So in case like this, what’s a proper rapid action? There’s zero forward speed, so what can be done when the engine fails ?
At the altitude they were at, not much can be done. It’s kind of a dead man’s zone. You can try to ease the bird forward to get a some forward speed, which will allow for a flare and cushion to some form of a touchdown. But in a power loss situation, everything you do to attempt to maintain useable Nr, comes with a cost that works against you. Be it a higher descent rate, a loss of speed, a further loss of Nr, etc. You need to either have kinetic energy (airspeed) or potential energy (altitude), to give you some chips to cash in, so to speak, as you attempt to preserve Nr with no engine power to maintain it.
Helicopters have what is known as a Height-Velocity, or H-V diagram. It details what combinations of speed and altitude, a successful autorotation in the event of power failure, is successful; thus, it defines the “dead man zone”, where a successful auto isn’t possible. In the case of the Astar series of helos, at 0 knots, or a hover, you have to be below 8’ AGL, or above 500’ AGL, in order to have the best chance at a successful autorotation. The Astar series are a low-inertia rotor system, meaning they don’t preserve Nr rotor speed well during a power loss. As opposed to a UH-1 Huey series, which is a very high inertia rotor system, that preserves Nr exceptionally well in the event of power loss, and thus has a different H-V diagram from the Astar. Essentially, in an Astar series, in the event of power loss, wheee you are going to be able to autorotate to, is somewhere that’s in your chin bubble window, or somewhere that’s nearly under you and not too far forward, and you have one chance at a good landing. In a Huey, in the event of power loss, where you can land is somewhere out the front windscreen, ie- distant, comparatively. And at the bottom end of the auto, you can touch down smoothly, pick back up airborne again to a few feet AGL with no engine power, move 40 yards or so laterally, turn 180 degrees, and touchdown smoothly again, all with the remaining Nr you were left with after the power loss, it preserves Nr so well.
In a high OGE hover situation, you’re already losing Nr due to the power loss. If you rapidly lower the collective to prevent the Nr loss, the cost for that is now incurring a very high descent rate. If you try to give forward cyclic to gain some speed, you unload the rotor system……which one of the effects of unloading a rotor system, is Nr loss, loss that can be normally checked with engine power; but if there’s no power or there’s no power getting to the rotor system, an power loss that causes Nr loss, will be highly exacerbated by the application of forward cyclic, resulting in an even more rapid loss of Nr, since there’s no power getting to the rotor system to keep the loss in check. You basically have very little, to,no choice in putting the aircraft down anywhere other than directly below you.
BL is, from where they were, the options were few, even if an instant reaction had been accomplished to whatever was the cause of the Nr loss. It’s why I’m never fully comfortable performing a high hover like this; even though for what I might be doing at the given moment, might require it.
From scene video, it appears the Astar came down on top of a shipping container that was on the ground in a shipping dock area, crushed part of the container and rolled to the right off of the container and to the ground next to the container, where it ignited in flames. Had the containers not been there, it’s entirely possible the flat impact on the ground would’ve spread the skids, and possibly not caused the post-crash fire. B2 models, as stock, have a non-crashworthy fuel tank located under the main rotor transmission and behind the rear seats; however, many also have conversions to an aftermarket crash-resistant fuel cell.
