Oracle, it’s taken over a year, but we’ve now received the amendment from Bell on 97% Nr, although it is slightly different then what you’ve described. You can bet as the largest 412EP operator in the world we’ve been in regular contact with Bell and we’ve only recently received the amendments. The amendment to our flight manual now states power on Nr continuous operation 97-100% below 6000’ DA, continuous operation 100% Nr 6000’ DA or above.
The reason I wanted to see your correspondence with Bell is because our flight manual has always restricted us to 100%Nr when operating with any external kit installed (hoist, night sun, FLIR, skis etc). Since you stated that all your aircraft have external kits it made us 412CF operators sceptical. It’s hard to suppress that scepticism when our flight manual has guidance such as:
Operation at 97% is considered best for cruise flight while hovering and other flight manoeuvres are best accomplished at 100% NR.
Cruise charts are presented for 100% engine and rotor rpm and standard configuration. Often better range (higher speed and reduced fuel consumption) may be achieved by reducing rpm to 97%. If rpm is beeped down 3% and speed remains same at a constant torque setting, then fuel consumption is reduced by over 1% and actual SHP required has been reduced by 3%. At certain operating conditions, speed actually increases 6 to 8 knots while beeping down at a constant torque. When this happens, range gain can be over 5%.
For your “ground resonance” example #1 above, were your autopilots engaged? Were you in SAS or ATT mode? Our flight manual was the following caution: “Run-on landings may result in roll oscillations while on the ground. If this occurs, lowering the collective full down or disengaging AP1 and AP2 will stop the oscillation”. Holding the force trim release button also helps.
It feels very similar to landing on very rough terrain and could be what you’ve described.