Flight delays are significantly undercounted at some of the biggest U.S. hub airports, skewing airlines’ on-time performance statistics disclosed to travelers, according to an inspector general’s report.

Statistics for carriers such as American Airlines Group Inc., Delta Air Lines Inc. and United Continental Holdings Inc. don’t include late arrivals by regional partners that account for almost one-fourth of U.S. flights and are exempt from reporting delays, the Transportation Department report said.

Philadelphia International Airport, for example, had 38,000 tardy flights in 2012, more than twice the official count of 13,800 published by the Department of Transportation’s Bureau of Transportation Statistics, the study found.

“BTS’s published flight delay data present the public with an incomplete picture of the number of delays that actually occur at a given airport or are generated by all carriers,” the report said.

Republic Airways Holdings Inc.’s Chautauqua Airlines, which flies for all three of those major carriers, isn’t required to report delays because its passenger revenues are less than 1 percent of the industry total, according to the report.

While regional airlines conduct more than half of all flights, they carry fewer people per plane and therefore generate less revenue than major carriers. Carriers that don’t report delays operate 24 percent of scheduled flights, the inspector general said.

178,000 Delays

In 2012, almost 178,000 flights that arrived at least 15 minutes late at the U.S.’s largest 35 airports weren’t included in the DOT’s reports, the inspector general found. The number of delays at those airports would swell by 25 percent if all late flights were included.

Delays would swell by at least 50 percent at 10 of those 35 airports if all carriers were included, the IG found. Those airports include Detroit Metropolitan, Charlotte/Douglas International and Washington’s Reagan National.

The Transportation Department will propose changing its rules after agreeing with the report’s recommendations to expand its data collection requirement, the IG said.

If reporting requirements were changed to include all regional airlines flying for a major carrier, along with airlines that have as much as 0.5 percent of total industry revenues, the agency would capture 92 percent of scheduled passenger flights, according to the report. It now receives 76 percent of flight data.

Fifteen airlines report delay data to the BTS. One regional carrier, Mesa Air Group Inc.’s Mesa Airlines, voluntarily reports its late flights.

Delays Declined

The report says tardy flights fell, 33 percent from 2000 to 2012, using the government’s flawed data. It didn’t say what the trend would be if all delays had been counted.

The decline occurred as airlines shrunk, airports added runways and the U.S. Federal Aviation Administration introduced new air-traffic technology.

Delay totals are also depressed because carriers have added time to their published schedules. The IG found that airlines added gate-to-gate times on 73 percent of flights it studied between 2000 and 2012.

Editors: Bernard Kohn and Elizabeth Wasserman.

To contact the reporter on this story: Alan Levin in Washington at alevin24@bloomberg.net. To contact the editor responsible for this story: Bernard Kohn at bkohn2@bloomberg.net.