Narrative:

While level at 5000 ft at an assigned speed of 180 KTS and having intercepted the runway 22R localizer for ord, we were configured with the flaps at 15 degrees and slats at full extend. Shortly after this intercept we were cleared for the ILS runway 22R approach. As the PF, I armed the ILS mode on the flight guidance control panel and noted that we were still greater than 15 mi from the airport. Although the surface winds were reported to be gusty (180 degrees at 21 KTS gusting 28 KTS) our ride conditions were still smooth. Just as the GS was about to capture (approximately 13.7 DME from ord) I noticed a light aerodynamic buffet, that I recognized to be associated with wake turbulence. Immediately following this the aircraft began to roll left and the autoplt counteracted with right aileron and nose down inputs as the GS had also captured. I then quickly placed my hands on the control yoke in the event of autoplt disengagement. At that instant the autoplt did in fact disengage and I continued to manually command right aileron. The roll to the left reached approximately 25-30 degrees and then changed directions with an uncommanded roll to the right. Again, I counteracted with left aileron inputs and increased forward pressure on the yoke. I believe the aircraft rolled to approximately 35-40 degrees before again changing directions and rolling left. As the aircraft began the second uncommanded roll to the left I immediately pushed both throttles up and held the nose down to increase performance and exit what I believed to be wake turbulence. As the aircraft rolled left the heading also changed to the left by approximately 5-10 degrees, which I felt was positive, as this would assist in tracking upwind of the disturbance that we had encountered. As the aircraft rolled to the left I had near full aileron deflection to the right and considerable nose down pressure. After the aircraft reached approximately 45-50 degrees of roll to the left the aircraft then exited the disturbance and all control forces and inputs were again normal. I noted that we were at approximately 4600 ft and just slightly left or south of the localizer. At that moment, I voiced to the captain that I would continue to hand fly the remainder of the approach, and then turned back to the right to reestablish on the localizer centerline (although we had only deviated just slightly south of the localizer centerline). Additionally, I climbed approximately 150 ft to regain the GS. Shortly after this the captain questioned ATC as to the aircraft that we had been following and expressed that we had a wake turbulence encounter earlier on the localizer. ATC replied with a 'boeing 737' and also added that he had received a number of reports of windshear. He then instructed us to slow to 170 KTS. The captain then reported that this was not just windshear and the ATC, while checking, replied with something along the line of, it being just a little B737. Other than the turbulence and windshear associated with the gusty surface conditions the remainder of the approach was flown without incident and nothing abnormal was noted. During the entire event, the onset and rate of roll both to the left and the right were slow in nature and imposed no apparent stress on the aircraft and therefore, no aerodynamic or other aircraft limitations were exceeded. Additionally, the B737 that we were sequenced behind began their approach from an altitude greater than our altitude of 5000 ft that we had intercepted the localizer from. This possibly could have been the cause for our wake turbulence encounter.

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Original NASA ASRS Text

Title: ENCOUNTER WITH WAKE TURB AND SUBSEQUENT LOSS OF ACFT CTL EXPERIENCED BY A DC9-83 FLT CREW WHEN ON APCH TO RWY 22R AT ORD, IL.

