Narrative:

While inbound to hiney on the eagul 2 arrival; we encountered the wake turbulence of another aircraft. The autoplt fought against it and bank angles never exceeded more than 20 degrees. Unbeknownst to us; this disengaged the LNAV portion of the autoplt leaving us in cws roll and altitude hold. My AFDS panel was MEL'd inoperative so I was required to look across the cockpit all day to confirm that we were in the correct modes. The captain did not notice his panel annunciations change and neither did I. After passing hiney (I'm not sure how much time had passed); I looked down and noticed my CDI was deflected to the right. I confirmed that my navigation select switch was in navigation. I looked over at the captain's AFDS panel and noticed that LNAV had disengaged. I was just about to verbalize this when approach called us and said that it looked like we had missed our turn. The controller then assigned us a heading. I'm only assuming that the autoplt's reaction to the wake turbulence caused LNAV to disengage. But I am 100% positive it was engaged before the event because I remember the aircraft turning at the point prior to hiney. Obviously; extra vigilance is required by the pilot with the inoperative AFDS panel. I had been making a concerted effort all day to stay on top of this. Advise crews to check their AFDS panel if the autoplt is flying during a wake turbulence encounter; or simply click it off and hand fly the aircraft. Once everything is stabilized; then re-engage everything from the beginning. Also; try to keep the CDI in your crosscheck. It's really the first indication in a -300/-500 that you're wandering off course.supplemental information from acn 779663: on the eagul 2 arrival into phx my aircraft flew through wake turbulence. During the brief wake turbulence encounter I think I grabbed the control column momentarily. In looking back; I must have disengaged LNAV. Very shortly after this phx approach said we had passed a waypoint and assigned a heading. We were then cleared for a visual approach to runway 8. I should have scanned the cockpit better after the wake turbulence for any deviations. Both pilots should have scanned the instruments and annunciators after encountering wake turbulence. Aircraft went through wake turbulence and LNAV disengaged. Aircraft missed waypoint on eagul arrival.

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Original NASA ASRS Text

Title: B737 FLT CREW ENCOUNTERS WAKE TURBULENCE DURING ARR AND BUMPS AUTOPLT INTO CWS INADVERTENTLY. ACFT FAILS TO MAKE NEXT TURN ON ARR AND IS NOTICED BY ATC.

Narrative: WHILE INBOUND TO HINEY ON THE EAGUL 2 ARR; WE ENCOUNTERED THE WAKE TURBULENCE OF ANOTHER ACFT. THE AUTOPLT FOUGHT AGAINST IT AND BANK ANGLES NEVER EXCEEDED MORE THAN 20 DEGS. UNBEKNOWNST TO US; THIS DISENGAGED THE LNAV PORTION OF THE AUTOPLT LEAVING US IN CWS ROLL AND ALT HOLD. MY AFDS PANEL WAS MEL'D INOPERATIVE SO I WAS REQUIRED TO LOOK ACROSS THE COCKPIT ALL DAY TO CONFIRM THAT WE WERE IN THE CORRECT MODES. THE CAPT DID NOT NOTICE HIS PANEL ANNUNCIATIONS CHANGE AND NEITHER DID I. AFTER PASSING HINEY (I'M NOT SURE HOW MUCH TIME HAD PASSED); I LOOKED DOWN AND NOTICED MY CDI WAS DEFLECTED TO THE RIGHT. I CONFIRMED THAT MY NAV SELECT SWITCH WAS IN NAV. I LOOKED OVER AT THE CAPT'S AFDS PANEL AND NOTICED THAT LNAV HAD DISENGAGED. I WAS JUST ABOUT TO VERBALIZE THIS WHEN APCH CALLED US AND SAID THAT IT LOOKED LIKE WE HAD MISSED OUR TURN. THE CTLR THEN ASSIGNED US A HDG. I'M ONLY ASSUMING THAT THE AUTOPLT'S REACTION TO THE WAKE TURBULENCE CAUSED LNAV TO DISENGAGE. BUT I AM 100% POSITIVE IT WAS ENGAGED BEFORE THE EVENT BECAUSE I REMEMBER THE ACFT TURNING AT THE POINT PRIOR TO HINEY. OBVIOUSLY; EXTRA VIGILANCE IS REQUIRED BY THE PLT WITH THE INOPERATIVE AFDS PANEL. I HAD BEEN MAKING A CONCERTED EFFORT ALL DAY TO STAY ON TOP OF THIS. ADVISE CREWS TO CHK THEIR AFDS PANEL IF THE AUTOPLT IS FLYING DURING A WAKE TURBULENCE ENCOUNTER; OR SIMPLY CLICK IT OFF AND HAND FLY THE ACFT. ONCE EVERYTHING IS STABILIZED; THEN RE-ENGAGE EVERYTHING FROM THE BEGINNING. ALSO; TRY TO KEEP THE CDI IN YOUR CROSSCHECK. IT'S REALLY THE FIRST INDICATION IN A -300/-500 THAT YOU'RE WANDERING OFF COURSE.SUPPLEMENTAL INFO FROM ACN 779663: ON THE EAGUL 2 ARR INTO PHX MY ACFT FLEW THROUGH WAKE TURBULENCE. DURING THE BRIEF WAKE TURB ENCOUNTER I THINK I GRABBED THE CONTROL COLUMN MOMENTARILY. IN LOOKING BACK; I MUST HAVE DISENGAGED LNAV. VERY SHORTLY AFTER THIS PHX APCH SAID WE HAD PASSED A WAYPOINT AND ASSIGNED A HDG. WE WERE THEN CLRED FOR A VISUAL APCH TO RWY 8. I SHOULD HAVE SCANNED THE COCKPIT BETTER AFTER THE WAKE TURBULENCE FOR ANY DEVS. BOTH PLTS SHOULD HAVE SCANNED THE INSTRUMENTS AND ANNUNCIATORS AFTER ENCOUNTERING WAKE TURBULENCE. ACFT WENT THROUGH WAKE TURBULENCE AND LNAV DISENGAGED. ACFT MISSED WAYPOINT ON EAGUL ARR.

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