The release of president Donald Trump’s fiscal year 2018 budget, which continues millions of dollars in funding for ongoing upgrades to the Boeing F-15C – as well as a new service life-extension programme (SLEP) for the aircraft’s longerons – has allowed the US Air Force to officially quash rumours of a near-term retirement for the fighter.

In order for the F-15C/D fleet to meet its planned service life within acceptable risk margins, the proposed budget sets aside $7 million in FY2018 for a new longeron SLEP activity.

“The longeron SLEP entails replacing 14 primary tension members in the structure of the forward fuselage and is critical to the safety of flight of these aircraft,” the USAF says in budget documents. “There are other structural issues with the F-15Cs besides the longerons, and full-scale fatigue testing is ongoing to assess these matters.”

Boeing is conducting fatigue testing with a full-scale F-15C and F-15E article, Steve Parker, vice-president of F-15 programmes at Boeing, told reporters in St. Louis, Missouri last week. The aircraft have gone through 30,000h of testing, he says. By replacing the longerons on the fighter – an endeavor that would cost about $1 million per aircraft – Boeing could push the F-15C's service life into the 2030s.

An earlier USAF analysis examined updating the F-15C’s fuselage, wings and landing gear, which could stretch the aircraft’s life into the 2040s – but would cost $30 million per airframe, Parker says.

“We believe that’s the most costly scenario,” he says. “We don’t believe the scenario is required – we don’t believe the air force is even looking at that scenario, because for $1 million they can replace the longerons.”

Rumours of the F-15C/D fleet’s possible retirement swirled around Washington DC earlier this year after a USAF official mentioned an analysis to replace the fighters with Lockheed Martin F-16s.

But Trump's proposed budget would continue modifications to keep the F-15C flying into the mid-2020s, including the Eagle passive active warning and survivability system and an integrated defensive electronic countermeasures system. These would autonomously locate radio frequency threats and deny RF threat systems.

The proposed budget also requests $57 million for an infrared search and track system in FY2018, and $16.7 million for upgrades to the platform's Raytheon APG-63(V)3 radar.