Narrative:

While descending into lax on the mitts 2 arrival; ATC advised us we were low on the profile. We were showing exactly on profile using GPS. Lax DME showed a discrepancy with GPS. It appeared to us that the GPS was 'anticipating' crossing a fix and caused us to step down early. Once ATC informed us of the situation; it was immediately recognized and corrected. Recommendations: 1) fatigue played a role. We were forced to deadhead (all night) to departure city; then fly to lax. Long duty day with no rest. Approximately over 12 hours duty -- all night flying. Operating trip from rest (hotel) would have helped. 2) putting a procedural note on STAR not to use GPS to identify stepdown fixes. Although STAR uses localizer DME to identify fixes; it is very convenient to overlook DME and use GPS. Subsequent arrs have shown nearly 3 NM difference between GPS and DME. This could be very serious if not addressed. Callback conversation with reporter revealed the following information: reporter stated that the A300-B4 aircraft is first generation airbus and has no FMC. The GPS installed is an after market system that couples with the autoplt. The aircraft manual calls this particular navigation system an 'en route navigation system.' the reporter is not certain that it is intended for use on arrs. He has seen this same behavior at dal and believes that by design the GPS anticipates fixes based on time/speed to the fix. This same system will begin a +1G dive for a new altitude selected during the altitude capture phase. This freight air carrier deadheads crews from east to west or west to east and that crew then flies the return trip to the opposite coast that same night. He believes that the type of flying his company does affects the crew's alertness.

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Original NASA ASRS Text

Title: AN A300 PLT WAS ADVISED BY ATC OF A 3 NM XING RESTR ALT ERROR. THE AUTOPLT WAS FLYING GPS INPUTS WHILE DME WAS ACCURATE.

Narrative: WHILE DSNDING INTO LAX ON THE MITTS 2 ARR; ATC ADVISED US WE WERE LOW ON THE PROFILE. WE WERE SHOWING EXACTLY ON PROFILE USING GPS. LAX DME SHOWED A DISCREPANCY WITH GPS. IT APPEARED TO US THAT THE GPS WAS 'ANTICIPATING' XING A FIX AND CAUSED US TO STEP DOWN EARLY. ONCE ATC INFORMED US OF THE SIT; IT WAS IMMEDIATELY RECOGNIZED AND CORRECTED. RECOMMENDATIONS: 1) FATIGUE PLAYED A ROLE. WE WERE FORCED TO DEADHEAD (ALL NIGHT) TO DEP CITY; THEN FLY TO LAX. LONG DUTY DAY WITH NO REST. APPROX OVER 12 HRS DUTY -- ALL NIGHT FLYING. OPERATING TRIP FROM REST (HOTEL) WOULD HAVE HELPED. 2) PUTTING A PROCEDURAL NOTE ON STAR NOT TO USE GPS TO IDENT STEPDOWN FIXES. ALTHOUGH STAR USES LOC DME TO IDENT FIXES; IT IS VERY CONVENIENT TO OVERLOOK DME AND USE GPS. SUBSEQUENT ARRS HAVE SHOWN NEARLY 3 NM DIFFERENCE BTWN GPS AND DME. THIS COULD BE VERY SERIOUS IF NOT ADDRESSED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THE A300-B4 ACFT IS FIRST GENERATION AIRBUS AND HAS NO FMC. THE GPS INSTALLED IS AN AFTER MARKET SYS THAT COUPLES WITH THE AUTOPLT. THE ACFT MANUAL CALLS THIS PARTICULAR NAV SYS AN 'ENRTE NAV SYS.' THE RPTR IS NOT CERTAIN THAT IT IS INTENDED FOR USE ON ARRS. HE HAS SEEN THIS SAME BEHAVIOR AT DAL AND BELIEVES THAT BY DESIGN THE GPS ANTICIPATES FIXES BASED ON TIME/SPD TO THE FIX. THIS SAME SYS WILL BEGIN A +1G DIVE FOR A NEW ALT SELECTED DURING THE ALT CAPTURE PHASE. THIS FREIGHT ACR DEADHEADS CREWS FROM E TO W OR W TO E AND THAT CREW THEN FLIES THE RETURN TRIP TO THE OPPOSITE COAST THAT SAME NIGHT. HE BELIEVES THAT THE TYPE OF FLYING HIS COMPANY DOES AFFECTS THE CREW'S ALERTNESS.

Data retrieved from NASA's ASRS site as of January 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.