A lack of flight test data was a contributing factor in an April 2015 accident that lea​d to an AC-130J nosediving 5,000 feet in an inverted position ft. ​before the pilots could recover, according to an Air Force Accident Investigation Board said​.

The flight dataSpecifically, flight data ​could have helped the pilots better understand been used could’ve warned the pilots about ​the limits of the aircraft’s handling while performing tight maneuvers. But defense contractor Lockheed Martin declined to provide the proprietary information without a contract, and the Air Force decided not to purchase it, according to the report.

The AC-130J was being tested by the 413th Flight Test Squadron at Eglin Air Force Base, Florida. The crew was performing various maneuvers to collect data on the flight envelope of the aircraft, the report said.

The pilot performed a "sideslip" maneuver by lowering one wing of the aircraft and applying opposite rudder. The action is usually used to aid aircraft landing in a strong crosswind.

The cause of the accident, the investigation board said, was that the pilot pushed the rudder too far while performing the sideslip, and then didn’t apply enough pressure to the rudder to stop the plane from rotating once it started to go went ​out of control.

However, the report noted, performing the sideslip and pushing the aircraft to its limits was the point of the test flight, and the pilot had been given special permission orders ​to carry out the maneuver.

With the flight data, however, the pilot might have had a better idea of when where​ control of the plane would be lost.

The pilot, "reflecting on the events in hindsight, thought perhaps they should not have proceeded beyond the second alert without predictions," the report said.

The basic information provided by Lockheed Martin was also inaccurate, the report said. It noted that warning alarms would sound when the aircraft reached a 16-degrees​ angle of sideslip. But instead the alarms sounded at 14.5 degrees.

"The error amounted to a reasonable expectation that the edge of the envelope was 16 degrees [angle of sideslip] when in fact it was only 14.5 degrees," the report said. "Had the correct limits been used, a safety monitor may have called ‘terminate’ earlier, possibly preventing [the accident] … admittedly, this is speculation."

Even so, However,​ investigators said they don’t believe it contributed to the accident because the pilot only used the information as a guideline and still relied on his own judgment.

"It was more up to me to use the cues and the [heads-up display] HUD ​to determine what rudder position would be required for me to get those alerts," the test pilot said.

When the pilot But the pilot started to turn too much, who had received prior permission from his superiors to execute the maneuver, extended it too far and ​was unable to regain control of the aircraft, , and the pilot wasn’t able to steer the aircraft back. T​the AC-130J inverted and started diving towards​ the Gulf of Mexico, plummeting for almost a mile dropping 5,000 ft. ​before the crew was able to recover, the report said.

The pilot and co-pilot were able to safely land the plane and no one was injured. But the high forces the airframe was subjected to — estimated at more than over ​3 Gs — rendered the plane a total loss. The Air Force says the cost of the ​damage was estimated at is ​$115.6 million dollars.

When the military develops an aircraft in conjunction with a private contractor, access to flight data is often written into the contract. But instead ​the C-130 airframe was developed commercially by Lockheed Martin.

The company performed and paid for the engineering and tests to develop data on the flight envelop of the aircraft and its acceptable safety limits. So the information is considered ​proprietary,​ and is only made available accessible ​to the U.S. military if it's they’re ​willing to pay for it.

"When the Air Force started procuring C-130Js and associated variants, they did not procure these proprietary data rights," said Lt. Cmdr. Matt Allen, a spokesman for U.S. Special Operations Command. "In the end, the government decided not to procure the engineer and test data models, and instead decided to rely on the new flying and handling qualities test program to obtain the data."

The test pilots were, in fact, doing just that, and trying to determine the exact flying characteristics of an the ​AC-130J loaded with new upgrades and equipment, and how it differed from a basic C-130J frame.

Lockheed did provide some basic data predicting how the aircraft would fly, and Allen said that 23 successful test flights were carried out before the accident.

But the accident investigation report said a contributing factor to the accident was "the fact the test team was provided inadequate procedural guidance or publications."

Not having that information meant the pilots were guessing how far they could push the craft past the sounding of alarm bells. And it meant they didn’t know "how much rudder force would be required to achieve" a successful sideslip, the report said.

"With a lack of predictive data, members of the team admitted they did not understand how much stability margin existed at the second special alert, but admitted it could potentially be very little," the accident investigation report said.

After the accident, the Air Force decided to purchase the data from Lockheed Martin. But because the information is considered ​proprietary to Lockheed’s business​, no one has disclosed how much it the information ​cost.

"The U.S. government has already put Lockheed on contract to provide predictive flight test data to support safe and effective flight test execution," Allen said. "Additionally, the government has contracted with Lockheed to support the Integrated Flight Test Team and help with planning, execution, analysis and reporting of future flight test efforts."

A spokesperson for Lockheed Martin said the company is directing questions about the incident to the Air Force.

More flight tests are being planned after the AC-130J is upgraded with a 105mm large caliber ​gun later this year, Allen said.

No modifications to the aircraft are being planned as a result of the accident, he added. The AC-130J airframe is considered stable, and the accident report indicates the accident occurred in part because the test pilots were pushing the plane to its limits.

— Lara Seligman contributed to this report.

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