Des Moines, Iowa ? Cheryl Allegretti’s husband was a meticulous pilot with more than 20 years experience when the plane he was piloting crashed in a northwest Iowa cornfield, killing him and two passengers, apparently because it ran out of gas.

“It’s still hard for me to believe it at all,” said Allegretti, of Cambridge, Wis. “Like everybody has told me, he was the most cautious, safety pilot that they ever knew.”

National Transportation Safety Board officials say what baffles them is the frequency with which pilots run out of gas. In the past five years, according to the NTSB, fuel exhaustion was the cause or a factor in 238 small plane crashes in the U.S., killing 29 people.

“It’s surprising to me that there’s a group of pilots who will knowingly push it, thinking ‘I can make it the last couple of miles’ and come up short,” said Tom Haueter, director of the NTSB’s Office of Aviation Safety.

There were 8,016 crashes of civilian planes — a category that excludes commercial and military flights — from 2004 through 2008, according to the NTSB. Pilot error is blamed in about 75 percent of those crashes, which killed 2,640 people on board.

In accidents where pilots were at fault, 3,909 happened during takeoffs or landings and 1,500 were because of mistakes made during bad weather, according to the NTSB.

Comparatively, the 238 small planes that crashed because they ran out of gas isn’t a large number, but aviation experts say it shouldn’t happen at all.

“There’s a certain group of accidents out there that are inexplicable. You just go ‘What are you thinking? What are you doing?’ They’re hard for us to get a handle on,”‘ Haueter said. “It seems like it’s an easily preventable accident.”

The June 23 Iowa crash that killed Frank Allegretti, 64, Thomas Boos, 60, of Fort Atkinson, Wis., and Malcolm McMillan, 65, of Milton, Wis., happened just a few miles from a small airport in the farm town of Sheldon. The men were flying from Wisconsin to South Dakota on a hunting trip.

Witnesses reported that the plane flew low and the engine sputtered before the crash.

When planes run out of fuel, the NTSB usually points to mistakes by pilots — inadequate preflight inspections, mistaken fuel planning or not checking the fuel caps.

A pilot who refueled during a November 2007 flight down the West Coast had filled up on a slope, leading him to mistakenly believe his tanks were full, according the NTSB. About two hours after taking off, the plane crashed, in McFarland, Calif., killing the pilot and two passengers.