Flight over a hostile environment (such as the North Sea) is regulated in JAR-OPS 3 by addressing the two issues: certification for tasks; and consequence in the event of a forced landing. Flight over a hostile environment requires the helicopter to be certificated in Category A (a subject which has been dealt with adequately in another thread) and, performance aside, is the ability to fly for extended periods with the probability that no failure will cause a catastrophic outcome. In addition to this, flight over water requires the fitting of additional safety equipment and, if that water is considered to constitute a hostile environment, certification for ditching (flight for oil support also calls for a number of practical additions to these requirements).
With specific regard to the Sea State, there is no specific boundary but the following is considered to apply:
Judgment is required when applying the definition of safe forced landing to over-water flights (the boundary between hostile and non-hostile) as injury to persons in the aircraft extends beyond the touchdown it also includes capsize and, the subsequent evacuation and access to safety equipment (such as life vests and liferafts) under difficult conditions. When attempting to allocate the boundary between hostile and non-hostile for over water flights, it is best to revert to the two main elements for Risk Assessment: the probability of the event; and the consequence:
The probability of the event will depend upon the Performance Class in which the helicopter is being operated.
The consequence of the event will depend on the Sea State of the sea over which the operation is being performance and the certification of the flotation equipment.
Over water helicopter operations are permitted in the knowledge that emergency situations may arise which may require an immediate and forced landing. Accordingly, (at amendment 9) ICAO Annex 6 Part 3 paragraphs 2.2.11 and 4.5.1, and national operating rules specify those circumstances where approved flotation and safety equipment must be carried; ICAO further states that Sea State shall be an integral part of ditching information.
Requirements for ditching approval are contained in FAR/JAR 27/29.801:
Paragraph (b) requires that measures must be taken to minimise the probability that in an emergency landing on water, the behaviour of the rotorcraft would cause immediate injury to the occupants or would make it impossible for them to escape.
Paragraph (d) requires that flotation and stability must be demonstrated in reasonably probable sea conditions.
Experience suggests that the greatest risk to the occupants in a ditching is drowning due to inability to evacuate the aircraft following capsize and subsequent flooding of the hull. FARs/JARs require the designer to select the reasonably probable wave condition for the area in which the helicopter is expected to operate and to demonstrate that the probability of capsize has been minimised. FAA and JAA have adopted an interpretation (AC29-2A para 337(a)(3)) which states that Sea State 4 is considered to satisfy the reasonably probable requirement. Most helicopters that apply for ditching approval are therefore certificated to Sea State 4.
Sea State 4 represents a wave height of 1.25 m - 2.5 m (4 ft - 8 ft) and wind speed of 17 kts - 21 kts.
The weather conditions up to Sea State 4 provide a pragmatic limit for operations within the non-hostile classification - particularly as it aligns with the standard for Certification for Ditching satisfying reasonably probable conditions. It is therefore suggested that, for operations over open sea areas, the boundary for ‘hostile’ should be set to above Sea State 4.
A recent study (CAA Paper 2005-06) of wave climates along a representative selection of main helicopter routes in the northern North Sea and West of Shetland 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%. Clearly when considering routine offshore operations in the specified area, they can be (and are) regarded as permanently hostile and any regulation should make sufficient provision to ensure that the probability of ditching is minimised.
Because the probability of flying over a Sea State of 4 is exceeded for a large proportion of the time, it has been a long standing recommendation of the JAA that AC 29 be amended to reflect those conditions (this recommendation can be seen in CAA Paper 2005-06 which can be found at -
http://www.caa.co.uk/docs/33/2005_06.PDF).
Prohibition from flight when the Sea State exceeds the certification standard has never been seriously considered as it would have caused a great deal of disruption to the offshore industry. The Oil Companies have however, been instrumental in ensuring that the certification of floats is raised above the norm of Sea State 4 - hence certification for ditching on the EC225, the S92 and the A139 which is now to Sea State 6.