the first officer or the jumpseater could figure out why the FMS dumped the arrival. We were then told by ATC to go direct greki. Then after greki to duffi and resume the argal 4 arrival. However my FMS did not show the argal 4 in the FMS flight plan arrival page anymore; but it was in the flight plan. We could not figure out why this happened as well. I then sat there and watched the first officer's FMS add a ground track without anyone touching the FMS's. This confused us even more. There were 3 highly experienced emb pilots in the cockpit and we had never seen these FMS anomalies before. As a result of both FMS's doing things that none of us in the cockpit could explain or understand; I elected to inform ATC of the FMS -- GPS problems and informed them that we felt the FMS's were unreliable. We then converted to VOR navigation and requested ATC to give us vectors and monitor our flight path more closely. We as a crew had lost confidence in the aircraft's FMS -- GPS system. We as a crew of 3 highly qualified and experienced emb pilots tried to correct the FMS's but they both kept on creating uncommanded ground tracks. We all sat and watched the first officer's FMS add a waypoint from duham which is a holding fix for the ILS runway 23R in rdu to greki without anyone touching an FMS key. The only thing I can think of is the fact that the aircraft had 4 avionics related MEL's. This might have caused some other avionics problem that caused both FMS's to do things that over 25000 hours of experience in aviation between all 3 of us in the cockpit could not explain. Make every attempt to repair aircraft with multiple avionics issues before sending it into the most congested complex airspace in the world. Operating this highly automated aircraft in a degraded mode lowers the safety margin in an environment where there is very little if any room for error. We had no TCAS; no radio altimeter; no egpws; no terrain alert. We were legally dispatchable according to MEL 37-10 to fly in the most congested airspace in the world without any of this critical safety equipment. This creates a situation were the airline's operational risk management practices are questionable.

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Original NASA ASRS Text

Title: EMB135 FO REPORTS STRANGE FMC MALFUNCTION DURING ARGAL ARRIVAL TO RDU CAUSING TRACK DEV.

Narrative: WE WERE AT CRUISE ALT AND WERE TOLD TO GO DIRECT TO HPW. WE DID; THEN WITHOUT ANY COCKPIT INDICATIONS THE L SEAT FMS AND R SEAT FMS DUMPED THE ARGAL 4 ARR AND WENT FROM HPW DIRECT TO RDU. WHEN THE ACFT PASSED HPW IS WHEN WE THINK THE FMS'S DUMPED THE ARR. THEN ATC CALLED US AND SAID YOU ARE OFF COURSE AND TO TURN 30 DEGS L. WE DID. NEITHER MYSELF; THE FO OR THE JUMPSEATER COULD FIGURE OUT WHY THE FMS DUMPED THE ARR. WE WERE THEN TOLD BY ATC TO GO DIRECT GREKI. THEN AFTER GREKI TO DUFFI AND RESUME THE ARGAL 4 ARR. HOWEVER MY FMS DID NOT SHOW THE ARGAL 4 IN THE FMS FLT PLAN ARR PAGE ANYMORE; BUT IT WAS IN THE FLT PLAN. WE COULD NOT FIGURE OUT WHY THIS HAPPENED AS WELL. I THEN SAT THERE AND WATCHED THE FO'S FMS ADD A GND TRACK WITHOUT ANYONE TOUCHING THE FMS'S. THIS CONFUSED US EVEN MORE. THERE WERE 3 HIGHLY EXPERIENCED EMB PLTS IN THE COCKPIT AND WE HAD NEVER SEEN THESE FMS ANOMALIES BEFORE. AS A RESULT OF BOTH FMS'S DOING THINGS THAT NONE OF US IN THE COCKPIT COULD EXPLAIN OR UNDERSTAND; I ELECTED TO INFORM ATC OF THE FMS -- GPS PROBS AND INFORMED THEM THAT WE FELT THE FMS'S WERE UNRELIABLE. WE THEN CONVERTED TO VOR NAV AND REQUESTED ATC TO GIVE US VECTORS AND MONITOR OUR FLT PATH MORE CLOSELY. WE AS A CREW HAD LOST CONFIDENCE IN THE ACFT'S FMS -- GPS SYS. WE AS A CREW OF 3 HIGHLY QUALIFIED AND EXPERIENCED EMB PLTS TRIED TO CORRECT THE FMS'S BUT THEY BOTH KEPT ON CREATING UNCOMMANDED GND TRACKS. WE ALL SAT AND WATCHED THE FO'S FMS ADD A WAYPOINT FROM DUHAM WHICH IS A HOLDING FIX FOR THE ILS RWY 23R IN RDU TO GREKI WITHOUT ANYONE TOUCHING AN FMS KEY. THE ONLY THING I CAN THINK OF IS THE FACT THAT THE ACFT HAD 4 AVIONICS RELATED MEL'S. THIS MIGHT HAVE CAUSED SOME OTHER AVIONICS PROB THAT CAUSED BOTH FMS'S TO DO THINGS THAT OVER 25000 HRS OF EXPERIENCE IN AVIATION BTWN ALL 3 OF US IN THE COCKPIT COULD NOT EXPLAIN. MAKE EVERY ATTEMPT TO REPAIR ACFT WITH MULTIPLE AVIONICS ISSUES BEFORE SENDING IT INTO THE MOST CONGESTED COMPLEX AIRSPACE IN THE WORLD. OPERATING THIS HIGHLY AUTOMATED ACFT IN A DEGRADED MODE LOWERS THE SAFETY MARGIN IN AN ENVIRONMENT WHERE THERE IS VERY LITTLE IF ANY ROOM FOR ERROR. WE HAD NO TCAS; NO RADIO ALTIMETER; NO EGPWS; NO TERRAIN ALERT. WE WERE LEGALLY DISPATCHABLE ACCORDING TO MEL 37-10 TO FLY IN THE MOST CONGESTED AIRSPACE IN THE WORLD WITHOUT ANY OF THIS CRITICAL SAFETY EQUIP. THIS CREATES A SITUATION WERE THE AIRLINE'S OPERATIONAL RISK MGMNT PRACTICES ARE QUESTIONABLE.

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.