The Aircraft Owners and Pilots Association (AOPA) is urging the U.S. National Transportation Safety Board (NTSB) to undertake an internal review of its approach to general aviation accident reports, citing concerns that the agency is approving “speculative” probable causes on little to no evidence.

“We remain concerned that the longstanding data-driven, facts-based standards continue to erode at the NTSB,” said AOPA senior vice president James Coon in a March 24 letter to NTSB acting chairman Bella Dinh-Zarr. “General aviation remains a safe form of travel…at substantially less risk than other modes, yet continues to get counterproductive scrutiny from the NTSB.”

The number of automotive traffic fatalities is 7,500 percent greater than that for general aviation, and boating results in 150 percent more deaths, Coon contended. “The industry is perplexed that general aviation remains on the board’s top 10 list despite the enormous disparity in fatalities as compared to other modes of transportation.”

General aviation in-flight loss of control has made the NTSB’s “Most Wanted” list of transportation safety improvements for three years in a row, and in recent years the agency has posted a series of safety alerts and held forums on general aviation safety.

The NTSB is preparing a response to the AOPA’s letter and expects to have it ready within the next several days. But the AOPA letter escalates the same concerns that the association’s Air Safety Institute and president Mark Baker raised last fall to then-chairman and current Board member Christopher Hart.

Association executives pointed to references to the pilot’s “impairment or incapacitation due to an acute cardiac event” in the probable cause of a 2015 accident of Piper PA46. AOPA noted this was cited even though the autopsy revealed no evidence of recent or old infarction.

Hart had acknowledged this in his previous response, but noted the autopsy of the pilot identified significant heart disease and that “a hyperacute infarction, occurring only over a few minutes may cause acute symptoms and leave no evidence if the victim dies traumatically.”

But Coon countered in the most recent letter that the lack of evidence of a hyperacute infarction “clearly suggests the probable cause of the accident in question was and is purely speculative.” He added that this speculation appears “to be finding its way into the culture of NTSB,” citing two more recent reports citing cardiovascular events without evidence. In one case, AOPA noted, the pilot had completed a checkup two days earlier and the physician documented no cardiovascular symptoms and found blood pressure “well controlled.”

In the earlier exchange, Hart had stressed that the NTSB uses “all available factual information identified in our investigation, including available operational evidence of human and aircraft performance, as well as the medical fitness of the pilot” in determining the probable cause. This evidence is “thoroughly analyzed,” he said, and added that the Safety Board continues to advocate for tools to obtain more information, such as crash resistant-flight recorders. He also said interested parties can petition for reconsideration of the Board’s findings if new evidence is found.

Coon, however, responded that “when there is not definitive evidence to determine the probable cause of an accident, it should be acceptable to make a ‘no determination of cause’ finding.” That would have been more accurate in all three of the accidents, he said.

“I am dismayed that the Board’s chief medical examiner allows this speculative practice to continue,” he added. “We hope the Board would work toward a more data-driven approach similar to that which the FAA has embraced.” An internal review could lead to such an approach, Coon said.

AOPA’s latest letter was sent a little more than a week after Dinh-Zarr stepped in as acting chairman, following the end of Hart’s term on March 15. Dinh-Zarr has a background in highway safety, and AOPA also pointed out that she has been a public health scientist specializing in injury prevention.