412 RRPM & Vertical Vibes.
BJC,
Thanks for your comments - it has been a long frustrating wait for the final word from Bell, although they gave the same advice over three years ago in a letter.
If you read the full text of the ALERT SERVICE BULLETIN 412-07-125, the last paragraph goes on to say:
"Bell Helicopter recommends standardizing operations by operating the aircraft at 100% NR at all altitudes during all flight operations. In the past operators operated at 97% NR for a perceived ride quality improvement of the 4/rev vibrations and a slight fuel burn improvement. The 4/rev vibration levels vary with the aircraft configuration, if operators wish to reduce these vibration levels they may install FRAHM damper per BHT-SI- 47 as required."
Having contacted Bell about placing our own Decal on the dash to reflect this policy (100%NR throughout the AEO envelope) they have no objection as long as the '6000 FT HD' decal also remains displayed iaw the ASB. I suspect that the original letter they gave us in 2005 was a reflection of their knowledge of problems with excessive head rotating component wear on new EP Models being climbed to cruise at 97%NR, particularly with external floats kit on the SLED-type offshore skids. Anyway - hope you have found life easier also by moving to 100% throughout. This saves time and transient wear on the gearboxes through twitchy beep actuators and avoids all prospective transient mast torque exceedences by pilots beeping back up to 100%NR WITHOUT first reducing mast torque to a safe level (70-75% or less) on the EP.
Oh - and NO-ONE in their right mind EVER runs on a 412 in ATT Mode, unless they are Japanese and are wearing a headband!! If you ever do it once by mistake (like not checking your newby has selected an HP/SP to SAS) then you will NEVER do it ever, ever again! More clean underwear required!
As regards our good friend JUANK, - hey mano, I suspect your aircraft are in sore need of a good Autopilot overhaul and some real engineering TLC.
Firstly no-one in their right mind operates a 412 on only ONE autopilot at a time - this feeds back at least 50% more Pilot Induced Oscillations into the system, depending on how good your actuators are. You should definitely not get the random inputs you described. However, if your aircraft is old and tired and has weak FLUX UNITS on the compasses, this coupled with worn Autopilot Trim Actuator motors can give you a sudden jump in YAW in ATT Mode. Of course, with a Bell 205/212 & 412 (without a 4-axis Autopilot) you should NEVER - as handling pilot - take your feet off the yaw pedals in flight. The only thing holding this puppy straight is the yaw trim part of the Stick Trim system. If you have a large disagreement between FD HDG Hold and YAW Trim/Stick Trim, or the compass 'suddenly' kicks large degree changes due to erratic compass system inputs/corrections/problems, you will get either a very fast repetitive shunting of yaw left and right up to 90 degrees, or may actually (unless you are quick) lose effective tail rotor control momentarily - which will scare you !!!!less unless you have your feet on the controls. I have had this happen three times in 412's enev with my feet on the pedals -usually older SP/HP machines with very tired compass systems which have wandered abnormally and very quickly offshore on pig-iron rigs. This just shows their susceptibility to yaw instability. However, it also happened once when the HDG Hold and Compass fell out with each other, (compass jumped 170 degrees in 1 second)resulting in a very quick and nasty 5 seconds of wrestling before the AP's were disengaged.
With regard to the hover problems, the 412 just cannot cope with OGE high-hover lifting situations out of the wind. Once out of a safe 4 feet ground cushion and out of wind, you are inviting 'Big G' to smite you mightily and undoubtedly he will! Hover the 412 into wind. Best with the wind between RED 75 and Green 45 relative to the nose. For underslungs essential. For slope landings - this is essentially a RIGID-ROTOR System. You just can't fly it like a 212 and it will NOT behave like one. Approach the lateral cyclic control stops and you will run out of everything , including breath/life. Dynamic Rollover is further complicated by the crappy out-of wind/crosswind performance as published for this beast. Low hover into wind and let the buggers jump out off the skids - far safer and it will preserve your life longer.
I won't lecture you on sloping ground landings, but it's always safer to be pilot's side (starboard) UP slope and to land 45 degrees across the slope if you can.
Lastly, if you do have significant wear in your Lift Link (a favourite for horrible vertical vibes), worn swachplate assembly, PLUS Autopilot problems AND perhaps a loose seat/light collective friction, the cumulative totals of all of these vertical vibes might send you to meet your maker earlier than you had previously expected. Methinks you need some serious money spent on spares, Amigo!
Find a new company to fly for with better aircraft!