There is a world of difference between the performance required for a helideck category A departure, HEC Class D and SAR HHO.
Whilst Nick is likely correct in his estimation of a correlation of 1.2 between the OEI HOGE and the departure from a hover whilst hoisting for SAR (where a fly-away with drop-down might be permitted) it does not necessarily carry across to rig departures.
HEC Class D is totally different in that the permitted reduction in height following an engine failure must not be more than the greater of 4ft, or 10% of the height established at the time of the failure.
When modelling the PC2e rig departures, establishing an accurate 'single' correlation between OEI HOGE and RTOM (for all types modelled) was quite difficult.
The rig departure has to take account of two elements: the deck-edge miss; and the single engine fly-away and recovery.
The deck-edge miss is sensitive to a number of elements:
The AEO vertical speed attained at the Rotation Point (TDP);
The height of the Rotation Point (TDP);
The rate and magnitude of the pitch down (- PA); and
The length of time negative PA is held before recovery is started.
These elements also have a bearing on the drop-down in addition to the reserve of power discussed by Nick. The length of fuselage also has an effect on the maximum mass for deck-edge miss - a smaller helicopter benefitting in this respect (for fairly obvious reasons).
The profile flown can also have a detrimental effect; this was shown in modelling by varying elements of the manoeuvre – e.g. the rate and magnitude of the PA as well as the time negative PA is held.
Variability was also observed in the flight trials of one of the types flown offshore; two flights with the same mass and environmental conditions (flown by different pilots) had a reduction of half the deck-edge miss and an increase of drop-down of up to 50 ft when negative PA was held for about a second longer. This difference could be explained by the lack of accuracy of the required cue (a set airspeed indication on the ASI) rather than just pilot variability.
Later procedures appear to address this inaccuracy by using ground speed on the PFD as the pitch up cue.
Jim