Narrative:

In clouds with autoplt engaged and tracking runway 25L localizer in LNAV. All is well, but gaate approach fix is not in FMS, so first officer selects ILS mode on ehsi for ILS DME. About 2 NM later, aircraft begins a right turn away from localizer (toward other runways and aircraft). As PF, I switch to heading select mode of autoplt and correct back. While on course, map shows an increasing track deviation away from course. At FAF (based on DME), the map/FMC shows us deviating 10 degrees off course and still 3 mi to go. Stopped on ground on taxiway M between runways 25L&right, map/FMC shows us 1.3 NM out on final, -3036 ft on VNAV path, and a ground speed of 20 KTS. At gate xx, last FMS position is 3 mins off in longitude from commercial chart coordinates. Per normal operation, FMC finally dropped out of navigation mode awaiting new route. All operations normal on return flight to phx. For some reason, this aircraft FMS radio updated away from course at a critical point in our flight. The only 'alert' of this malfunction was our situational awareness. Callback conversation with reporter revealed the following information: the reporter was flying a B757-200 when it turned off of the ILS course. He made an entry in the maintenance log concerning an FMS error, but the maintenance personnel were unable to duplicate the incident. His chief pilot later informed him, according to the reporter, that the lax controllers had admitted that both the runway 25L and the runway 7R ILS transmitters were operating at the same time and therefore the FMS probably updated by using the DME and localizer signal from the nearest transmitter, that of runway 7R. The captain said that his company's standard practice is to have both pilots select map mode for the approach. This prevents the FMS from updating its position. Requiring the PNF to tune the ILS frequency and track it in the ILS mode allows the FMS to use automatic tuning to vors in the area while tracking the ILS's raw data. The other NAVAID's data then continuously updates the FMS's position. The chief pilot, allegedly told the captain that the company's approach policy was under review.

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Original NASA ASRS Text

Title: AN ACR B757 FLC HAS THEIR ACFT TURN OFF COURSE DURING AN ILS APCH TO RWY 25L AT LAX. THE FLC RECOGNIZED THE DEV AND CORRECTED. LATER, THE FLC FOUND THAT THERE HAD BEEN A CHANGE IN THE APCH DIRECTION AND ALTHOUGH THE NEW ILS WAS OPERATING, APPARENTLY SO WAS THE OLD XMITTER.

Narrative: IN CLOUDS WITH AUTOPLT ENGAGED AND TRACKING RWY 25L LOC IN LNAV. ALL IS WELL, BUT GAATE APCH FIX IS NOT IN FMS, SO FO SELECTS ILS MODE ON EHSI FOR ILS DME. ABOUT 2 NM LATER, ACFT BEGINS A R TURN AWAY FROM LOC (TOWARD OTHER RWYS AND ACFT). AS PF, I SWITCH TO HDG SELECT MODE OF AUTOPLT AND CORRECT BACK. WHILE ON COURSE, MAP SHOWS AN INCREASING TRACK DEV AWAY FROM COURSE. AT FAF (BASED ON DME), THE MAP/FMC SHOWS US DEVIATING 10 DEGS OFF COURSE AND STILL 3 MI TO GO. STOPPED ON GND ON TXWY M BTWN RWYS 25L&R, MAP/FMC SHOWS US 1.3 NM OUT ON FINAL, -3036 FT ON VNAV PATH, AND A GND SPD OF 20 KTS. AT GATE XX, LAST FMS POS IS 3 MINS OFF IN LONGITUDE FROM COMMERCIAL CHART COORDINATES. PER NORMAL OP, FMC FINALLY DROPPED OUT OF NAV MODE AWAITING NEW RTE. ALL OPS NORMAL ON RETURN FLT TO PHX. FOR SOME REASON, THIS ACFT FMS RADIO UPDATED AWAY FROM COURSE AT A CRITICAL POINT IN OUR FLT. THE ONLY 'ALERT' OF THIS MALFUNCTION WAS OUR SITUATIONAL AWARENESS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR WAS FLYING A B757-200 WHEN IT TURNED OFF OF THE ILS COURSE. HE MADE AN ENTRY IN THE MAINT LOG CONCERNING AN FMS ERROR, BUT THE MAINT PERSONNEL WERE UNABLE TO DUPLICATE THE INCIDENT. HIS CHIEF PLT LATER INFORMED HIM, ACCORDING TO THE RPTR, THAT THE LAX CTLRS HAD ADMITTED THAT BOTH THE RWY 25L AND THE RWY 7R ILS XMITTERS WERE OPERATING AT THE SAME TIME AND THEREFORE THE FMS PROBABLY UPDATED BY USING THE DME AND LOC SIGNAL FROM THE NEAREST XMITTER, THAT OF RWY 7R. THE CAPT SAID THAT HIS COMPANY'S STANDARD PRACTICE IS TO HAVE BOTH PLTS SELECT MAP MODE FOR THE APCH. THIS PREVENTS THE FMS FROM UPDATING ITS POS. REQUIRING THE PNF TO TUNE THE ILS FREQ AND TRACK IT IN THE ILS MODE ALLOWS THE FMS TO USE AUTOMATIC TUNING TO VORS IN THE AREA WHILE TRACKING THE ILS'S RAW DATA. THE OTHER NAVAID'S DATA THEN CONTINUOUSLY UPDATES THE FMS'S POS. THE CHIEF PLT, ALLEGEDLY TOLD THE CAPT THAT THE COMPANY'S APCH POLICY WAS UNDER REVIEW.

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.