Narrative: WHILE LEVEL AT 5000 FT AT AN ASSIGNED SPD OF 180 KTS AND HAVING INTERCEPTED THE RWY 22R LOC FOR ORD, WE WERE CONFIGURED WITH THE FLAPS AT 15 DEGS AND SLATS AT FULL EXTEND. SHORTLY AFTER THIS INTERCEPT WE WERE CLRED FOR THE ILS RWY 22R APCH. AS THE PF, I ARMED THE ILS MODE ON THE FLT GUIDANCE CTL PANEL AND NOTED THAT WE WERE STILL GREATER THAN 15 MI FROM THE ARPT. ALTHOUGH THE SURFACE WINDS WERE RPTED TO BE GUSTY (180 DEGS AT 21 KTS GUSTING 28 KTS) OUR RIDE CONDITIONS WERE STILL SMOOTH. JUST AS THE GS WAS ABOUT TO CAPTURE (APPROX 13.7 DME FROM ORD) I NOTICED A LIGHT AERODYNAMIC BUFFET, THAT I RECOGNIZED TO BE ASSOCIATED WITH WAKE TURB. IMMEDIATELY FOLLOWING THIS THE ACFT BEGAN TO ROLL L AND THE AUTOPLT COUNTERACTED WITH R AILERON AND NOSE DOWN INPUTS AS THE GS HAD ALSO CAPTURED. I THEN QUICKLY PLACED MY HANDS ON THE CTL YOKE IN THE EVENT OF AUTOPLT DISENGAGEMENT. AT THAT INSTANT THE AUTOPLT DID IN FACT DISENGAGE AND I CONTINUED TO MANUALLY COMMAND R AILERON. THE ROLL TO THE L REACHED APPROX 25-30 DEGS AND THEN CHANGED DIRECTIONS WITH AN UNCOMMANDED ROLL TO THE R. AGAIN, I COUNTERACTED WITH L AILERON INPUTS AND INCREASED FORWARD PRESSURE ON THE YOKE. I BELIEVE THE ACFT ROLLED TO APPROX 35-40 DEGS BEFORE AGAIN CHANGING DIRECTIONS AND ROLLING L. AS THE ACFT BEGAN THE SECOND UNCOMMANDED ROLL TO THE L I IMMEDIATELY PUSHED BOTH THROTTLES UP AND HELD THE NOSE DOWN TO INCREASE PERFORMANCE AND EXIT WHAT I BELIEVED TO BE WAKE TURB. AS THE ACFT ROLLED L THE HDG ALSO CHANGED TO THE L BY APPROX 5-10 DEGS, WHICH I FELT WAS POSITIVE, AS THIS WOULD ASSIST IN TRACKING UPWIND OF THE DISTURBANCE THAT WE HAD ENCOUNTERED. AS THE ACFT ROLLED TO THE L I HAD NEAR FULL AILERON DEFLECTION TO THE R AND CONSIDERABLE NOSE DOWN PRESSURE. AFTER THE ACFT REACHED APPROX 45-50 DEGS OF ROLL TO THE L THE ACFT THEN EXITED THE DISTURBANCE AND ALL CTL FORCES AND INPUTS WERE AGAIN NORMAL. I NOTED THAT WE WERE AT APPROX 4600 FT AND JUST SLIGHTLY L OR S OF THE LOC. AT THAT MOMENT, I VOICED TO THE CAPT THAT I WOULD CONTINUE TO HAND FLY THE REMAINDER OF THE APCH, AND THEN TURNED BACK TO THE R TO REESTABLISH ON THE LOC CTRLINE (ALTHOUGH WE HAD ONLY DEVIATED JUST SLIGHTLY S OF THE LOC CTRLINE). ADDITIONALLY, I CLBED APPROX 150 FT TO REGAIN THE GS. SHORTLY AFTER THIS THE CAPT QUESTIONED ATC AS TO THE ACFT THAT WE HAD BEEN FOLLOWING AND EXPRESSED THAT WE HAD A WAKE TURB ENCOUNTER EARLIER ON THE LOC. ATC REPLIED WITH A 'BOEING 737' AND ALSO ADDED THAT HE HAD RECEIVED A NUMBER OF RPTS OF WINDSHEAR. HE THEN INSTRUCTED US TO SLOW TO 170 KTS. THE CAPT THEN RPTED THAT THIS WAS NOT JUST WINDSHEAR AND THE ATC, WHILE CHKING, REPLIED WITH SOMETHING ALONG THE LINE OF, IT BEING JUST A LITTLE B737. OTHER THAN THE TURB AND WINDSHEAR ASSOCIATED WITH THE GUSTY SURFACE CONDITIONS THE REMAINDER OF THE APCH WAS FLOWN WITHOUT INCIDENT AND NOTHING ABNORMAL WAS NOTED. DURING THE ENTIRE EVENT, THE ONSET AND RATE OF ROLL BOTH TO THE L AND THE R WERE SLOW IN NATURE AND IMPOSED NO APPARENT STRESS ON THE ACFT AND THEREFORE, NO AERODYNAMIC OR OTHER ACFT LIMITATIONS WERE EXCEEDED. ADDITIONALLY, THE B737 THAT WE WERE SEQUENCED BEHIND BEGAN THEIR APCH FROM AN ALT GREATER THAN OUR ALT OF 5000 FT THAT WE HAD INTERCEPTED THE LOC FROM. THIS POSSIBLY COULD HAVE BEEN THE CAUSE FOR OUR WAKE TURB ENCOUNTER.

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.