To aid further understanding here are some extracts from and a link to the minister's letter.
The causal factor identified in this instance was that PWLS starboard shaft was misaligned from build, and that the incorrect installation of key components resulted in the defect suffered.
Several contributory factors lead to the misalignment and the subsequent incorrect assessment that the starboard shaft and couplings would perform as required. Following reviews of the impact of the shaft misalignment, a decision was made that PWLS should proceed to sea for trials and that further monitoring would be conducted.
During PWLS’ sea trials, both shafts were extensively tested over several weeks and thousands of nautical miles in the Northern North Sea and UK South Coast Exercise Areas. Vibration readings remained within limits at all times. Accordingly, no issues, conditions of class or actionable items were transferred into service from the Aircraft Carrier Alliance (ACA) to the Ministry of Defence (MOD) in 2019 at Vessel Acceptance.
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Thereafter, during PWLS’ subsequent operational time at sea, which included exercising and NATO tasking in the Mediterranean in June 2022, no abnormalities associated with either shaft were reported.
Following the coupling failure in August 2022, an alignment issue was also identified with her port shaft. Rectification work was initiated and is expected to be completed in time for PWLS to commence her operational programme, as planned, this autumn.
PWLS was always scheduled to be alongside and undergo upgrade work for the majority of 2023 in preparation for taking over as the UK Very High Readiness Strike Carrier in 2024. Such work includes updates to the flight deck to expand F35 operations upgrades to communications and information systems and upgrades to combat management systems architectures. Consequently, over this period capability upgrades have been conducted in parallel to the shaft repairs to ensure efficiency and value for money.
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The financial cost is estimated to be approximately £25 million. Responsibility for payment has not yet been established therefore it would be inappropriate to comment further at this stage.
Letter dated 3rd April from the Minister of State for Defence Procurement to Chair regarding HMS Prince of Wales' Defect Investigation Outcome