Chinook ditching North Sea
A friend of mine ditched a commercial Chinook in the North Sea back in the '80's They had a problem with the hydraulics which caused a severe control malfunction and wildly fluctuating Nr.
In the Chinook varying the rotor speed caused an increased vibration level that took some time to even out. My friend said the vibration was so enormous that he thought the whole control panel was coming off. Not only did they complete a successful water landing but all 47 SOB were evacuated and picked up by boats from a nearby oil installation.
The cause, if I remember rightly, the memory's not what it used to be) was discovered to be contamination of hydraulic oil. A compressor wash was carried out at the end of each days flying due to operating in a salt-laden atmosphere. Apparently the high levels of fluoride in the local water had not been anticipated and some inportant seals had rotted.