Narrative:

During takeoff from ZZZ at about 100 KIAS the aircraft veered to the left for about 1 second and then corrected itself without any pilot input. All engine indications; flight control indications and all other system indications were normal. During climb out I began to question the first officer who was flying the airplane. I asked him if he had kicked the rudder. He said that he had not. As we climbed out I called dispatch and asked for a phone patch to ZZZ1 maintenance. I discussed the situation with maintenance at length to find out if they had any ideas as to what it might be. Nothing definitive was decided. I then talked to dispatch about where we should land the airplane. We decided that ZZZ1 was not much further than ZZZ2 and that the WX and maintenance at ZZZ1 were vastly superior. We diverted into ZZZ1 and declared an emergency asking for the arff equipment to be standing by. An uneventful landing was made. In retrospect if I had to guess what happened I would speculate that a plastic bag that was on the runway just prior to the swerve was ingested by the left engine. The engine lost power for just enough time to cause the aircraft to swerve without the engine indications changing. Maintenance is downloading the flight data recorder. I will be very interested in their conclusions. Callback conversation with reporter revealed the following information: the reporter stated that the flight data recorder indicated a 10-15 KT crosswind gust caused a momentary 10-15 KT air speed decrease. There was some weather in the vicinity but not close enough that the crew would suspect a weather component to this event. The aircraft movement was rapid and severe enough that they suspected either an engine malfunction or a momentary rudder hard over.

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Original NASA ASRS Text

Title: A B737-800 EXPERIENCED A WIND GUST AT 100 KTS ON TKOF CAUSING THE ACFT TO VEER RAPIDLY LEFT FOR A ABOUT 1 SECOND.

Narrative: DURING TKOF FROM ZZZ AT ABOUT 100 KIAS THE ACFT VEERED TO THE L FOR ABOUT 1 SECOND AND THEN CORRECTED ITSELF WITHOUT ANY PLT INPUT. ALL ENG INDICATIONS; FLT CTL INDICATIONS AND ALL OTHER SYS INDICATIONS WERE NORMAL. DURING CLBOUT I BEGAN TO QUESTION THE FO WHO WAS FLYING THE AIRPLANE. I ASKED HIM IF HE HAD KICKED THE RUDDER. HE SAID THAT HE HAD NOT. AS WE CLBED OUT I CALLED DISPATCH AND ASKED FOR A PHONE PATCH TO ZZZ1 MAINT. I DISCUSSED THE SITUATION WITH MAINT AT LENGTH TO FIND OUT IF THEY HAD ANY IDEAS AS TO WHAT IT MIGHT BE. NOTHING DEFINITIVE WAS DECIDED. I THEN TALKED TO DISPATCH ABOUT WHERE WE SHOULD LAND THE AIRPLANE. WE DECIDED THAT ZZZ1 WAS NOT MUCH FURTHER THAN ZZZ2 AND THAT THE WX AND MAINT AT ZZZ1 WERE VASTLY SUPERIOR. WE DIVERTED INTO ZZZ1 AND DECLARED AN EMER ASKING FOR THE ARFF EQUIP TO BE STANDING BY. AN UNEVENTFUL LNDG WAS MADE. IN RETROSPECT IF I HAD TO GUESS WHAT HAPPENED I WOULD SPECULATE THAT A PLASTIC BAG THAT WAS ON THE RWY JUST PRIOR TO THE SWERVE WAS INGESTED BY THE L ENG. THE ENG LOST PWR FOR JUST ENOUGH TIME TO CAUSE THE ACFT TO SWERVE WITHOUT THE ENG INDICATIONS CHANGING. MAINT IS DOWNLOADING THE FLT DATA RECORDER. I WILL BE VERY INTERESTED IN THEIR CONCLUSIONS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER STATED THAT THE FLT DATA RECORDER INDICATED A 10-15 KT CROSSWIND GUST CAUSED A MOMENTARY 10-15 KT AIR SPEED DECREASE. THERE WAS SOME WEATHER IN THE VICINITY BUT NOT CLOSE ENOUGH THAT THE CREW WOULD SUSPECT A WEATHER COMPONENT TO THIS EVENT. THE ACFT MOVEMENT WAS RAPID AND SEVERE ENOUGH THAT THEY SUSPECTED EITHER AN ENG MALFUNCTION OR A MOMENTARY RUDDER HARD OVER.

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.