A B737-400 in cruise at FL330 with yaw damper on upon engaging autoplt the aircraft began to yaw lef...

Attributes
ACN	466611
Time
Date	200003
Day	Sun
Local Time Of Day	0601 To 1200
Place
Locale Reference	navaid : olm.vortac
State Reference	WA
Altitude	msl single value : 33000
Environment
Flight Conditions	VMC
Weather Elements	Turbulence
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zse.artcc
Operator	common carrier : air carrier
Make Model Name	B737-400
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 160 flight time total : 10000 flight time type : 1500
ASRS Report	466611
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 250 flight time total : 7000 flight time type : 1000
ASRS Report	466588
Events
Anomaly	aircraft equipment problem : critical other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : yaw damper off light other flight crewa other flight crewb
Resolutory Action	flight crew : overcame equipment problem
Consequence	other other
Factors
Maintenance	performance deficiency : fault isolation performance deficiency : testing performance deficiency : repair
Supplementary
Problem Areas	Maintenance Human Performance Aircraft Flight Crew Human Performance
Primary Problem	Aircraft

Narrative:

At top of climb (hand flown) engaged autoplt and immediately the aircraft began to yaw left and right. Disengaged autoplt and yaw damper disengaged simultaneously. This should not happen with normal yaw damper. Ran the uncommanded yaw checklist then the yaw damper abnormal checklist. Re-engaged yaw damper and I experienced no yaw. Then re-engaged the autoplt and aircraft began to yaw immediately as reported above. Disengaged autoplt, yaw damper disengaged and flew to sfo. Ferried with appropriate MEL's. Met by honeywell, our engineering and maintenance personnel. They admitted this aircraft has a new digital yaw damper and had the same problem during flight test but could not duplicate problem so released to revenue service. Subsequent flight test produced same results. Callback conversation with reporter revealed the following information: reporter stated when the autoplt was engaged the left to right yaw was minor to moderate and could be felt very slightly in the rudder pedals. The reporter said the other odd thing was when disengaging the autoplt the yaw damper trimmed off. The reporter stated this airplane had two previous maintenance reports on the autoplt and some units were replaced. The reporter said with the yaw damper turned off the autoplt functioned normally the rest of the trip to sfo and on the maintenance ferry to boeing. The reporter said on arrival at bfi the crew was debriefed by honeywell engineers who advised a digital yaw damper coupler and pressure reducer had been installed in this aircraft. The reporter stated the honeywell engineers also advised when this modification was tested it had some problems but when flown afterward they could not duplicate the reports. Callback conversation with reporter acn 466588 revealed the following information: reporter stated the yaw damper when the autoplt was engaged initially was one half rudder input with wings 10 degrees of bank. The reporter said the rudder pedals did move slightly. The reporter stated the yaw damper operation was normal with no autoplt engagement. Conversely, the autoplt operated normally with no yaw damper engagement. The reporter said this same crew maintenance ferried the aircraft to boeing and were advised by the honeywell engineers of two modifications, a power unit pressure reducer and a digital yaw damper coupler.

Original NASA ASRS Text

Title: A B737-400 IN CRUISE AT FL330 WITH YAW DAMPER ON UPON ENGAGING AUTOPLT THE ACFT BEGAN TO YAW L AND R. WHEN AUTOPLT DISENGAGED THE YAW DAMPER DISCONNECTED. TESTED YAW DAMPER AND RE-ENGAGED AND OPERATION NORMAL AND RE-ENGAGED AUTOPLT AND ACFT BEGAN TO YAW IMMEDIATELY.

Narrative: AT TOP OF CLB (HAND FLOWN) ENGAGED AUTOPLT AND IMMEDIATELY THE ACFT BEGAN TO YAW L AND R. DISENGAGED AUTOPLT AND YAW DAMPER DISENGAGED SIMULTANEOUSLY. THIS SHOULD NOT HAPPEN WITH NORMAL YAW DAMPER. RAN THE UNCOMMANDED YAW CHECKLIST THEN THE YAW DAMPER ABNORMAL CHECKLIST. RE-ENGAGED YAW DAMPER AND I EXPERIENCED NO YAW. THEN RE-ENGAGED THE AUTOPLT AND ACFT BEGAN TO YAW IMMEDIATELY AS RPTED ABOVE. DISENGAGED AUTOPLT, YAW DAMPER DISENGAGED AND FLEW TO SFO. FERRIED WITH APPROPRIATE MEL'S. MET BY HONEYWELL, OUR ENGINEERING AND MAINT PERSONNEL. THEY ADMITTED THIS ACFT HAS A NEW DIGITAL YAW DAMPER AND HAD THE SAME PROB DURING FLT TEST BUT COULD NOT DUPLICATE PROB SO RELEASED TO REVENUE SVC. SUBSEQUENT FLT TEST PRODUCED SAME RESULTS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED WHEN THE AUTOPLT WAS ENGAGED THE L TO R YAW WAS MINOR TO MODERATE AND COULD BE FELT VERY SLIGHTLY IN THE RUDDER PEDALS. THE RPTR SAID THE OTHER ODD THING WAS WHEN DISENGAGING THE AUTOPLT THE YAW DAMPER TRIMMED OFF. THE RPTR STATED THIS AIRPLANE HAD TWO PREVIOUS MAINT RPTS ON THE AUTOPLT AND SOME UNITS WERE REPLACED. THE RPTR SAID WITH THE YAW DAMPER TURNED OFF THE AUTOPLT FUNCTIONED NORMALLY THE REST OF THE TRIP TO SFO AND ON THE MAINT FERRY TO BOEING. THE RPTR SAID ON ARR AT BFI THE CREW WAS DEBRIEFED BY HONEYWELL ENGINEERS WHO ADVISED A DIGITAL YAW DAMPER COUPLER AND PRESSURE REDUCER HAD BEEN INSTALLED IN THIS ACFT. THE RPTR STATED THE HONEYWELL ENGINEERS ALSO ADVISED WHEN THIS MODIFICATION WAS TESTED IT HAD SOME PROBS BUT WHEN FLOWN AFTERWARD THEY COULD NOT DUPLICATE THE RPTS. CALLBACK CONVERSATION WITH RPTR ACN 466588 REVEALED THE FOLLOWING INFO: RPTR STATED THE YAW DAMPER WHEN THE AUTOPLT WAS ENGAGED INITIALLY WAS ONE HALF RUDDER INPUT WITH WINGS 10 DEGS OF BANK. THE RPTR SAID THE RUDDER PEDALS DID MOVE SLIGHTLY. THE RPTR STATED THE YAW DAMPER OP WAS NORMAL WITH NO AUTOPLT ENGAGEMENT. CONVERSELY, THE AUTOPLT OPERATED NORMALLY WITH NO YAW DAMPER ENGAGEMENT. THE RPTR SAID THIS SAME CREW MAINT FERRIED THE ACFT TO BOEING AND WERE ADVISED BY THE HONEYWELL ENGINEERS OF TWO MODIFICATIONS, A PWR UNIT PRESSURE REDUCER AND A DIGITAL YAW DAMPER COUPLER.

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.

A B737-400 in cruise at FL330 with yaw damper on upon engaging autoplt the aircraft began to yaw left and right. When autoplt disengaged the yaw damper disconnected. Tested yaw damper and re-engaged and operation normal and re-engaged autoplt and aircraft began to yaw immediately.