In May, US airlines reported four domestic flights with tarmac delays exceeding three hours and one international flight with tarmac delays exceeding four hours, according to the US Department of Transportation’s (DOT) Air Travel Consumer Report and the Bureau of Transportation Statistics.

All four domestic flights were operated by United Airlines. Two occurred at Chicago O’Hare Airport (ORD) on May 12; both flights were canceled after the tarmac delay extended beyond three hours. The longest domestic delay involved a May 9 United Airlines flight from Los Angeles International Airport (LAX) to Houston Bush Intercontinental (IAH) that was diverted to Corpus Christi International Airport (CRP); the flight remained on the CRP tarmac for 222 minutes before continuing on to Houston.

Similarly, the excessive international flight delay for the month involved an ExpressJet Airlines flight from Monterrey, Mexico bound for IAH on May 9. That flight was also diverted to CRP, where the aircraft remained on the tarmac for 261 minutes before continuing on to IAH. The Houston area was battered by strong thunderstorms May 9.

The 12 carriers filing on-time performance data with DOT reported a 76.9% on-time arrival rate in May, falling 2.5 points from May 2013’s 79.4% on-time rate, and down 2.7 points from April.

US passenger airlines canceled 1.9% of scheduled domestic flights in May, a 0.8 point hike above April’s rate, and a 0.8 point increase over May 2013. 9,513 out of 499,278 scheduled flights were canceled in May.

US airlines reporting the lowest rate of canceled flights in May were Delta Air Lines and Alaska Airlines (both 0.1%), Virgin America (0.2%), Hawaiian Airlines (0.4%), Frontier Airlines (0.6%) and Southwest Airlines (0.7%). ExpressJet Airlines had the highest percentage of May cancellations (5.8%), followed by Envoy Airlines at 4.3% and JetBlue Airways (2.4%).

In May, 8.2% of systemwide delays were due to late-arriving aircraft (i.e., the previous flight with the same aircraft arrived late, causing the present flight to depart late); 6% of the month’s flights were affected by national aviation system delays (i.e., non-extreme weather conditions, airport operations, heavy traffic volume, air traffic control, etc.); 6% of delays were caused by the air carrier (i.e., circumstances within the airline’s control, such as maintenance or crew problems); 0.6% of delays were caused by extreme weather; and delays occurring due to security reasons were 0.03% of the month’s total.