The U.S. Air Force has released the final request for proposal to replac its fleet of Bell Helicopter UH-1Ns. Top Air Force officials have said the legacy Hueys lack the performance for their current mission sets and are becoming more difficult and costly to maintain.

“This acquisition will replac the fleet of Vietnam-era UH-1N helicopters with an airframe that addresses shortfalls in speed, range, endurance, payload and survivability,” the Air Force said.

The service previously scrapped plans to issue final request for proposals, citing industry comments that it was unfeasible to offer an off-the-shelf aircraft for the competition. Last month, the Air Force added ADS-B Out technology as a requirement for the new aircraft. This was prompted by the FAA’s 2020 ADS-B mandate.

Contract award is expected for the third quarter of fiscal year 2018, with an initial operational fielding in FY 2020 or 2021. Potential bidders include a Boeing-Leonardo team, Sikorsky and Bell.