Flight crew of B737-300 report autoflt LNAV function disengaged during boach RNAV SID from las. Airc...

Attributes
ACN	691803
Time
Date	200603
Local Time Of Day	0601 To 1200
Place
Locale Reference	intersection : rbell
State Reference	NV
Altitude	msl single value : 3267
Environment
Flight Conditions	VMC
Light	Daylight
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	climbout : initial
Route In Use	departure sid : roach
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Experience	flight time last 90 days : 97 flight time total : 15000 flight time type : 7000
ASRS Report	691803
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Experience	flight time last 90 days : 220 flight time total : 220 flight time type : 220
ASRS Report	691801
Events
Anomaly	aircraft equipment problem : critical non adherence : published procedure non adherence : clearance other spatial deviation
Independent Detector	other flight crewa other flight crewb other other : 3
Resolutory Action	controller : issued alert flight crew : overrode automation flight crew : returned to intended or assigned course flight crew : overcame equipment problem
Supplementary
Problem Areas	ATC Human Performance Aircraft Chart Or Publication Flight Crew Human Performance
Primary Problem	Ambiguous

Narrative:

Departing las on the boach 2 RNAV departure climbing through 1200 ft AGL with LNAV and autoplt engaged; the LNAV dropped off and the autoplt started a right turn where the heading bug was selected. First officer was flying and I instructed him to disengage autoplt and turn back on course. ATC noticed our turn and we advised him our LNAV dropped off and we accepted a left turn to recapture the departure. While established on climb I was given a telephone number to contact las departure as soon as possible. I contacted ATC via my cell once we landed. I was advised by las ATC that we (company) seem to have a significant number of departures dropping off course as we had experienced. I apologized for any inconvenience or disruption we may have caused. I told him I would take this up with our air carrier and would also be filing a report. He was familiar with the reports. When turning over the aircraft over to the new crew; I advised them of our event and to be cognizant of the same.supplemental information from acn 691801: as the captain and I debriefed what had happened we tried to come to some sort of conclusion what had happened. She thought maybe the aircraft was flying to what was selected on the heading bug once the LNAV disengaged. Although a good guess; I don't believe we can be sure. We did not see if the LNAV push tile was de-illuminated nor did we see the hsel push tile illuminate when the LNAV dropped off. We also did not let the aircraft go far enough to see if it would roll out on the selected heading.

Original NASA ASRS Text

Title: FLT CREW OF B737-300 RPT AUTOFLT LNAV FUNCTION DISENGAGED DURING BOACH RNAV SID FROM LAS. ACFT APPEARED TO BE TURNING TO A HDG IN THE OPPOSITE DIRECTION OF THE SID TRACK. MANUALLY INTERVENED AND CORRECTED.

Narrative: DEPARTING LAS ON THE BOACH 2 RNAV DEP CLBING THROUGH 1200 FT AGL WITH LNAV AND AUTOPLT ENGAGED; THE LNAV DROPPED OFF AND THE AUTOPLT STARTED A R TURN WHERE THE HDG BUG WAS SELECTED. FO WAS FLYING AND I INSTRUCTED HIM TO DISENGAGE AUTOPLT AND TURN BACK ON COURSE. ATC NOTICED OUR TURN AND WE ADVISED HIM OUR LNAV DROPPED OFF AND WE ACCEPTED A L TURN TO RECAPTURE THE DEP. WHILE ESTABLISHED ON CLB I WAS GIVEN A TELEPHONE NUMBER TO CONTACT LAS DEP ASAP. I CONTACTED ATC VIA MY CELL ONCE WE LANDED. I WAS ADVISED BY LAS ATC THAT WE (COMPANY) SEEM TO HAVE A SIGNIFICANT NUMBER OF DEPS DROPPING OFF COURSE AS WE HAD EXPERIENCED. I APOLOGIZED FOR ANY INCONVENIENCE OR DISRUPTION WE MAY HAVE CAUSED. I TOLD HIM I WOULD TAKE THIS UP WITH OUR ACR AND WOULD ALSO BE FILING A RPT. HE WAS FAMILIAR WITH THE RPTS. WHEN TURNING OVER THE ACFT OVER TO THE NEW CREW; I ADVISED THEM OF OUR EVENT AND TO BE COGNIZANT OF THE SAME.SUPPLEMENTAL INFO FROM ACN 691801: AS THE CAPT AND I DEBRIEFED WHAT HAD HAPPENED WE TRIED TO COME TO SOME SORT OF CONCLUSION WHAT HAD HAPPENED. SHE THOUGHT MAYBE THE ACFT WAS FLYING TO WHAT WAS SELECTED ON THE HDG BUG ONCE THE LNAV DISENGAGED. ALTHOUGH A GOOD GUESS; I DON'T BELIEVE WE CAN BE SURE. WE DID NOT SEE IF THE LNAV PUSH TILE WAS DE-ILLUMINATED NOR DID WE SEE THE HSEL PUSH TILE ILLUMINATE WHEN THE LNAV DROPPED OFF. WE ALSO DID NOT LET THE ACFT GO FAR ENOUGH TO SEE IF IT WOULD ROLL OUT ON THE SELECTED HDG.

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.

Flight crew of B737-300 report autoflt LNAV function disengaged during boach RNAV SID from las. Aircraft appeared to be turning to a heading in the opposite direction of the SID track. Manually intervened and corrected.