NYSE, United Airlines downed by tech glitches

A technical glitch forced the New York Stock Exchange to suspend trading for more than three hours yesterday in the biggest outage to strike a US financial market in nearly two years, unnerving investors already rattled by the meltdown in Chinese stocks and the Greek debt crisis.

The exchange said the halt, which occurred shortly after 11:30 am EDT and lasted until 3:10 p.m., was not the result of a cyberattack. Other exchanges were trading normally.

"It's not a good day, and I don't feel good for our customers who are having to deal with the fallout," NYSE president Thomas Farley told CNBC during the halt.

Traders awaited the reopening anxiously because much of the business on the NYSE happens when portfolio managers put in orders designed to occur at the exact market close to ensure end-of-day pricing.

The NYSE accounts for more than 60 percent of S&P 500 .SPX volume at the close of the market, according to Credit Suisse analyst Ana Avramovic. Most of this is at the "market-on-close," when those orders are processed for funds and institutions.

"If you don't have all the orders on that marketplace on the close, the pricing on the close would be definitely not accurate," said Empire Executions president Peter Costa, who trades on the NYSE floor.

The issues at NYSE came on the same day that technical problems led United Airlines to ground all its flights for about two hours.

The US Department of Homeland Security said there were no signs that the problems at NYSE and United Airlines stemmed from "malicious activity."

Nearly all US trading is done electronically, and the NYSE outage again raised questions about the technology at exchanges after major glitches in recent years.

A technical problem at NYSE's Arca exchange in March caused some of the most popular exchange-traded funds to be temporarily unavailable for trading.

And in August 2013, trading of all Nasdaq-listed stocks was frozen for three hours, leading US Securities and Exchange Commission chair Mary Jo White to call for a meeting of Wall Street executives to insure "continuous and orderly" functioning of the markets.

The SEC said it was closely monitoring the situation at NYSE. The White House said President Barack Obama had been briefed on the matter.

The New York Stock Exchange accounted for about 13.4 percent of all equities volume last month and 12.5 percent Tuesday according to BATS Global Markets data.