Here is an attempt to quantify the benefit of the extant regulation with respect to the reduction in Exposure; in order to do this it is necessary to establish those operations which are potentially:
without exposure; and
where exposure might remain.
Firstly we need a reasonable basis for assessment. During the modelling of PC2e, access was provided to an operator’s database of weather reports (INTOPS).
INTOPS provided us with 39,131 North Sea wind data points (reports) for a four year period. Using this data we established that:
Wind <= 20kts occurred in 22,725 reports or 58%
Wind >20kts but <30kts occurred in 8,577 or 22%
Wind >30kts occurred in 7,829 reports or 20%
The following assessment assumes a helideck which is compliant with Annex 14 Volume II – as implemented in Cap 437. The orientation is not assessed but it is assumed that, if the CAP 437 recommendations have been followed, the Obstacle Free Sector is oriented into the prevailing wind; if this is the case, the figures shown below will be pessimistic.
Using the figures shown above, we are able to estimate the likely proportions of: non-exposed; potentially non-exposed; and potentially exposed, operations.
Non-Exposed
Wind at or below 20kts in all sectors:
Pure PC2 can be flown in 58% of all take-off or landing cases
Wind in excess of 20kts occurring in the 180° sector:
PC2e can be flown in 21% of all take-off or landing cases
This accounts for 79% of take-off and landings.
However, each aircraft has a zero drop down mass that is associated with an unfactored wind - if we assume that this wind is 30kts (not an unreasonable assumption given the figures provided in the calculated masses) then PC2e (without exposure) can be flown for an additional (30/360 x 20%) 1.7% of flights – i.e. when the wind is > 30kts and in the (30°) LOS (no obstacles above deck height).
This raises the non-exposed operations to 80.7%.
Potentially Non-Exposed
Wind >30kts and in the 150° obstacle sector
PC2e with zero drop down occurs for (150/360 x 20%) 8.3% of flights
In these cases, exposure is undefined but is tending to zero. As the wind increases all aircraft reach the point where OEI HOGE is achieved (just below the lowest Vtoss).
The exception to this occurs on those decks with turbulent sectors; this is, and should continue to be, dealt with in the HLL.
Potentially Exposed
Wind is >20 and <30 and in the 180° sector
We have 11% of flights where, potentially, there would be exposure.
In Summary
Operations without exposure are likely to be 80.7% of the total.
Operations where exposure tends to zero is likely to be 8.3% of the total.
Operations with exposure is likely to be 11% of the total.
Conclusions
Whilst there are operations in the North Sea where there is de facto no exposure, this has never been quantified (apart from operations with the AW139). Using the actual wind data and the examples of masses shown previously this can now be done.
We continue to base the take-off and landing masses on the clear area WAT just so we can establish the second segment climb – this is no longer satisfactory.
There were several reasons why there were change to the regulations, the most important of which was to provide a signal to manufacturers that future aircraft should be
capable of providing exposure free operations.
To do nothing but justify what is already done will result in the continuation in operation of existing underpowered models and hold out the prospect of no change in the future.
Jim