Yes, the rudder boost assists in the objective of keeping below the 150lb, but the root cause is the asymmetry. As the Kingair increased in size over the years and got more powerful engines the asymmetric forces increased at OEI T/O. It wasn't fitted to the early KingAirs with smaller engines, but on later models / variants it's mandatory. It may also be that the aircraft actually just runs out of rudder without AFX. That's a Beech test pilot question.

Rudder Boost
 

Just in reference to the above post and not to the accident. I was once conducting a pre-buy survey C of A test flight on a B200. I was not rated so had a rated guy fly the aircraft. When I shut down the right engine, at altitude, the rudder boost ran away in the wrong direction towards the failed engine. The pilot flying, a normal type rated pilot, did not cope and I recovered the aircraft when I took control from him at an extreme attitude. However it was controllable if you are sharp. The fault was a capillary hole in the rubber boost valve membrane so it acted like a venturi and the rubber membrane was sucked rather than pushed.

NTSB Docket

A question that has me puzzled. The KingAir docket (Post 43) mentions that at the instant of engine failure the pilot initially applied wrong corrective rudder before reverting to correct rudder. At least, that is how I read it.

Memories fade or can fool you, but at one stage in my career I was a flying instructor on the turbo-prop HS 748.

It was equipped with Rolls Royce Dart engines. I was training another pilot. After lift off I closed the left engine throttle quite quickly to simulate an engine failure.
The pilot was on instruments. The instant I closed the throttle the aircraft initially swung in the opposite direction to that expected. I expected a yaw to the left. Instead for a second or so the aircraft yawed slightly right which was instantly corrected by the pilot.

The swing was less than two seonds but the pilot under training saw the compass card move and applied instant corrective rudder. The result was an immediate sharp roll caused by his corrective rudder which was made worse when the drag cut in from the closed throttle. In other words the simulated dead engine propeller seemed to momentarily increase thrust as its throttle hit the idle stop then a fraction of a second later the windmilling drag cut in and the direction of swing was as expected.

Being on instruments at lift off, the pilot felt the aircraft momentarily yaw in the opposite of the direction expected and quick as a flash applied corrective rudder. It caught me by surprise especially the fast rate of roll that cut in caused by the windmilling prop made worse by the pilots initial wrong rudder application.

Because of the obvious confusion which engine had simulated failed and which had caught both the pilot under instruction and myself by surprise, I decided to discontinue the exercise by taking over control, levelling the wings as well as re-introducing power to the "failed' engine. The rest of the flight was normal.

I wondered if the KingAir accident was made worse by the initial mis-identification of which engine had failed? But in the KingAir accident there was no apparent pulling back of a throttle to simulate a engine failure. As I said earlier, memories of long ago (in the HS758 case it was 53 years ago} are unreliable but something unusual must have happened to make the incident stick in my mind.
So the question is:
Can rapidly closing a throttle to simulate an engine failure in a turbo-prop aircraft, momentarily cause an apparent increase of power on the "dead" engine due coarsening of its propeller?

Maybe there’s a short moment, while the prop is moving to coarse, when the prop is still producing ‘quite a bit’ of thrust at the same time as the drag has reduced ‘quite a bit’.

But anything is possible on a Turbo (or Draggie, depending on one’s pedigree) being - as they are - the devil’s work.

There is a suggestion in the report that the pedestal engine/prop friction knobs are getting a little extra interest, as having a history of being temperamental. Could be an uncommanded throttle movement.

However, there are many pieces to the puzzle, and among others things I don't see why a simulated engine failure and a real engine failure would not both produce the same reverse-yaw effect you mention.

Is this effect a consequence of the manner in which the propeller is powered? If the engine is a two shaft type then unlike with a piston engine failure there is initially nothing to retard the rotation of the propeller until the torque-sensing power reduction and/or auto-feather starts to take effect.

As I mentioned in an earlier post, the King Air 200/300 have a special condition associated with their certification, which acknowledges very high yaw angles which can be achieved. Yaw angles approaching 40 degrees are possible, in more or less level flight, and the effect is more pronounced with the nose gear extended. I can't help thinking this could have been a factor. I have explored these high yaw angles while flight testing a modified 200, and the characteristic is real. I was warned (and required) to test for it. However, if a pilot were unaware of this characteristic, and blundered into it, it would be really surprising. That coupled with an unintended power lever roll back (also a known event in King Airs, if the friction is not set), and single pilot, so task saturation, would create these circumstances...

The feathering and autofeather of the PT6 is very smooth and effective. There are few "power" effects during feathering, it just winds down and feathers. Unfeather, on the other hand, must be handled very gently!

Certainly a grim video.
As someone who has never flown a twin I have a naive question:-
The pilots evidently never had this under control; would a twin pilot ever consider shutting down (both) engines and aiming for an open space?
Sure this action is an admission of pilot failure but they might have managed a forced landing without loss of life?

In all King Air models, power levers —the left one in particular, have a tendency to roll back to idle if the friction knob is not tightened. As per the “before takeoff checklist,” the friction knobs have to be set (tightened).

Note also that if any power lever comes back to a certain position, the autofeather system is disabled, this is to prevent a propeller to feather if the pilot willingly retards a power lever.

In this case, and if the power lever rolled back towards idle, the pilot needed to add power by bringing or pushing the power lever(s) forward.

And, “step on the heading bug” to keep directional control (assuming he did set the bug correctly for takeoff). Instead, he briefly applied force on the wrong rudder, that exacerbated the situation.

The NTSB docket states many pilot’s shortcomings:

The CVR recorded no calls by the pilot or copilot for any checklists, no takeoff briefing was made for any emergency contingency, no calls for V-speeds, etc.

According to testimonies retrieved by the NTSB, the pilot did not normally use checklists, but liked to just “jumped in the airplane and went.”

Etc.

I have certainly included this in my plan for certain operations were performance or control were not as certain (some testing of external mods on twins, where the control characteristics were not fully known). If I must return to earth, the more in control I am when I do it, the better it will turn out. I am willing to take criticism about not trying to climb away on one, if that were in question. Happily, whenever I have discussed this intended action with the boss/owner of the twin, I have not been resisted.