Boeing has been forced to ground its entire test fleet on its long-delayed flagship aircraft, in the latest hit to the troubled plane manufacturer.

Boeing has confirmed it has halted flight tests on its 777-9 fleet after discovering the failure of an engine-related structural component.

The setback to the 777X’s already delayed certification program came as trouble developed on another front for Boeing.

The US Federal Aviation Administration has also ordered inspections into Boeing’s 787 Dreamliners, following a horrifying incident in March when 50 people were injured after a pilot unexpectedly slammed forward into the controls on a LATAM Airlines flight from Sydney.

The FAA had only recently cleared the way for certification tests on the long-awaited 777X, which Boeing describes as “the world’s largest and most fuel-efficient twin-engine jet”.

Earlier this week, a routine inspection after a test flight in Hawaii found a severed link on one of Boeing’s five test aircraft.

Inspections of two other planes found cracks in the assembly that transfers thrust between the engines and the pylons that secure the engine to the air frame.

“During scheduled maintenance, we identified a component that did not perform as designed,” a Boeing spokesperson said.

“Our team is replacing the part and capturing any learnings from the component and will resume flight testing when ready.”

The company said the failed part was a “structural component between the engine and the airplane structure”, and is unique to the 777-9.

“We are inspecting the flight-test fleet for this condition. No near-term flight tests were planned on the other flight-test airplanes, which have scheduled maintenance and lay-up activities,” it said.

The company has notified the FAA of the faults.

Travel industry publication One Mile at a Time reports there are 481 orders for the 777X, whose commercial rollout – originally slated for 2020 – has been pushed back until 2025.

Almost half are from Emirates, which has a $US52 billion order for the wide-body planes to replace its retiring Airbus A380s.

Rival Qatar Airways also recently placed an order for 20 777X aircraft.

Emirates president Sir Tim Clark has said he doesn’t expect Boeing can deliver before 2026 – six years behind schedule.

Meanwhile, Boeing also faces further issues with its Dreamliners.

Under an airworthiness directive issued by the FAA earlier this week, nearly 1000 planes worldwide must have their captain’s and first officer’s seats inspected within 30 days amid concerns about the mid-air LATAM plunge.

The FAA said the apparent reason for the dive while the plane was crossing the Tasman was the involuntary movement of the captain’s seat, which caused the auto-pilot to disconnect.

A dozen people, including passengers and crew, were taken to a hospital after the plane landed in New Zealand following the mid-flight incident.

“The plane, unannounced, just dropped. I mean it dropped unlike anything I’ve ever experienced on any kind of minor turbulence, and people were thrown out of their seats, hit the top of the roof of the plane, thrown down the aisles,” passenger Brian Jokat said at the time.

The FAA said such movement of an occupied seat could lead to a rapid descent and serious injury to passengers and crew.

It said it had received five reports of similar problems with the captain and first officer seats on 787s, the most recent in June.

Two remain under investigation.