HMS Gannet's duty observer, Lieutenant Angela Lewis said: "Conditions were extremely challenging.
"Sea spray from the waves was being whipped up to a height of about 60ft in places and we were in the hover at about 80ft, so it was quite nerve-wracking."
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Mladic, now 75, was the military commander of Bosnian Serb forces in the 1990s, who fought against the Bosnian Croat and Bosniak armies.
Weather: Heavy rain and strong winds in the north
His representatives said Cassidy, who was hospitalised last week, died "surrounded by those he loved."