The US Marine Corps will have to wait until at least March for its new heavy lift helicopter, the Sikorsky CH-53K King Stallion, to make its maiden flight.

Originally planned for summer 2014, the King Stallion’s first flight was pushed to the end of the year when a crack was found in one of the four gear boxes of a ground test article. US Naval Air Systems Command now says the aircraft will enter flight testing “sometime between March and May next year.”

“First flight is driven by the current ground test vehicle (GTV) test events,” says Kelly Burdick, a spokesman for the navy’s programme executive office for aviation. “The GTV is currently undergoing powered ground tests to measure and verify the ability of the drive system, transmissions and engines and flight control system to safely fly the CH-53K helicopter across multiple flight scenarios.”

Engineers at NAVAIR and Sikorsky have made adjustments to the CH-53K’s main rotor gearbox to improve load distribution and have been retesting the fix “to ensure optimal performance prior to the flight test phase”, Burdick says.

“All issues discovered to this point have a technical solution and are typical of developmental programs - this is why we do this testing,” she says. “These tests, their data, and their schedule all drive the timeframe for first flight and discoveries are typical during this phase of testing.”

A Sikorsky spokesman says the static ground test article has undergone nearly 200h of testing, including subjecting the airframe to 115% of its maximum load. Sikorsky also has completed vibration and ultimate load testing on two conditions, in which the main rotors are overloaded by 150% compared to design loads.

The USMC has plans to buy 200 King Stallions to replace its entire fleet of smaller CH-53E Super Stallions. The CH-53K’s structural integrity was officially cleared for flight in April, then rolled out during a 5 May ceremony at Sikorsky’s West Palm Beach, Florida, manufacturing facility.

The King Stallion shares its designation and much of its exterior design with the Super Stallion, but is in reality a clean-sheet aircraft with new rotor, engines, transmission, cockpit, cabin and tail rotor. With a maximum takeoff weight of 39,900kg (88,000lb), the CH-53K will be the US military’s largest helicopter when it enters service. It will be capable of ferrying two Humvees at once compared to the CH-53E’s one, as well as make multiple combat drops on a single flight.

The USMC originally had plans for the King Stallion to enter service in 2015, but developmental delays have caused initial operational capability to slide at least until 2018.