B737-300 crew had the FMC position updating DME's lockup on 108 pt 0, allowing the required navigati...

Attributes
ACN	602136
Time
Date	200312
Day	Sun
Place
Locale Reference	airport : las.airport
State Reference	NV
Environment
Flight Conditions	VMC
Aircraft 1
Controlling Facilities	tracon : l30.tracon
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Flight Phase	descent : approach descent : intermediate altitude
Route In Use	arrival star : tyssn
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
ASRS Report	602136
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	aircraft equipment problem : less severe
Independent Detector	aircraft equipment other aircraft equipment : fmc legs page other flight crewa
Resolutory Action	none taken : unable
Consequence	other
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft

Narrative:

LNAV glitch noted several times, often in vicinity las: one or both VOR's will lock onto invalid VOR frequency, 108.0, while in 'automatic,' and stay there. If both VOR's do this at same time, the 'rnp (required navigation performance)/actual' (as shown on the 'legs' page) will start to increase, sometimes exceeding 2.00, which then obligates pilots to discontinue the RNAV approach or departure. Suspect this occurs because of a lack of 'good' VOR's in the area, especially at lower altitudes, combined with strong radio signal on that frequency, from local radio station. Usually can get this to clear by cycling from 'navigation' to 'VOR/ILS' and back on the HSI switch which seems to reinitiate the automatic search mode. Easy to miss unless monitoring 'rnp/actual' or if flying an older aircraft with audio bleedover from VOR into communication system. Not sure this is a problem in '-700' aircraft with GPS. Suspect the 'fix' for this is not easy or cheap. Suggest pilots are made aware of the issue.

Original NASA ASRS Text

Title: B737-300 CREW HAD THE FMC POS UPDATING DME'S LOCKUP ON 108 PT 0, ALLOWING THE REQUIRED NAV PERFORMANCE 'RNP' TO GO ABOVE THE LIMIT FOR RNAV OP. THIS OCCURS IN THE LAS AREA.

Narrative: LNAV GLITCH NOTED SEVERAL TIMES, OFTEN IN VICINITY LAS: ONE OR BOTH VOR'S WILL LOCK ONTO INVALID VOR FREQ, 108.0, WHILE IN 'AUTO,' AND STAY THERE. IF BOTH VOR'S DO THIS AT SAME TIME, THE 'RNP (REQUIRED NAV PERFORMANCE)/ACTUAL' (AS SHOWN ON THE 'LEGS' PAGE) WILL START TO INCREASE, SOMETIMES EXCEEDING 2.00, WHICH THEN OBLIGATES PLTS TO DISCONTINUE THE RNAV APCH OR DEP. SUSPECT THIS OCCURS BECAUSE OF A LACK OF 'GOOD' VOR'S IN THE AREA, ESPECIALLY AT LOWER ALTS, COMBINED WITH STRONG RADIO SIGNAL ON THAT FREQ, FROM LCL RADIO STATION. USUALLY CAN GET THIS TO CLR BY CYCLING FROM 'NAV' TO 'VOR/ILS' AND BACK ON THE HSI SWITCH WHICH SEEMS TO REINITIATE THE AUTO SEARCH MODE. EASY TO MISS UNLESS MONITORING 'RNP/ACTUAL' OR IF FLYING AN OLDER ACFT WITH AUDIO BLEEDOVER FROM VOR INTO COM SYS. NOT SURE THIS IS A PROB IN '-700' ACFT WITH GPS. SUSPECT THE 'FIX' FOR THIS IS NOT EASY OR CHEAP. SUGGEST PLTS ARE MADE AWARE OF THE ISSUE.

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.

B737-300 crew had the FMC position updating DME's lockup on 108 pt 0, allowing the required navigation performance 'rnp' to go above the limit for RNAV operation. This occurs in the las area.