* Corn, wheat contracts affected, soybean spared
* About 95 percent of volumes usually trade electronically
* Problem fixed after close of trading, says CME (Writes through, adds trader comment)
By Julie Ingwersen and Tom Polansek
CHICAGO, April 8 The world's largest futures markets operator had to shut electronic trade for leading agricultural contracts on Tuesday, just a day before key U.S. crop data, as rare technical problems hit and sent traders scrambling to get back on the floor.
CME Group, which has just won a court case giving it the right to use electronic trades as well as open outcry business in calculating end-of-day prices, had to rely solely on the centuries-old floor trading for these on Tuesday.
The electronic system handles around 95 percent of volumes on a typical trading day and the outage struck grains contracts in the last hour of business, just as traders squared positions ahead of Wednesday's release of monthly grains stocks data.
"Think of how much volume there is on the screen. To have that stifled, 15 minutes before the close, the day before a crop report - there was a logjam," a wheat options trader said.
Traders flocked back to the old octagonal "pit," packing shoulder to shoulder and shouting orders across its graduated floor. Traditional hand signals came to the rescue to communicate in the chaos.
"It was just a blast from the past," said Jerome Israelov, a 25-year veteran of the grain floor, who lost his voice in the shouting. "It was just a fun, fun hour."
The problem was fixed after the close of trading, CME spokesman Chris Grams said. CME listed contracts affected on its website here
Oils and metals trading was not affected, the CME said. Soybean contracts were also spared. By late afternoon, the exchange had no new details on the cause of the outage nor why it affected corn and wheat markets but not soybeans.
"I'd love to say it's not a big deal, but it's the benchmark for grain prices around the world," said Ken Smithmier, who trades the grain markets electronically.
The outage was the longest in recent memory, according to Chess Obermeier, a broker in the corn options pit.
While the severity of the outage and failure to track its cause could dent confidence, the CME Group holds a reliable track record and has built up goodwill among investors.
"These guys were famous in terms of keeping the lights on and being boringly good," said Brad Hintz, analyst for Bernstein Research. "Everybody gets one mistake. You just don't want to see continuing mistakes."
Other venues experienced a spate of disruptions in trading last year.
The Chicago Board Options Exchange had to shut down for a half day in April 2013 because of a problem connected to expanding its trading hours. In August, Nasdaq OMX Group suffered a three-hour disruption.
(Additional reporting by Mark Weinraub, Meredith Davis and Michael Hirtzer; Editing by Jonathan Oatis, Grant McCool and Eric Walsh)