F-15E Strike Eagle lost in Libya yields its secrets ... sort of

[Zabaniyah]

An F-15E lost last March during the Libya campaign has yielded its secrets. At least the experts think it has.

Since the jet went down in a war zone, examining crash debris was not possible. But aerial photography, computer models and accounts from the two-man flight crew point to a critical weight imbalance caused by failure of a right pylon pressure regulator.

Jerry Byrd, engineering branch chief for the Eagle Division at Robins Air Force Base, believes fuel stopped transferring from the right fuel drop tank creating a critical weight differential after the pilot dropped a 500-pound bomb from the left side. The Warner Robins Air Logistics Center agency is the focal point for sustainment of the Air Force’s F-15 fleet.

According to reports, after the pilot dropped the ordnance he banked 100 degrees and made a left turn away from the target at 380 knots. That threw the aircraft into an uncontrollable spin. Both crew members successfully ejected.

Byrd said the system is designed to feed fuel from the right and left fuel drop tanks in an even, controlled manner so that weight is equal or essentially equal on both wings.

“Over time, you will develop somewhat of an imbalance,” he concedes, “but it’s not supposed to do anything major.”

Prior to the crash, the pilot reported fuel had stopped transferring from the right side after an aerial refueling. A pressure regulator in the pylon is key. It controls the forcing of air into the tank that, in turn, pushes fuel into the internal fuselage tank to be burned by the fighter’s two engines.

“Some times, those regulators freeze. Some times there is debris,” Byrd said. “For whatever reason, some times the regulators stick, the tank doesn’t pressurize and the fuel doesn’t feed.”

In this case, the pilot descended to 20,000 feet, possibly to thaw the regulator, and reported movement of fuel from the right side. However, the transfer did not continue, Byrd believes, creating a significant weight imbalance of up to 2,500 pounds since the left drop tank was empty at this point.

As the F-15 sits on the ground empty, the right wing is heavier by 1,850 pounds due to the gun.

“The pilot in this case took off with an imbalance of 8,168 foot-pounds with all the stores that were added,” Byrd reported. “Then when he dropped the 500-pound bomb and you add 2,500 pounds of fuel, he could have had over 30,000 foot-pounds on that right side. So when you have a big lateral asymmetry like that, abrupt maneuvering can lead to a departure.”

Boeing, manufacturer of the F-15, assisted Robins with the accident investigation.

“They looked at air speed, angle of attack, altitude … a whole bunch of factors,” the chief engineer noted.

The conclusion? The aircraft dash-one -- the operational bible for the air crew -- needs to be modified.

“When you have a huge lateral asymmetry, we need to clarify the dash-one to talk about critical angle of attack and abrupt maneuvering,” Byrd said. “That’s where our attention has been.”

Source: http://warnerrobinspatriot.com/book...agle-lost-in-Libya-yields-its-secrets-sort-of