#1 INS FAILURE CAUSES A FALSE HARD OVER YAW DAMPER SIGNAL. THIS CAUSED A TURN; SOME BUFFET AND ALT LOSS.

Narrative

FLYING INTL FLT FROM BOS-LGW (GATWICK). AFTER LEVEL OFF AT FL330 THE #1 INS WARNING LIGHT CAME ON. TROUBLE- SHOOTING THE MALFUNCTION CODE INDICATED A NAVIGATIONAL ERROR PROB. THE FO MADE 2 SEPARATE ACCURACY CHKS AND CONFIRMED THAT THE #1 INS WAS OFF ABOUT 20 MI FROM THE POS OF INS #2 AND #3. THE #1 INS WAS TAKEN OUT OF TRIPLE MIX AND THE CAPT'S NAV WAS TRANSFERRED TO INS #3. THE #1 INS WARNING LIGHT WENT OUT. THE #1 INS WAS LEFT IN THE ON POS (IN ACCORDANCE WITH COMPANY POLICY) TO PROVIDE AN ATTITUDE BACKUP IN CASE TOTAL FAILURE OF INS #3. A LITTLE OVER 1 HR LATER THE ACFT HEELED HARDOVER TO THE L ABOUT 15 DEGS AND THE ACFT BEGAN TO BUFFET. AT THE SAME TIME; THE #1 INS WARNING LIGHT CAME BACK ON AND CAPT'S VERT GYRO TUMBLED AND SEVERAL WARNING FLAGS CAME INTO VIEW ON THE INSTS. THE CAPT IMMEDIATELY GAVE CTL OF THE ACFT TO THE COPLT. THE CAPT TURNED OFF BOTH YAW DAMPERS AND THE #1 INS. THE COPLT WAS THEN ABLE TO IMMEDIATELY GAIN CTL OF THE ACFT AFTER YAW DAMPERS AND INS #1 WERE TURNED OFF. THE WHOLE EPISODE TOOK NO MORE THAN ABOUT 5 SECONDS. THE AIRSPD INCREASED TO ABOUT .84 MACH (WE WERE CRUISING AT .83) AND WE LOST ABOUT 250-300 FT. BUFFETING WAS SIGNIFICANT AND WE BELIEVE IT WAS DUE TO A HIGH SIDESLIP FORCE CAUSING THE VERT STABILIZER TO REACH A HIGH ANGLE OF ATTACK BROUGHT ABOUT BY FALSE SIGNALS TO THE YAW DAMPER SYS BY INS #1. ALL CAPT'S FUNCTIONS WERE TRANSFERRED TO INS #3 AND THE YAW DAMPERS TURNED BACK ON WITH NO FURTHER PROBS. ONE FEMALE PAX HURT AN ANKLE WHILE WALKING TO THE LAV. AFTER CONFERRING WITH DISPATCH; WE CONTINUED TO DEST. ALL 3 CREW MEMBERS WERE FAMILIAR WITH A SIMILAR INCIDENT WHICH OCCURRED THIS SUMMER ON ANOTHER OF OUR WDB'S. WE THEREFORE WERE ABLE TO IMMEDIATELY RECOGNIZE THE PROB AND TAKE CORRECTIVE ACTION EVEN THOUGH WE HAVE NO FORMAT GUIDANCE IN ANY OF OUR MANUALS OR TRAINING PROCS. AN ANALYSIS OF THE PREVIOUS INCIDENT WAS PUBLISHED IN A COMPANY FLT OPS MONTHLY NEWSLETTER BUT WAS NOT PUBLISHED AS AN OFFICIAL NOTIFICATION TO ALL CREW MEMBERS. IF WE HAD NOT READ THAT ARTICLE; IT MAY HAVE TAKEN MUCH LONGER TO ANALYZE AND REACT TO THIS PROB AND MADE THE RECOVERY MUCH MORE DIFFICULT. I RECOMMEND THAT AFTER AN APPROPRIATE ANALYSIS OF THIS INCIDENT; ALL WDB USERS BE FORMALLY NOTIFIED OF THE POSSIBILITY OF THIS PROB AND PROCS TO DEAL WITH IT. