G-ANDY:
Don’t feel obliged to sum up the discussion because you instituted the thread - your assumption about the doors and windows is incorrect. (It is also not clear that the thread has exhausted itself yet.)
Each aircraft type for which an application for certification for ditching is requested has to show compliance with the requirement for
emergency exits; the number and type of these exits is governed by the passenger capacity and all required emergency exits must be above the water line. Specifically, FAR/JAR 807(d)(3) states that
Flotation devices, whether stowed or deployed, may not interfere with or obstruct the exits.
The presence of ‘pop-out-windows’ is an additional safety element - they are not the emergency exits that are required by certification; rather they are related to JimL’s remark:
Because it is reasonably probably that the helicopter will invert following a ditching in high sea states, all helicopters operating for oil support in the North Sea have a 'pop-out-window' at each seat row. These windows help to ensure that all passengers and crew members can evacuate within 'breath hold' time following an inversion.
The breath hold time following cold-shock (even when wearing survival suits) is in single figures.
Emergency exits and doors can cause additional problems; when G-TIGK’s door (see the picture above) was released using the emergency handle, it floated upwards and snagged; the consequence was a jagged edge which punctured one of the liferafts.
Additionally, JimL was not quite correct when he referenced the sea state:
…analysis indicates that the North Sea has a sea state higher than 4 for 30% of the time in winter.
in fact:
A recent study of wave climates along a representative selection of main helicopter routes in the northern North Sea and West of Shetland (regarded in JAR-OPS 3 as a Hostile environment), indicates that Sea State 4 will be exceeded on 26-36% of occasions over the whole year. During the winter period between December-February, this increases to between 51-65%. If the certification requirement was raised to Sea State 6, the risk of exceedance would fall to a maximum of approximately 3% over the whole year and 3-7% in the winter months.
Hence the attempt by the manufacturers to improve on sea state 4 for ditching approval; for example the S92 moves from sea state 4 to 6 by adding two additional bags one on each side of the fuselage at the back of the cabin.
TC is also correct in that the modelling of stability is done with tanks generating a regular wave form - it is the breaking of the wave that causes capsize not necessarily its size.