A report into the Rescue 116 helicopter crash in which all four crew members died on a search and rescue mission will raise significant questions about why a number of crucial navigation systems failed.

While the sole objective of an Air Accident Investigation Unit (AAIU) investigation is the prevention of accidents and incidents and they do not assign fault or blame or determine liability, the report, which is expected to be published today, will also cast doubt over whether the March 2017 rescue mission was necessary.

Paul Ormsby, Dara Fitzpatrick, Ciarán Smith and Mark Duffy died when their helicopter crashed into an island off the coast of Mayo just after midnight on 14 March 2017.

Meanwhile, the widow of helicopter pilot Captain Mark Duffy has welcomed the pending publication saying it would deliver her family closure.

In a statement ahead of the report’s publication, Hermione Duffy reflected on the “many long and lonely days and nights without Mark in the nearly five years since the night our worlds were thrown upside down and Mark lost his life”.

“Esmé, Fionn and I are very relieved to see the publication of this report,” she said, referencing their children.

She appealed for people to remember that “four honourable souls lost their precious lives that night in the service of others, and in circumstances which are harrowing and traumatic to read of and which have left wives, children, parents and extended families bereft.”