Losses and Aviation Safety

17 years after a USAF F-15 crashed due to cracks in the jet’s longeron the same issue appeared on 90 F-16 fighters across the entire service inventory

Cracks in canopy sill longeron appeared on 90 USAF F-16 fighters

The Colorado Air National Guard’s 140th Maintenance Group (MXG) at Buckley Air Force Base recently had two of their own F-16s grounded simultaneously due to cracked canopy sill longerons (CSLs).

As the airframe has aged, it required these airmen to incorporate new and innovative maintenance practices to remain at the forefront of discovering new means of increasing airpower efficiency to ensure the fighter jets remain mission capable and ready to defend the nation 24/7.

US Air Force (USAF) Staff Sgt. Danielle M. Morgan and Tech. Sgt. Taylar M. Reilly, led one of these new practices as they repaired canopy sill longerons. The CSLs run the full length of the cockpit and support the structure between the frame and skin of the aircraft, preventing tension and bending of the fuselage.

“The aircraft is immediately grounded when the canopies crack,” Reilly said to Staff Sgt. Luccario Lovato, 140th Wing Public Affairs, for the article Colorado Air National Guard Maintenance Group creates innovative solution to aging F-16. “A crack can spread, and if both sides break, the whole nose could fall off.”

A US Air Force (USAF) F-16C Fighting Falcon, 120th Fighter Squadron (FS), 140th Fighter Wing (FW), Colorado Air National Guard (COANG).

Cracks in F-16 canopy sill longeron can become a fleet-wide issue

Canopy sill longeron cracking has already appeared in 90 aircraft across the USAF inventory of F-16s within the past year.

The F-16 can fly at twice the speed of sound, pulling up to 9-G turns (9 times the earth’s gravitational force) to accomplish air-to-air and air-to-ground missions, applying significant strain to an aging aircraft. While structural concerns are not new, structural maintenance is usually done at a depot facility.

“While this isn’t the first time we’ve done this specific task, this is the first time that we’ve had two aircraft with this issue at the same time,” Reilly said.

Reilly also explained that the structural integrity of the longeron is getting worse, and suspects it’ll become a fleet-wide issue.

Noteworthy, according to Alert 5, similar cracking issues plagued the USAF F-15 Eagle fleet in the past. In 2007 an F-15C suffered an in-flight break-up because of a defective longeron supplied by Boeing. The mishap, that took place during a training mission, resulted in a significant grounding event.

Similar cracking issues plagued the USAF F-15 Eagle fleet in the past

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The Eagle destroyed in that incident was from the 131st Fighter Wing of the Missouri Air National Guard. On Nov. 2, 2007 the aircraft departed on a standard training sortie. The F-15C, flown by Maj. Stephen Stilwell, experienced a violent shudder in its forward fuselage approximately 20 minutes after departing from an airfield near St. Louis. Then the aircraft broke up at an altitude of 18,000 feet above the ground. Major Stilwell managed to eject safely (despite sustaining a dislocated left shoulder and a fractured left arm) just before the aircraft broke apart, scattering debris across the Missouri countryside.

As reported by The Seattle Times, investigators traced the cause of the accident to the jet’s longeron. The investigation revealed that the longeron in question exhibited a non-uniform thickness, failing to meet the 0.10-inch (0.25 centimeter) specification outlined in the Boeing contract. The measured thickness ranged between 0.039 and 0.073 inches, a significant deviation that compromised the structural integrity of the longeron.

A comprehensive inspection program for all F-15 A-D models in the USAF followed the investigation into the crash of the F-15C. As a result, all 441 of the F-15 fighter interceptors were temporarily sidelined. The inspections revealed that a concerning number, a total of 182 aircraft, had longerons that didn’t meet original manufacturing specifications.

After undergoing additional inspections as the investigation was completed, most of the grounded F-15s were cleared to return to flight by February 2008.

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Photo credit: SMSGT JOHN P. ROHRER / U.S. Air Force

Dario Leone

Dario Leone is an aviation, defense and military writer. He is the Founder and Editor of “The Aviation Geek Club” one of the world’s most read military aviation blogs. His writing has appeared in The National Interest and other news media. He has reported from Europe and flown Super Puma and Cougar helicopters with the Swiss Air Force.

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