Flipster
<<It is also apparent that there is some confusion as to the speed and ROC of the ac as the crew penetrated cloud, as they eventually did do - even Boeing's best guess seems like a stab in the dark. Helos do not zoom climb like jets, as you well know - but this is a fact that may have been lost on some? >>
I should wait until someone else qualifies this but what the hell.
While a helo does not have that combination of momentum and aerofoils that can, with a pull on the stick, execute a “zoom climb” regardless of power (eg as a glider can), it has two sources of “energy” that can indeed allow a zoom climb:
the stored energy in the rotors;
the thrust in excess of that for holding it up in the air (lift = weight) that is used for propelling it along at its cruising speed that is balanced by the drag.
You only have to look at air displays by Chinooks to realise how agile they can be when their payload is not excessive (and ZD576 that day was not heavily loaded with respect to two engines operating).
If you go through Boeing's “Analysis of Available Data”, by Mitchel, you would appreciate that it was a fair effort of rational analysis attending to speed/distance calculations in great detail; for a start contrary to the picture of excessive speed that seemed to be presented by many, they had simply covered their route at thecruising speed for an HC2 Chinook;
further, and I suggest of great significance to understanding this crash, after the turn at waypoint change their airspeed on that final leg had dropped significantly (this being masked by the increase in wind speed as the landmass was approached), the Boeing analyst concluding that they were slowing down; that the engine powers were found matched suggests that this was a steady state for a while in turn suggesting a planned slow down.
If the a/c had been continuing at its cruise speed, it would have had the thrust component used for pushing it along through the air available, at a pull on the cyclic, to push it up.
Indeed this does cause unwanted ballooning when not complimented carefully with the requisite working on the thrust lever when you want to do a fast approach/landing, does it not? That they had not zoomed up significantly despite having cranked her back (nose up 30 deg from memory? - path 20 deg up?) suggests that this extra thrust was not available reinforcing Boeing's conclusion that they were slowing down. The bit that they had climbed seems due to the stored rotor energy alone (reduced to about 90%?). The thrust lever, having been pulled fully up, had acted as collective but the engine system had not had time to respond as the power was still matched. They had been taken by surprise.
So how could they have been surprised by entering the ground mist when they had turned towards ground that was higher than them and they had started to slow down? Only that they had got there sooner than they expected is surely the only logical explanation.
They had turned onto a heading, reflected in the HP's course selector, that was the optimum approach for a known landing area, for which waypoint A would have been an obvious inner marker, with a RADALT warning set appropriately for an imminent landing, with the HP's baro alt subscale setting appropriate as a QFE for the elevation of that landing area, and they had started slowing down. And they were using a call sign appropriate for an exercise.
Looks like it is at least worth some consideration that, for whatever reason, they were doing a fast approach to that landing area but, for whatever reason, misjudged their closing rate.
Because of the nature of the ground there (topography, lack of features, nature of vegetation, etc) a fast approach to that landing area on that track is difficult to judge in the best of conditions – in the conditions of the day, it would have been folly to approach at speed without some other trusted reference other than the mk1 eyeball or the SuperTANS – and had that other reference been for whatever reason been misguiding them, it would have been hard to pick up on the error from visual cues until it was too late.