Some musing on AW189 performance.
Leaving aside the issue of the requirement for the H-V diagram when Category A is not mandated, let’s see what the performance data tells us.
For Category A: we can see that the ‘clear area’ procedure permits maximum mass operations up to 4000ft and 20⁰C. The ‘vertical’ procedure at MSL and 15⁰C permits a RTOM of 8,050 kg with nil wind increasing to 8,200 kg with 20 kts (all accountable headwinds are already factored at 50% in the graphs). Like most others I am baffled by the ‘rolling take-off’ clear area procedure (as stated earlier); some explanation would be extremely helpful.
There is a strong correlation between the Category A ‘vertical’ procedure and the Category B graphs for ‘reject’ and ‘fly-away’ (if one take the Category A ‘vertical’ procedure with its TDP at 110ft and reconstructs it with the Category B ‘reject’ and ‘fly-away’ graphs with the same drop down, one mirrors the other). The reject graph however, has a maximum height of 200ft!
With respect to Performance Class 2; the flight manual states that the second segment climb performance (150ft/min required) is assured if the Category B ‘fly-away’ graph is used. However, that doesn’t tell us enough (because that may not be the profile that is used). What can be seen from the performance graphs however is with the offshore regime (the ISA conditions of +15C and MSL) at Vy with MCTOM of 8300 kg:
- At OEI 2 minute power, the ROC is in excess of 700ft/min (to above 50⁰C);
- At OEI MCP, the ROC is in excess of 350ft/min (to above 35⁰C);
The Category B ‘reject graph’ and ‘fly-away’ graphs can be used to tailor most vertical profiles and, specifically, the HAPS helideck departure and arrival. The one thing that is missing is the achievement of deck-edge clearance – for this we must await the Category A ‘helideck’ procedure.
We already know that the ‘reject’ graph provides us with sufficient data (reliably) to predict what will result from an engine-failure before the ‘Rotation Point’ (RP - which we can choose) or after ‘Committal Point’ (CP - which we can also choose). The ‘fly-away’ graph permits us to tailor our take-off mass to the drop-down – both for take-off and landing. The ‘fly-away’ graph has wind accountability (without factoring) - although the first 20 kts of wind is ‘shaded’ because it is said that the airspeed system is unreliable up to that speed (without knowing whether that might be a factor in performing the fly-away’ profile, it is difficult to assess whether it is or is not an issue; realistically, we have a reasonable accurate wind vector from the rig which we could factor and apply). Working backwards from the drop-down height, we can establish safe* Performance Class 2 take-off or landing masses.
* With the proviso that we cannot assure deck-edge clearance – so what’s new?
Now to calculate some figures: using the established average deck height in the North Sea of 98ft, and the height of the RP and CP as our sea-miss distances (once again using ISA conditions) – the PC2 take-off mass and landing masses (without ditching exposure) will be:
- Zero wind = 7,800 kg;
- 20 kts wind (factored) = 8,200 kg
Jim