212man:
Not quite - both engines were driving the rotor but with a limited power to simulate OEI.
Yeahhhhh, I get that. In fact the NTSB report describes what they did, but not *how* they did it. And...not being familiar with all this fly-by-wire stuff, I just wasn't sure how they could simulate a single-engine failure and "fool" the FADECs into
not trying to maintain 100% NR, which is their primary task. But I guess they did.
Then again, we understand that in a twin, if you pull the power of one engine completely off at a very high-speed cruise, the good engine is going to try very hard to keep the NR up, to the point of overtemping itself - or- if you have overtemp protection provided by FADEC, letting the rotor RPM droop.
And Bell obviously wanted to avoid unnecessary damage to those expensive CT-7's. Thus, GE must've come up with some program that removed/disabled the governing feature and limiting power to the equivalent of OEI. Simple!
When they did that at 185 knots, things did not go according to plan. The rotor RPM drooped to 91%. The pilots only lowered the collective to a 58% index, not even as low as they'd pushed it down in prior tests at lower speeds. But the NR did not come back up. Then what they're calling an unanticipated "scissor-mode" started in the main rotor and the pilots got a bad vibration. As the pilot's inputs got out-of-phase with the vibration, the NR decayed to about 80%. Then the AHRS, which was supposed to
help with stability, started working against them. (Shades of the 609 crash!)
Yowsa!
From the NTSB report:
Interviews with the helicopter manufacturer test pilots and engineers suggest that there were two ways for the pilots to exit the low Nr and, correspondingly, the vibration condition: (1) lower/reduce the collective to increase Nr or (2) exit OEI training mode, which would increase power available from the engines.
I suggest that there was a third way: RAISING THE NOSE. Every helicopter pilot knows that simply raising the nose and loading the rotor results in an increase in NR. But the crew probably didn't know how low the NR was. There was only one master caution tone, and it was the same for *21* separate items. The little PSI (power situation indicator) screen just had a green arc with no numbers between 90 and 100, AND the engineers at Bell admitted (presumably with some throat-clearing and shoe-shuffling) that the information for that green arc were taken from another model helicopter! Whaaat? See, they hadn't actually tested the 525 at really low NR's yet, powered or otherwise.
Which might be an indictment of Bell or might not. In a test program for a new-design aircraft, you can only test for so many things on any given flight. Maybe Bell just hadn't gotten around to testing the rotor at really low NR's yet. Althoooooough...didn't they spend a lot of time flying that rotor around on a 214ST?
The NTSB goes on and on about why the pilots did not exit the "OEI training mode" when they realized that not only was their NR really low but that it was causing this horrible vertical vibration. NTSB give the impression that the crew had all the time in the world to troubleshoot and respond to the problem. In reality, when the bad vibration started, the flight would be over in another nine seconds. Those nine seconds must've been a wild ride.
Finally, it is simply inconceivable that in 2016 a company like Bell would send a sophisticated, experimental ship like the 525 up on an envelope-expanding test flight with no CVFDR, no camera in the cockpit and no way to record the pilots' intercom channel. The mind boggles at Bell's arrogance and negligence there. I mean, couldn't someone have mounted a GoPro in the cabin of that thing? It seems that every damn R-44 comes equipped with such a device. That flight test department does not seem to be very well-managed.
When I'm critical of things like the goofy 505, people often say to me, "Don't you think Bell knows how to design and build a helicopter?" Programs like the 525 development really make me wonder. This ain't the 1940's; test pilots shouldn't be expendable anymore.