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: WHEN THE ANOMALY OCCURRED; THERE WAS HARD YAW TO THE L. THE CAPT'S ATTITUDE INDICATOR FAILED WITH FLAGS. THE STANDBY ATTITUDE INDICATOR WAS NOT REFED AND THE ACFT WAS IMMEDIATELY GIVEN TO THE FO TO FLY. THERE HAD BEEN A RECENT ARTICLE IN THE FLT OPS MAGAZINE PRINTED BY THAT AIRLINE ABOUT A PREVIOUS SIMILAR INCIDENT. THE ARTICLE SUGGESTED THAT BOTH YAW DAMPERS BE TURNED OFF IN A CASE LIKE THIS AND THE FLC DID SO. PRESENT PROC CALLS FOR TURNING OFF THE FAULTY INS. RPTR IS NOT SURE IF THAT INS STILL PROVIDES SIGNALS TO THE YAW DAMPER OR IF THE YAW DAMPER IS ALSO RENDERED INOP. CALLBACK CONVERSATION WITH REPORTER ACN #257720 REVEALED THE FOLLOWING: THE RPTING CAPT STATES THAT HIS ACR SHOWS AN APPALLING LACK OF INTEREST IN THIS PROB; EVEN THOUGH THIS IS THE SECOND SUCH INCIDENT IN 1993. HE BELIEVES THAT HIS CREW WAS ABLE TO HANDLE THIS INCIDENT SO WELL BECAUSE THEY ALL HAD READ AN ARTICLE IN THE ACR'S MAGAZINE FOR ITS CREW MEMBERS DEALING WITH THE OTHER INCIDENT OF THIS TYPE. THE RPTR STATES THAT APPARENTLY THE #1 INS SENDS INFO TO THE YAW DAMPERS WHICH CAUSED THE HARD OVER RUDDER SIGNAL. THE RPTR DOES NOT KNOW IF THE #2 INS SENDS INFO TO THE OTHER SET OF YAW DAMPERS; OR WHETHER IT ALL COMES FROM THE #1 INS WHEN IT IS NOT TURNED OFF. SOP FOR OPERATING THE INS IS TO PUT ANY INS THAT SHOWS A NAV FAULT INTO THE ATTITUDE MODE WHICH STILL ALLOWS THE INS TO SEND ATTITUDE INFO TO THE HSI AND THE YAW DAMPERS. THE RPTING CAPT SAYS THAT 'HE IS BULLET PROOF' WITH LESS THAN 2 YRS TO FLY; SO HE IS GOING TO GO TO THE OFFICE AND MAKE SOME WAVES OVER THIS INCIDENT. HE HAS NOT INFORMED THE FAA OF THIS INCIDENT; AND BELIEVES THAT THEACR HAS NOT DONE SO EITHER. THIS TYPE SYS HAS A 'TRIPLE MIX' NAV SYS THAT AVERAGES THE NAV SIGNALS OF THE 3 INS BUT DOES NOT AVERAGE ATTITUDE SIGNALS. SUPPLEMENTAL INFORMATION FROM ACN 257721: IN AUGUST WE (ACR) HAD A WDB THAT WAS 'OUT OF CONTROL' DUE TO AN INS PROB WHICH WAS BROUGHT UNDER CONTROL AFTER THE CAPT'S GYRO TUMBLED AND THE #1 INS WAS TURNED OFF. THE #1 INS FEEDS INPUTS TO THE LOWER YAW DAMPERS THROUGH THE CADC. THIS INFO WAS NOT INCLUDED IN OUR MANUALS. THE FAULTY INS CAUSED THE PROB. I SAID TO TURN OFF THE #1 INS. THE CAPT HAD ALREADY REACHED UP AND TURNED OFF THE YAW DAMPENERS. THE OCCURRENCE LASTED APPROX 5 SECONDS. PROCS SHOULD BE DEVELOPED AND INCORPORATED INTO THE MANUAL. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: THE RPTING FO STATES THAT HE UNDERSTANDS THAT HIS ACR IS PLANNING TO REPLACE THE CURRENT INS UNITS WITH OTHERS FROM ANOTHER MANUFACTURER THAT ARE MORE RELIABLE. THE RPTR STATES THAT THE LOWER YAW DAMPERS GET THEIR INFO FROM THE #1 INS. THE PLT'S MANUALS DO NOT STATE WHERE THE UPPER YAW DAMPERS GET THEIR INFO.