LONDON — Nearly 20 years after the British government first decided to build two large aircraft carriers to revive the Royal Navy's global air strike capability, the first of those warships, HMS Queen Elizabeth, has left the dock at Rosyth, Scotland, to start sea trials.

The 70,000 ton carrier slowly emerged from the dock today where the BAE Systems-led Aircraft Carrier Alliance has assembled the warship from modules built by shipyards around Britain in a program set to cost around £6.2 billion (U.S. $7.8 billion) by the time the second carrier, HMS Prince of Wales, is completed in 2019.

The Queen Elizabeth squeezed out of the dock with just 14 inches clearance either side to start initial trials in the North Sea.

The contractor trials are expected to last for around 11 weeks before the carrier heads for the naval base at Portsmouth, which will be her home for the next 50 years or so. 

If things go according to plan, the carrier is expected to be accepted off contract by the Royal Navy around the end of the year.

Unspecified technical issues have delayed the start of the trials by about two months, but Ministry of Defense officials have previously said it is a minor setback and within the tolerance of the program to remain on track.

The carriers are the largest warships ever operated by the Royal Navy.

The vessels are 280 meters long and can embark up to 40 Lightning II short take-off vertical landing jets and helicopters — although senior naval officers have said they could actually carry many more aircraft if required.

The most notable design feature is the carriers two island sections instead of one. The design provides independent control of navigation in the forward island with an air traffic operations in aft island.

The hangar deck measures 155 meters by 33 meters with lifts capable of lifting two aircraft onto the flight deck simultaneously in about 60 seconds. The vessels are powered by an integrated electric propulsion system employing two Rolls-Royce MT30 gas turbine units and two Wärtsilä diesel generators. 

Although not referring directly to the sea trials process, the National Audit Office, the government's financial watchdog, warned in a report recently that the MoD was now entering a "high risk phase" of the project as it sought to bring together various core carrier strike programs between now and full flexible operating capability in 2026.

"The next three years will be critical to establishing the carrier strike capability. The MoD must bring together the carriers, F-35B Lightning II jets, and Crowsnest [airborne early warning ] radar [helicopters] with trained crews and supporting infrastructure, logistics, communications and surveillance. It needs to test and operate all these elements together in preparation for deploying in 2021," said the NAO.

"It [the MoD] is focusing on managing strategic risks across the program over the next three years which could have a significant impact on delivery," said the NAO report on delivering the Royal Navy's carrier strike.

The report highlighted shortages of skilled naval personnel, tight schedules with limited contingency on the three core carrier related programs and operational unknowns that would only become clear after equipment testing as being the prime risks.

Queen Elizabeth's emergence from the dock at Rosyth has been a long time in the making. The Labour Government led by Prime Minister Tony Blair mandated the revivial of the Royal Navy's large deck carrier capability in the defense review of 1998.

The contract with the Aircraft Carrier Alliance saw the first metal cut on the warships around eight years ago. To finally get this far is no mean achievement for the program. Labour and Conservatives have both sought to reduce, delay or cut the program altogether at various times. 

At one point, the Conservatives even sought to have parts of the carrier redesigned during construction when they opted to swap the F-35 B for the conventional take-off and landing carrier variant. ‎That idea was eventually scrapped due to the high cost of amending the carrier design. It will be the first time the British have had a large deck carrier at sea since 1984 when the Royal Navy pensioned off HMS Hermes. Since the Conservative government's Strategic Defence and Security Review of 2010, the British haven't operated a naval air strike capability at all after the Invincible-class light carriers with their Harrier jets were controversially scrapped.

British pilot and carrier operational skills have been retained primarily by seconding personnel to the U.S. Navy

Initial operating capability of a force of F-35B short take and vertical landing strike jets, along with Crowsnest radar-equipped, airborne early warning-equipped helicopters, onboard HMS Queen Elizabeth are expected by 2020.

Fixed-wing flight trials with three British F-35Bs off the East coast of the United States are slated for next year following earlier helicopter tests with Merlin and Chinooks.

The cash-strapped British won't have sufficient F-35Bs available initially to fully equip the warship and the British government announced late last year that U.S. Marine Corp jets would also be deployed on the Queen Elizabeth during it's first operational deployment scheduled for 2021.

The British don't have the resources to operate both of their carriers at once but the completion of HMS Prince of Wales will enable the Royal Navy to have one carrier available for operations at all times.

The jets are also being flown by the Royal Air Force on land-based operations.

The British government has pledged to buy 48 jets and has given a vague commitment that it will eventually acquire 138 Lightning II's by the time the aircraft program ends.

Andrew Chuter is the United Kingdom correspondent for Defense News.

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