First officer of large transport reports a pilot induced oscillation on the bank axis at 30 ft AGL on a visual approach. Reporter took control of the aircraft to stabilize approach and the captain made the landing.

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Attributes
ACN	381208
Time
Date	199709
Day	Sat
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : phx
State Reference	AZ
Altitude	agl bound lower : 30 agl bound upper : 30
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : phx tower : mrb
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	descent : approach landing other
Route In Use	approach : visual
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument pilot : atp
Experience	flight time last 90 days : 300 flight time total : 16000 flight time type : 200
ASRS Report	381208
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	other anomaly other anomaly other
Independent Detector	other flight crewa
Resolutory Action	other
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	other

Narrative:

While on a visual approach to runway 26L at phoenix everything appeared normal, then at the 30 ft call on the radar altimeter, the captain (who was the PF) began to drop the right wing into a considerable bank of 15-20 degrees. He noticed this and began a correction to wings level. He over controled the aircraft and rolled into a 20-25 degree bank to the left. He once again corrected, but I could see he over corrected to the right and we rolled 20-25 degree bank to the right again. We were in a pilot induced oscillation about the roll axis. At this point I grabbed the controls, and neutralized them and held them in this position. The aircraft slowly stabilized, and the captain was able to make the landing. I have never in my flying career seen anything like this happen, that was pilot induced. The WX was good, the wind 220 degrees and 08 KTS and no wake turbulence. The captain later commented that he did not know what had happened, but it would never happen again. The matter has been reported to the chief pilot's office and is under investigation.

Original NASA ASRS Text

Title: FO OF LGT RPTS A PLT INDUCED OSCILLATION ON THE BANK AXIS AT 30 FT AGL ON A VISUAL APCH. RPTR TOOK CTL OF THE ACFT TO STABILIZE APCH AND THE CAPT MADE THE LNDG.

Narrative: WHILE ON A VISUAL APCH TO RWY 26L AT PHOENIX EVERYTHING APPEARED NORMAL, THEN AT THE 30 FT CALL ON THE RADAR ALTIMETER, THE CAPT (WHO WAS THE PF) BEGAN TO DROP THE R WING INTO A CONSIDERABLE BANK OF 15-20 DEGS. HE NOTICED THIS AND BEGAN A CORRECTION TO WINGS LEVEL. HE OVER CTLED THE ACFT AND ROLLED INTO A 20-25 DEG BANK TO THE L. HE ONCE AGAIN CORRECTED, BUT I COULD SEE HE OVER CORRECTED TO THE R AND WE ROLLED 20-25 DEG BANK TO THE R AGAIN. WE WERE IN A PLT INDUCED OSCILLATION ABOUT THE ROLL AXIS. AT THIS POINT I GRABBED THE CTLS, AND NEUTRALIZED THEM AND HELD THEM IN THIS POS. THE ACFT SLOWLY STABILIZED, AND THE CAPT WAS ABLE TO MAKE THE LNDG. I HAVE NEVER IN MY FLYING CAREER SEEN ANYTHING LIKE THIS HAPPEN, THAT WAS PLT INDUCED. THE WX WAS GOOD, THE WIND 220 DEGS AND 08 KTS AND NO WAKE TURB. THE CAPT LATER COMMENTED THAT HE DID NOT KNOW WHAT HAD HAPPENED, BUT IT WOULD NEVER HAPPEN AGAIN. THE MATTER HAS BEEN RPTED TO THE CHIEF PLT'S OFFICE AND IS UNDER INVESTIGATION.

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.