Elevated Heliupad Take-off Technique
CAT A (Group A) (Class 1) Elevated Heliport Techniques have traditionally ALWAYS employed a reverse flight profile from the helipad up to and between 80 and 200 feet dependant on the type or any obstacles that may be in front of you.
Offshore Helicopter Operations are conducted (generally) under the alleviations afforded by JAR-OPS 3.517 - Take-off and Landing with Exposure (ie. the inference being that should the engine fail during the "Exposed" period, a safe forced landing may not be assured).
This situation exists partly due to the vaguaries of the offshore environment, turbulence around helidecks and so forth, and partly because it is not really possible to determine the performance of the helicopter prior to arrival at the deck, due to the obstacle environment etc.
The "Exposure" period is also "Theoretical" because if it was not; the only time you would realise that you may or may not have exceeded the exposure period, is when it actually happens, and guess what, you are already exposed.
The implied risks associated with operating with an exposure period during take-off and landing are mitigated by the requirements of the approval, in that to be so approved you must operate the helicopter under an engine reliability programme, have a full HUMs kit and protocols fitted for trend monitoring, AND MOST SIGFNIFICANTLY operate the helicopter IAW the procedures acceptable to the authority.
These proicedures were first published under JAR-TGL 14 and follow in principle the older HAPs modelling studies for offshore operations. These procedures DO NOT incorporate a reverse flight take-off procedure. They are based on a vertical take-off to a TDP of approx 25 feet and then a rotation away from the deck using the momentum of the intitial vertical climb to hopefully clear the tail should an engine fail. There is no performance solution in play and safety is assured by the close monitoring of the 2 engines in play as indicated earlier to provide an assurance within defined acceptable probabilities that neither engine will fail .
I do not know what kind of approval the AW139 has for offshore, and although the Operator may be JAR-OPS, the Procedures they are required to operate must be acceptable to their responsible Authority which may differ from those currently applied in the UK-NS sector.
From a performance point of view, ignoring the turbulence environment for a moment, a helicopter conducting the CAT A approved Elevated Helipad Profile, at the correct mass for WAT, has the capability in the event of OEI to either, Re-land safely back on the helipad, or, continue the take-off safely, meeting all required safety margins, depending on the point in the profile at which the OEI occurred.
For a helicopter operating to TGL-14 and the HAPs model, this absolute level of safety in the event of OEI is not provided, but may be there depending on the ambient operating conditions and obstacle environment.
THEREFORE - if some offshore operators are electing to (or required to) use a CAT A technique offshore, from a performance point of view, provided the ambient conditions and obstacle environment is suitable, the helicopter is being operated to a greater level of safety than one using the JAR-OPS 3.517 Class 2 With Exposure alleviations.
Hope this helps.