Further musings on performance
In a previous post, reference was made to the ‘clear area’ procedure and the rolling take-off to 25 kts GS. On further examination, it is seen that this is not the only reference to ground speed in the Category A performance section; it is also used in: failure before TDP (rejected take-off); approach to LDP (balked landing); LDPs; and to refine the balked landing manoeuvres – in fact anywhere a reference speed is 40 kts or below.
For the flight testing regime, the ‘allowable wind’ is for the flight test team to decide but, for vertical take-off and landing, general guidance is that it should be at ‘0 to 3 knots’ (AC 29.45(b)). It is likely therefore that an instrumented helicopter has accurately demonstrated all the points/manoeuvres referred above. I can imagine that there was a discussion in certification with respect to speeds below 40 kts, where the (raw data) ASI is known to be inaccurate, leading to this inappropriate use of GS to be applied.
Most of the referenced points/manoeuvres are airspeed sensitive (not ground speed) and reference to GS is therefore non-sequitur. This unique way of describing reference points/manoeuvres either has to be conditioned by explanation along with examples of practical use or a reversion to airspeed applied. It strikes me that providing an RFM to suit the certification process and not the pilot is not such a good idea.
Flight operations in the higher latitudes rarely encounter calm conditions (luckily) – in fact the mean wind offshore is 20 kts. It is unlikely that the pilot will reference the ground speed during his scan of the instruments (it usually sits on its own in an undefined location somewhere in the PFD/MFD), and only the most sophisticated autopilots can fly to the lowest speeds referred to in the RFM. In fact only with respect to siting over/near the landing spot will GS be an issue.
For these reason, reference to ground speed in the RFM is of little use unless the wind conditions are calm – which they rarely are – and/or ground speed becomes a primary reference parameter (as it has become from some offshore approach procedures).
Jim