Narrative:

This aircraft had 1 yaw damper placarded inoperative and also required a lot of rudder trim during the flight; for example 4 units at 300 KTS (this was entered as a separate write-up). Although we checked the aileron trim before engine start; we did not check aileron trim in cruise by disconnecting the autoplt. Later in the flight we experienced a separate and unrelated problem; pitch pulses on short final. On a stable approach into bos (ILS runway 4R) with brag; a 20 KT quartering headwind; behind a heavy airbus; we noticed a rhythmic pulsing in the yoke that started at approximately 300 ft and continued into the flare (this was entered as a separate write-up). I had some concerns about aircraft control; because the yoke pulse was a very small pitch up; followed by a very small pitch down. It felt like some kind of an aircraft input to the yoke. However; I do not rule out the possibility of some kind of unusual windshear or vortex effect from the traffic ahead. The autoplts and autothrottles were off and there were no trim inputs; ice build-up or bird strikes. Due to this pitch anomaly; below 300 ft; I got high on the GS. Although I landed a little long (approximately 2500 ft past threshold); I elected to continue the landing as the safest course of action. I made smooth flare inputs; sensing that the little up and down pitch pulses would cancel each other out. That was the case and the flare and landing were smooth. I discussed the rudder trim and pitch event in detail with various mechanics; made 2 separate entries and even suggested that they pull the flight data recorder. Callback conversation with reporter revealed the following information: reporter stated that maintenance replaced the yaw damper servo the following day; but could not duplicate an elevator abnormality. Subsequent flts did not result in any pitch anomalies.

Related reports:

Google
 

Original NASA ASRS Text

Title: B757 REQUIRED EXCESSIVE RUDDER TRIM DURING CRUISE. THE ACFT ALSO EXHIBITED MINOR PITCH PULSING ON FINAL APCH.

Narrative: THIS ACFT HAD 1 YAW DAMPER PLACARDED INOP AND ALSO REQUIRED A LOT OF RUDDER TRIM DURING THE FLT; FOR EXAMPLE 4 UNITS AT 300 KTS (THIS WAS ENTERED AS A SEPARATE WRITE-UP). ALTHOUGH WE CHKED THE AILERON TRIM BEFORE ENG START; WE DID NOT CHK AILERON TRIM IN CRUISE BY DISCONNECTING THE AUTOPLT. LATER IN THE FLT WE EXPERIENCED A SEPARATE AND UNRELATED PROB; PITCH PULSES ON SHORT FINAL. ON A STABLE APCH INTO BOS (ILS RWY 4R) WITH BRAG; A 20 KT QUARTERING HEADWIND; BEHIND A HVY AIRBUS; WE NOTICED A RHYTHMIC PULSING IN THE YOKE THAT STARTED AT APPROX 300 FT AND CONTINUED INTO THE FLARE (THIS WAS ENTERED AS A SEPARATE WRITE-UP). I HAD SOME CONCERNS ABOUT ACFT CTL; BECAUSE THE YOKE PULSE WAS A VERY SMALL PITCH UP; FOLLOWED BY A VERY SMALL PITCH DOWN. IT FELT LIKE SOME KIND OF AN ACFT INPUT TO THE YOKE. HOWEVER; I DO NOT RULE OUT THE POSSIBILITY OF SOME KIND OF UNUSUAL WINDSHEAR OR VORTEX EFFECT FROM THE TFC AHEAD. THE AUTOPLTS AND AUTOTHROTTLES WERE OFF AND THERE WERE NO TRIM INPUTS; ICE BUILD-UP OR BIRD STRIKES. DUE TO THIS PITCH ANOMALY; BELOW 300 FT; I GOT HIGH ON THE GS. ALTHOUGH I LANDED A LITTLE LONG (APPROX 2500 FT PAST THRESHOLD); I ELECTED TO CONTINUE THE LNDG AS THE SAFEST COURSE OF ACTION. I MADE SMOOTH FLARE INPUTS; SENSING THAT THE LITTLE UP AND DOWN PITCH PULSES WOULD CANCEL EACH OTHER OUT. THAT WAS THE CASE AND THE FLARE AND LNDG WERE SMOOTH. I DISCUSSED THE RUDDER TRIM AND PITCH EVENT IN DETAIL WITH VARIOUS MECHS; MADE 2 SEPARATE ENTRIES AND EVEN SUGGESTED THAT THEY PULL THE FLT DATA RECORDER. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT MAINT REPLACED THE YAW DAMPER SERVO THE FOLLOWING DAY; BUT COULD NOT DUPLICATE AN ELEVATOR ABNORMALITY. SUBSEQUENT FLTS DID NOT RESULT IN ANY PITCH ANOMALIES.

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.