US officials say a routine upgrade that went awry caused the mass disruption of air travel. Airlines are still scrambling to return operations to normal after the longest and only second nationwide stop of flights in the United States since the attacks of 9/11 in 2001. More than 5,000 flights were cancelled in the US on Saturday after over 9,000 were scrapped a day earlier as the knock-on effects continued from Friday’s abrupt ground stop.