Getting messy, too many colours!
I in green now
Originally Posted by
HeliComparator 
Crab as you know I (used to) fly EC225 in oil and gas support. In that role perhaps 95% of the flight time is at MCP doing 145-150 in the cruise, and up to 165 on descent. So taking into account climb time, perhaps 90% at 145+. Obviously the cause of a double engine failure in the cruise is hard to envisage, but since we spend the vast majority of time in the cruise and double engine failure is something we are required to train for, it seems sensible to take the cruise case - and for example I guess the PHI 76 was probably doing 145 when it hit the bird.
Exactly how many Dual Engine failures have ever occurred on the North Sea? One that I can think of, another one where the failures were not simultaneous. If you are trying to say that it's rare and improbable then I quite agree, but nevertheless this is the circumstance we are considering.
Before we had the simulator we used to do this in the aircraft. But we were wimps and simulated auto entry purely by lowering the lever - engine control switches were not retarded due to the risk is using real OEI power. So the "failure" was pilot flying initiated - pretty benign you might think.
it was also not representative of an actual simultaneous Dual Engine Failure. Correct, with an actual simultaneous dual engine failure the Nr would likely be pretty low before the collective was lowered due to pilot reaction time.
Nearly everyone got it right, flaring the aircraft as the lever was lowered to maintain 1g+. But a few (mainly fairly junior IIRC) pilots just lowered the lever rapidly and all hell broke loose with things floating up at sub-zero g including the pilots, then they would react to that and pull the cyclic back a bit, the disc would bite and the Nr would wind up at a phenomenal rate. I recall on my TRE check having a trainee do this and I just caught the Nr with a massive pull as it reached the max transient power off Nr. Scary!
Had you had a real Dual Engine Failure....with a complete loss of power input to the MGB....with a resulting decay in Rotor RPM....would not that increase in RPM be what you needed to offset the Loss of RPM that the loss of input would have caused? Of course when the g came on, the increase in rpm is exactly what is needed. But the problem is that the effect is delayed by have low g for the first few moments of the recovery into auto. The pilot's inevitable delay in entering auto is bad enough and will quite probably result in the Nr getting perilously low. Add to that a pilot reaction that does not initially get an autorotative flow going, ie one that allows further Nr reduction after the recovery is initiated, and it could lead to the Nr becoming unrecoverably low. Let's also remember that the issue is blade stall, so if there is a period of zero g, lowering Nr, then aft cyclic is applied (with a bit of reaction time) to correct zero g, initially a poor response to the cyclic due to low g, then as the g starts to come on the cyclic really "bites" and well over 1g is produced. Whilst this would normally be a "good thing" as you say, to restore the Nr, in fact low Nr + high disc loading = increased propensity to stall, so those actions might make the unrecoverable Nr somewhat higher than it might otherwise be. Even if the blades don't stall, whilst you might get the Nr back you will get more than you bargained for as the Nr shoots way over the max transient Nr. If you are used to flying metal bladed helicopters, you will be amazed at how fast the Nr can rise on a plastic bladed helicopter under g loading.
Set against all that is leading with the cyclic to maintain 1g, and it all goes swimmingly well!
Regarding the bit about Nr continuing to decay at zero g, obviously the nature of our auto entry means I have never seen this, but I think the aerodynamics of it make the consequence clear.
How long can you maintain a Zero G condition? Seconds or Minutes?
I suppose if you really tried and didn't care about recovery, you could put the heli into a ballistic dive and hold zero g for as long as the heli could stand the speed increase. You would end up in a vertical dive but I suppose would run out of fwd cyclic fairly quickly if nothing else gave way.
But practically speaking I am only talking about a few seconds. Any Nr loss greater than that arising from pilot reaction time is to be avoided, maintaining 1g+ (for which you have to lead with the cyclic) minimises that. Just lowering the collective makes it a whole lot worse.
So in summary, from fast cruise rapid entry into autorotation can either be a benign affair if leading with the cyclic, or absolute mayhem if the collective is lowered without any cyclic input.
Thinking back to my days on the AS332L where we did retard the throttles to practice autos, with the entry from say 120kts the effect was less critical, but still noticeable. Entry without aft cyclic was messy rather than mayhem.
Perhaps "messy" actually describes a situation where transient effects were causing variations in the Rotor RPM which would only be "normal" considering the loss of drive from the Engines, the shift from the Rotors being driven by the engines to an autorotative descent, shifting airspeeds and aircraft attitudes, and change in Tail Rotor Torque demand?