Two key government watchdogs raised doubts Wednesday about the accuracy of the Federal Aviation Administration’s record-keeping on potentially disastrous close calls between planes on the runway and in flight.

One of them, Jeffrey B. Guizetti, assistant inspector general for aviation, told a House committee that he expected that an additional 300 to 600 incidents daily in which planes get too close would be revealed once new automatic tracking software is phased in.

“The chances of a collision on the ground or in the air is low, but if it does occur, there will be a catastrophic loss of life,” Guizetti told the subcommittee on aviation. “So that is why there is a lot of concern about tracking these near misses.”

The committee was also told that the biggest current danger to passengers may be on the ground rather than in the air, when mistakes are made at crowded airports or planes overshoot the runway.

The issue of mistakes and misconduct by air-traffic controllers caught public attention last year after several near collisions, an incident involving first lady Michelle Obama’s plane and instances­ of controllers caught sleeping on the job.

Critics have suggested that some controllers and their supervisors have kept mistakes off the books to avoid punishment.

David Grizzle, who supervises the FAA’s more than 14,000 controllers, acknowledged that under past reporting practices “there was an incentive” for supervisors to “manage down the number of incidents that had been reported.”

Managing millions of flights a year is a complex business, and so is the system that guides it. Sometimes when a plane get closer than the stipulated safe distance — generally three miles or 1,000 feet of altitude apart — it isn’t counted as an error. In some cases, for example, allowing two planes to get too close while on parallel courses in the same direction might not be deemed an error.

There were 1,234 recorded operational errors in fiscal 2009, and although there were more than a million fewer flights in 2010, the number reported jumped to a record 1,887.

“The reporting of incidents has increased, but we are not able to tell which part of the increase is the result of greater reporting and which part of the increase is the result of more incidents, in fact, occurring,” Grizzle said.

The majority of those official errors did not put passengers at great risk. But there were enough serious incidents that year that the National Transportation Safety Board stepped in to investigate.

Among the cases the NTSB reviewed were those in which a Boeing 737 nearly hit a helicopter while taking off from Houston; a Boeing 777 skimmed under a small plane on takeoff from San Francisco; a Boeing 737 nearly collided with a Cessna in Burbank, Calif.; an Airbus 319 passed 100 feet above the path of a Boeing 747 taking off in Anchorage; and an Embraer 135 taking off from Chicago took evasive action to avoid an in-bound twin-engine prop plane.

It also reviewed an incident near Reagan National Airport in which an airliner carrying Rep. F. James Sensenbrenner Jr. (R-Wis.) swerved to avoid another jet after the encounter activated the on-board collision avoidance system.

The FAA has moved to what it calls a non-punitive culture for reporting errors by air-traffic controllers for a strategic reason. Agency officials have argued that to implement a revolutionary $40 billion system known as ­NextGen, they need a complete picture of the mistakes controllers make.

In the past, controllers faced punishment for their errors, and supervisors recognized it was in their best interest to keep a lid on the number of errors that were reported.

Now, controllers are encouraged to report their own errors without fear of retribution.

The new system has coincided with a spike in the number of reported incidents. When that trend began, the FAA attributed the increase in reported errors to the new reporting system.

But the FAA backed off the contention and acknowledged that most self-reported mistakes were not included in the official count.

“It is unclear if the recent increase in operational error reports is due to more actual reporting or an actual increase in errors,” Gerald L. Dillingham of the Government Accountability Office told the committee. “FAA also lacks data [in] some areas that are important for monitoring safety risk.”

With the volume of air traffic growing faster than airports can expand, Dillingham said the FAA needs to speed up its control over planes that are rolling rather than flying.

“There’s beginning to be a consensus that the most dangerous part of the trip is when the wheels are still on the ground,” Dillingham said. “According to FAA, there are three runway incursions that occur each day at towered airports in the United States. Runway excursions can be just as dangerous. FAA does not have a process in place to track and evaluate runway excursions.”

An incursion is when an unauthorized aircraft, vehicle or person causes a safety issue on the runway. An excursion is when a plane veers off or overruns the runway surface.