Narrative:

Departed ord at am local. The ATIS listed runways 32L/T1, 9L and 4L for departures. We took off from 32L/T1. The takeoff roll required some left rudder to track the centerline which was consistent with the reported wind. At approximately 90-100 KTS, the aircraft very abruptly swerved to the left followed almost immediately with an abrupt swerve to the right. I glanced at the engine instruments and they all appeared normal. Rudder corrections were made but for a few moments I felt I did not have control of the aircraft. An abort was about to be initiated but the yawing stopped and the aircraft appeared normal and was tracking straight ahead so the takeoff was continued without further difficulty. In analyzing the situation later, I believe we had an encounter with thrust stream turbulence from a departing heavy aircraft on runway 4L. Out speed at the instant of the encounter would place us approximately abeam of the end of 4L. The distance from the end of 4L to runway 32L is approximately 2000'. The aim shows that an medium large transport or widebody transport produces a 25 mph velocity 2100' from the aircraft when at takeoff thrust. Because of a possible slipstreaming effect with a favorable wind as it was (030/16), I suspect that a gust factor greater than 25 mph could have existed at 32L. In any case, the velocity gradient we transversed would have been very steep accounting for the very abrupt yawing. The only difficulty I have with this theory is that the first yaw was to the left which would mean that the wind effect was on the fuselage and then the vertical stabilizer and I would not expect the first yaw to be as strong as it was. Shortly after takeoff, the new ATIS reported the wind as 030/10. Upon return to ord 2 days later, I called the ord tower (they were very cooperative and interested in pursuing an explanation) and they told me there had been a similar report recently. The tower also advised me that with this runway plan, they will also sometimes also use 4L for arrs after about XA30 local. A 3 degree glide path to the 1000' point on 4L places an arriving aircraft about 160' over runway 32L. This would also produce a wake hazard. I will adopt as a personal policy and recommend wide dissemination of a recommendation not to depart 32L without advisory of 4L departures and arrs and use of a reasonable time interval.

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Original NASA ASRS Text

Title: ACR MLG EXPERIENCES TEMPORARY LOSS OF CONTROL ON TKOF ROLL ABOUT 90-100 KTS. PIC BELIEVES IT WAS DUE TO JET BLAST FROM ACFT DEP 04L. THE MLG WAS ON 32L DIRECTLY DOWNLINE FROM CENTERLINE OF 04L.

Narrative: DEPARTED ORD AT AM LCL. THE ATIS LISTED RWYS 32L/T1, 9L AND 4L FOR DEPS. WE TOOK OFF FROM 32L/T1. THE TKOF ROLL REQUIRED SOME LEFT RUDDER TO TRACK THE CENTERLINE WHICH WAS CONSISTENT WITH THE REPORTED WIND. AT APPROX 90-100 KTS, THE ACFT VERY ABRUPTLY SWERVED TO THE LEFT FOLLOWED ALMOST IMMEDIATELY WITH AN ABRUPT SWERVE TO THE RIGHT. I GLANCED AT THE ENGINE INSTRUMENTS AND THEY ALL APPEARED NORMAL. RUDDER CORRECTIONS WERE MADE BUT FOR A FEW MOMENTS I FELT I DID NOT HAVE CONTROL OF THE ACFT. AN ABORT WAS ABOUT TO BE INITIATED BUT THE YAWING STOPPED AND THE ACFT APPEARED NORMAL AND WAS TRACKING STRAIGHT AHEAD SO THE TKOF WAS CONTINUED WITHOUT FURTHER DIFFICULTY. IN ANALYZING THE SITUATION LATER, I BELIEVE WE HAD AN ENCOUNTER WITH THRUST STREAM TURBULENCE FROM A DEPARTING HEAVY ACFT ON RWY 4L. OUT SPEED AT THE INSTANT OF THE ENCOUNTER WOULD PLACE US APPROX ABEAM OF THE END OF 4L. THE DISTANCE FROM THE END OF 4L TO RWY 32L IS APPROX 2000'. THE AIM SHOWS THAT AN MLG OR WDB PRODUCES A 25 MPH VELOCITY 2100' FROM THE ACFT WHEN AT TKOF THRUST. BECAUSE OF A POSSIBLE SLIPSTREAMING EFFECT WITH A FAVORABLE WIND AS IT WAS (030/16), I SUSPECT THAT A GUST FACTOR GREATER THAN 25 MPH COULD HAVE EXISTED AT 32L. IN ANY CASE, THE VELOCITY GRADIENT WE TRANSVERSED WOULD HAVE BEEN VERY STEEP ACCOUNTING FOR THE VERY ABRUPT YAWING. THE ONLY DIFFICULTY I HAVE WITH THIS THEORY IS THAT THE FIRST YAW WAS TO THE LEFT WHICH WOULD MEAN THAT THE WIND EFFECT WAS ON THE FUSELAGE AND THEN THE VERTICAL STABILIZER AND I WOULD NOT EXPECT THE FIRST YAW TO BE AS STRONG AS IT WAS. SHORTLY AFTER TKOF, THE NEW ATIS REPORTED THE WIND AS 030/10. UPON RETURN TO ORD 2 DAYS LATER, I CALLED THE ORD TWR (THEY WERE VERY COOPERATIVE AND INTERESTED IN PURSUING AN EXPLANATION) AND THEY TOLD ME THERE HAD BEEN A SIMILAR REPORT RECENTLY. THE TWR ALSO ADVISED ME THAT WITH THIS RWY PLAN, THEY WILL ALSO SOMETIMES ALSO USE 4L FOR ARRS AFTER ABOUT XA30 LCL. A 3 DEG GLIDE PATH TO THE 1000' POINT ON 4L PLACES AN ARRIVING ACFT ABOUT 160' OVER RWY 32L. THIS WOULD ALSO PRODUCE A WAKE HAZARD. I WILL ADOPT AS A PERSONAL POLICY AND RECOMMEND WIDE DISSEMINATION OF A RECOMMENDATION NOT TO DEPART 32L WITHOUT ADVISORY OF 4L DEPS AND ARRS AND USE OF A REASONABLE TIME INTERVAL.

Data retrieved from NASA's ASRS site as of August 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.