Chinook - Still Hitting Back 3 (Merged)

[walter kennedy]

Pulse1
re your post #5251 – how I wish S/L Burke would pop into this forum.
I am not exactly a fan of his after what he said (repeating a section of your post):
<<From the HoL Report:
112. Finally, Squadron Leader Burke commented on the rudder input of 77 per cent left yaw found in the wreck of ZD 576:

"That is an enormous rudder input. It is unthinkable to put that in at high speed. As I may have explained, particularly in the Chinook but in any helicopter, the helicopter does not use the yaw input for control once you have gone over 20 knots. It puts an enormous strain on the aircraft because you obtain yawing control in the simplest way by tilting the rotors one left, one right. You are spinning the aircraft about its middle. It is quite difficult to do. The rudder is quite heavy on a Chinook. You have to make a real effort to put that amount of control in. The only conceivable reason that I can think of for putting that voluntarily in as a pilot is if you have partially lost control coming out and you are trying to counteract a yaw one way or the other" (Q 719). >>

This statement supports the view that they were in a situation where they had not been in control and so may have eliminated other possibilities, such as an emergency evasive manoeuvre while under control, from consideration in the minds of others at the inquiry or reading the transcript.
I find the above difficult to reconcile with characteristics of tandemrotor a/c that I have read about and with what I have seen in videos of Chinooks at airshows. As they are naturally unstable about the vertical (yaw) axis, either the pilot or an automatic control system is busy keeping the a/c straight – I am aware of accounts and videos of Chinooks flying sideways at high speeds.
Mindfull of this, I wondered if turning sideways was a method of slowing down in a hurry without using power (eg while needing all immediately available power for lift) and read around to get support for my common sense feeling that it would have served this purpose; while I did find a few, I came across a significant further advantage for turning sideways in an emergency – you get greater translational lift. I quote here from “The Art of the Helicopter” by John Watkinson (sec 9.9 The tandem rotor, p 371-2):
<<The interference between the rotors can be minimized by flying the machine side-ways as this increases the diameter of the stream tube and improves the aspect ratio, giving greater translational lift. This technique works well in a steep climb. The drag of the broadside hull will be a problem as speed builds up.>>
The last point is actually an advantage if you are trying to lose speed anyway you can in an emergency.

If you are aware of the posts I have been making on this thread you would know that the basic scenario I have argued is that they were intending to do a fast approach to a known LZ – the following is just the sideways aspect.
I believe they had started to slow down (Boeing analysis) – on last leg tailwind had increased by 15 kts but ground speed had been calculated as constant such that air speed must have reduced from previous cruise level;
A good way to let the high speed wash off is to reduce power such that thrust is just enough to act against weight and coast for a while – the matched power levels found suggests such a steady state – it is a minimum power level in a flight profile and a worst case if you require to make a sudden emergency manoeuvre as it would take time for the FADEC to respond and the engines to spool up – you would be better off to have been maintaining a high cruise speed so as to have that thrust component that had been working against drag to vector more lift (cyclic climb) or a sudden turn without losing height;
I believe that they were somehow misled in their judgment of closing range (critical in a fast approach) and surprised at their proximity to the ground when it became apparent;
Along with the full application of the thrust lever, I believe that the handling pilot was showing good airmanship with his apparent reaction of aggressively starting to yaw the a/c side-ways, getting some immediate extra lift and greatly increasing the drag – a few more seconds and this may have made a difference at least reducing the severity of the impact.
I hope that you can now understand that the a/c attitude at impact together with the “rudder” position can suggest an evasive manouevre under control.
Perhaps Flt Lt Cook was better at operational flying than an Odiham test pilot as was implied by Group Captain Pulford; this does not mean that S/L Burke's experience with the Chinook's control problems should not be given full consideration but if his omission of such a critically significant manouevre option, when he had the opportunity to give his judgment where it counted, is anything to go by then perhaps indeed “... he therefore lacked the operational currency to provide relevant evidence to the inquiry" .