B737-300 crew had a track deviation departing las on an RNAV departure.

Attributes
ACN	611175
Time
Date	200403
Day	Tue
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : las.airport
State Reference	NV
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : l30.tracon tower : fll.tower
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Flight Phase	climbout : initial climbout : intermediate altitude
Route In Use	departure : on vectors departure sid : boach
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 150 flight time total : 11000 flight time type : 7500
ASRS Report	611175
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	aircraft equipment problem : less severe non adherence : far non adherence : published procedure other spatial deviation
Independent Detector	other flight crewa other flight crewb
Resolutory Action	controller : issued new clearance flight crew : overrode automation flight crew : became reoriented flight crew : overcame equipment problem
Consequence	other
Supplementary
Problem Areas	FAA Company Aircraft
Primary Problem	Company
Air Traffic Incident	Pilot Deviation

Narrative:

LNAV departure on the baoch 1 out of las. All switches set correctly for takeoff and runway update accomplished. CDI was centered on takeoff roll. First officer was PF. At about 1000 ft AGL with the LNAV engaged, the first officer selected autoplt and aircraft turned left although the first point, rebell, I believe, was supposed to be on the nose. With both switches in navigation, there was no way to back up the LNAV. The autoplt was disengaged and we continued on a heading of about 230 degrees. I selected direct to the fix on the CDU and it was still commanding to turn further left. I then noticed the required navigation performance was 1.0 and the actual navigation performance was 5.65. By this time, we were heading in a southwest direction and I informed ATC our LNAV was out of limits and we needed vectors. We were assigned a heading and the mccarran departure to hec. About 30 seconds later the unable required navigation performance message appeared on the CDU. Within 2 mins or so, the actual navigation performance came back down to less than 1 NM, well within limits. ATC inquired as to what happened on the initial turn and I told them we had a sudden problem with the LNAV on the departure. There were no traffic conflicts. We continued the remainder of the day and had no further problems with the LNAV system. Maintenance was informed of the problem when we landed at the end of the day. We concluded it was probably a hiccup with the LNAV computer. I asked the first officer if he was sure he did the runway update and he assured me that he did. I would like to note it was probably 2 mins before the message on the CDU said the actual navigation performance was out of limits. We were VMC, so it wasn't a real problem, but if we had been in the WX, with no backup to the LNAV departure, it could have been a bit more disconcerting. It took a while to figure out exactly what the problem was as it is not normal to look at the bottom of the legs page to check the actual navigation performance and required navigation performance.

Original NASA ASRS Text

Title: B737-300 CREW HAD A TRACK DEV DEPARTING LAS ON AN RNAV DEP.

Narrative: LNAV DEP ON THE BAOCH 1 OUT OF LAS. ALL SWITCHES SET CORRECTLY FOR TKOF AND RWY UPDATE ACCOMPLISHED. CDI WAS CTRED ON TKOF ROLL. FO WAS PF. AT ABOUT 1000 FT AGL WITH THE LNAV ENGAGED, THE FO SELECTED AUTOPLT AND ACFT TURNED L ALTHOUGH THE FIRST POINT, REBELL, I BELIEVE, WAS SUPPOSED TO BE ON THE NOSE. WITH BOTH SWITCHES IN NAV, THERE WAS NO WAY TO BACK UP THE LNAV. THE AUTOPLT WAS DISENGAGED AND WE CONTINUED ON A HDG OF ABOUT 230 DEGS. I SELECTED DIRECT TO THE FIX ON THE CDU AND IT WAS STILL COMMANDING TO TURN FURTHER L. I THEN NOTICED THE REQUIRED NAV PERFORMANCE WAS 1.0 AND THE ACTUAL NAV PERFORMANCE WAS 5.65. BY THIS TIME, WE WERE HEADING IN A SW DIRECTION AND I INFORMED ATC OUR LNAV WAS OUT OF LIMITS AND WE NEEDED VECTORS. WE WERE ASSIGNED A HDG AND THE MCCARRAN DEP TO HEC. ABOUT 30 SECONDS LATER THE UNABLE REQUIRED NAV PERFORMANCE MESSAGE APPEARED ON THE CDU. WITHIN 2 MINS OR SO, THE ACTUAL NAV PERFORMANCE CAME BACK DOWN TO LESS THAN 1 NM, WELL WITHIN LIMITS. ATC INQUIRED AS TO WHAT HAPPENED ON THE INITIAL TURN AND I TOLD THEM WE HAD A SUDDEN PROB WITH THE LNAV ON THE DEP. THERE WERE NO TFC CONFLICTS. WE CONTINUED THE REMAINDER OF THE DAY AND HAD NO FURTHER PROBS WITH THE LNAV SYS. MAINT WAS INFORMED OF THE PROB WHEN WE LANDED AT THE END OF THE DAY. WE CONCLUDED IT WAS PROBABLY A HICCUP WITH THE LNAV COMPUTER. I ASKED THE FO IF HE WAS SURE HE DID THE RWY UPDATE AND HE ASSURED ME THAT HE DID. I WOULD LIKE TO NOTE IT WAS PROBABLY 2 MINS BEFORE THE MESSAGE ON THE CDU SAID THE ACTUAL NAV PERFORMANCE WAS OUT OF LIMITS. WE WERE VMC, SO IT WASN'T A REAL PROB, BUT IF WE HAD BEEN IN THE WX, WITH NO BACKUP TO THE LNAV DEP, IT COULD HAVE BEEN A BIT MORE DISCONCERTING. IT TOOK A WHILE TO FIGURE OUT EXACTLY WHAT THE PROB WAS AS IT IS NOT NORMAL TO LOOK AT THE BOTTOM OF THE LEGS PAGE TO CHK THE ACTUAL NAV PERFORMANCE AND REQUIRED NAV PERFORMANCE.

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.