A B727-200 air carrier flight crew experiences a yawing movement and discovers later that the lower ...

Attributes
ACN	367176
Time
Date	199704
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : bna
State Reference	TN
Altitude	msl bound lower : 33000 msl bound upper : 33000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zid
Operator	common carrier : air carrier
Make Model Name	B727-200
Operating Under FAR Part	Part 121
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
ASRS Report	367176
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : critical other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	controller : issued new clearance other
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

On flight from ord to fll on apr/mon/97, at 33000 ft .82 mach, aircraft began to yaw excessively and spontaneously. I requested a lower altitude and we began a descent to FL240 as cleared by ATC. We slowed our speed to 250 KTS. Somewhere around FL260 I requested lower. We were cleared to 10000 ft. At about FL250 we noticed the lower rudder indices were deflecting left and right. Suspecting lower yaw damper malfunction, we disconnected lower yaw damper. Oscillations ceased and we leveled at FL240 and requested clearance to ord. All appropriate checklists, procedures and limitations were followed. Landing was made ord runway 4R without further incident. Callback conversation with reporter revealed the following information: the first officer of this B727-200 said that they were in smooth flight conditions when the aircraft began to yaw excessively. After they descended to a lower altitude they found that the lower rudder was moving left and right while the upper rudder was stable. After switching the lower yaw damper to off the movement of the lower conformed to that of the upper rudder and the yawing movement stopped. The reporter said that they were about 100 NM north of bna and after talking with the company they decided to return to ord for another aircraft. Maintenance found that the lower rudder's yaw damper computer malfunctioned and was inducing yaw. After the computer was replaced the aircraft flew normally.

Original NASA ASRS Text

Title: A B727-200 ACR FLC EXPERIENCES A YAWING MOVEMENT AND DISCOVERS LATER THAT THE LOWER RUDDER YAW DAMPER'S COMPUTER HAD MALFUNCTIONED CAUSING YAW TO BE INDUCED. WHEN THE LOWER YAW DAMPER SWITCH WAS SHUT OFF THE ACFT FLEW NORMALLY.

Narrative: ON FLT FROM ORD TO FLL ON APR/MON/97, AT 33000 FT .82 MACH, ACFT BEGAN TO YAW EXCESSIVELY AND SPONTANEOUSLY. I REQUESTED A LOWER ALT AND WE BEGAN A DSCNT TO FL240 AS CLRED BY ATC. WE SLOWED OUR SPD TO 250 KTS. SOMEWHERE AROUND FL260 I REQUESTED LOWER. WE WERE CLRED TO 10000 FT. AT ABOUT FL250 WE NOTICED THE LOWER RUDDER INDICES WERE DEFLECTING L AND R. SUSPECTING LOWER YAW DAMPER MALFUNCTION, WE DISCONNECTED LOWER YAW DAMPER. OSCILLATIONS CEASED AND WE LEVELED AT FL240 AND REQUESTED CLRNC TO ORD. ALL APPROPRIATE CHKLISTS, PROCS AND LIMITATIONS WERE FOLLOWED. LNDG WAS MADE ORD RWY 4R WITHOUT FURTHER INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE FO OF THIS B727-200 SAID THAT THEY WERE IN SMOOTH FLT CONDITIONS WHEN THE ACFT BEGAN TO YAW EXCESSIVELY. AFTER THEY DSNDED TO A LOWER ALT THEY FOUND THAT THE LOWER RUDDER WAS MOVING L AND R WHILE THE UPPER RUDDER WAS STABLE. AFTER SWITCHING THE LOWER YAW DAMPER TO OFF THE MOVEMENT OF THE LOWER CONFORMED TO THAT OF THE UPPER RUDDER AND THE YAWING MOVEMENT STOPPED. THE RPTR SAID THAT THEY WERE ABOUT 100 NM N OF BNA AND AFTER TALKING WITH THE COMPANY THEY DECIDED TO RETURN TO ORD FOR ANOTHER ACFT. MAINT FOUND THAT THE LOWER RUDDER'S YAW DAMPER COMPUTER MALFUNCTIONED AND WAS INDUCING YAW. AFTER THE COMPUTER WAS REPLACED THE ACFT FLEW NORMALLY.

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 B727-200 air carrier flight crew experiences a yawing movement and discovers later that the lower rudder yaw damper's computer had malfunctioned causing yaw to be induced. When the lower yaw damper switch was shut off the aircraft flew normally.