My main comment would be that problems are mostly associated with reduced visibility and, not necessarily, a descending cloud base. This leads to two additional problems:
1. How is visibility measured in flight (I don’t know the answer); and
2. What if the cloud is descending generally – executing the 180º will not result in a flight back into a clear area.
This is why I emphasized that the decision is never presented to the pilot in clear and unambiguous terms. On the other hand, if the aircraft is well equipped – either with two pilots or with an autopilot (but in any case with some form of augmentation); there is a buffer both in height and in control.
My view has always been that although Parts 27/29 have a clause that states in 2x.141(c):
The rotorcraft must: (c) Have any additional characteristics required for night or instrument operations, if certification for those kinds of operation is requested. Requirements for helicopter instrument flight are contained in appendix B.
It is well known that there are no enforced requirements for certification associated with night operations (or operations in a reduced visual cue environment by day). This is not such an issue in those States where aircraft used for night HEMS are twins certificated under Appendix B of Part 27/29, but in the USA where singles with no additional (stability) requirements are used, too much reliance is placed upon FAR 135.207:
Sec. 135.207 - VFR: Helicopter surface reference requirements.
No person may operate a helicopter under VFR unless that person has visual surface reference or, at night, visual surface light reference, sufficient to safely control the helicopter
which, in an unlit area, relies upon the use of NVG to meet the visual surface reference requirements.
Although the objective of the rule does state that visual cues must be “sufficient to safely control the helicopter”, we all know that this (subjective judgement) is totally reliant upon the other part of the equation - i.e. the stability of the helicopter. Whilst that stability is addressed by Appendix B to Parts 27/29 for any aircraft that is certificated for flight in IMC, it is not for most singles (and even some twins).
The handing qualities of the helicopter and the usable cue environment are inversely proportionate to each other. As the quality of handling increases, the requirement for visual cues reduces. At the extreme (with an auto-pilot), the only cues that are required, are those which provide for obstacle avoidance (not unimportant but of a secondary order).
This is not a message that is usually well received by regulators, or operators, as it really points to the necessity to address stability in a reduced visual cue environment – particularly at night. The traditional answer to this dilemma is to place a requirement, at night, for twins (knowing that most will come with certification for flight in IMC) - this is the European solution; in addition, in some States (the UK for one), airspace at night is designated IFR; which leads to the (JAR) requirement for an auto-pilot for single-pilot operations.
Night IFR in the UK does not mandate the full set of rules contained in ICAO Annex 2 but instead a quasi-night-VFR regime exists that permits operations below 3,000ft to be conducted much as they are for day VFR.
No simple answers but, if States wish to address the main issues, stability is key;
it does not have to be twins, Appendix B to Parts 27/29 can be used for singles.
It is only when the issue of stability has been addressed that EVS or SVS come into play; they address (the secondary issue mentioned above of) obstacle avoidance but can never replace the necessity for good handling qualities.