Bell 412
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Official Word From Bell On 412 Operating Rrpm
Greetings All,
This just in from my own Bell Rep this week for your edification:
On the issue operation at 97% NR, In order to standardize operations, Bell Helicopter will be issuing a rev to the flight manual that will limit rotor RPM to 100 % during all operations. Normal governor excursions above or below 100 % will be acceptable but continuous ops shall be at 100%.
At this point in time normal rotor RPM limits (Green arc) can be anywhere from 97 to 100 %. There is no restriction. There is a perceived ride quality 4/rev improvement at 97 % if the aircraft is not equipped with a FRAHM damper (412-SI-47) as well as a slight fuel burn improvement. At 100% rotor blade lift is optimized .At high altitude, high gross weights and airspeeds at or near VNE, control system loads increase dramatically as the rotor approaches retreating blade stall. Because retreating blade stall occurs at a lower airspeed at 97%, operation at 100 % will reduce these loads and will be stipulated as the RPM limit used for continuous ops.
A revision to the flight manual, plus a decal to install on the instrument panel will be release thru ASB shortly advising operators.
Hope this helps those of you who are still flogging about at 97% to start changing your checklists now!
Cheers,
Oracle
This just in from my own Bell Rep this week for your edification:
On the issue operation at 97% NR, In order to standardize operations, Bell Helicopter will be issuing a rev to the flight manual that will limit rotor RPM to 100 % during all operations. Normal governor excursions above or below 100 % will be acceptable but continuous ops shall be at 100%.
At this point in time normal rotor RPM limits (Green arc) can be anywhere from 97 to 100 %. There is no restriction. There is a perceived ride quality 4/rev improvement at 97 % if the aircraft is not equipped with a FRAHM damper (412-SI-47) as well as a slight fuel burn improvement. At 100% rotor blade lift is optimized .At high altitude, high gross weights and airspeeds at or near VNE, control system loads increase dramatically as the rotor approaches retreating blade stall. Because retreating blade stall occurs at a lower airspeed at 97%, operation at 100 % will reduce these loads and will be stipulated as the RPM limit used for continuous ops.
A revision to the flight manual, plus a decal to install on the instrument panel will be release thru ASB shortly advising operators.
Hope this helps those of you who are still flogging about at 97% to start changing your checklists now!
Cheers,
Oracle
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RSAF Bell 412
I would like to learn more about this unique aircraft. I notice, that in addition to somewhat unique engine fuel controller, it had (has) an unusual avionics suite. How many of these aircraft were built? Where are they today?
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Bell 412SA information
I have Bell 412EP RSAF Flight Manual and Bell 412EP-Specifications Neither has much detail on the Rogerson Kratos Avionics that seems to be somewhat unique to this limited production aircraft.
I'm particular interested to know about the digital bus structure and what specific data one might be able to capture by monitoring and recording bus activity.
I'm particular interested to know about the digital bus structure and what specific data one might be able to capture by monitoring and recording bus activity.
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ground resonance
hi all.
Excuse my english. I´m a spanish helicopter pilot, i was looking for any incident about groun resonance in 412 and find you.
In spain we have four incidents by ground resonance. well no ground resonance, i think there were greats bounces in ground, because when we try to take of whith the helicopter bounce and it separate of the ground it continue flying jumping (or bounce) and the only way to stop bounce was shutdown engines (autorrotacion).
The four helicopters were working in firefighting and all were without persons on board at 100% N/R.
I was flying two of there four (same helicopter)
The first time i complete succesfull the helicopter´s control fighting about one minute with it and maintenance don´t find something inussual. two days later in ground in a sand filed it become to bounce and this time i have to shutdown the engines in flight because it was imposible regain control.
in several flights with anothers 412 we have the same problems,with light and medium bounces and the only way to cut this situation was turnning throtles to idle and try to pass to flight gently aplly a lot of collective.
Excuse my english and thanks to all for your forum
Excuse my english. I´m a spanish helicopter pilot, i was looking for any incident about groun resonance in 412 and find you.
In spain we have four incidents by ground resonance. well no ground resonance, i think there were greats bounces in ground, because when we try to take of whith the helicopter bounce and it separate of the ground it continue flying jumping (or bounce) and the only way to stop bounce was shutdown engines (autorrotacion).
The four helicopters were working in firefighting and all were without persons on board at 100% N/R.
I was flying two of there four (same helicopter)
The first time i complete succesfull the helicopter´s control fighting about one minute with it and maintenance don´t find something inussual. two days later in ground in a sand filed it become to bounce and this time i have to shutdown the engines in flight because it was imposible regain control.
in several flights with anothers 412 we have the same problems,with light and medium bounces and the only way to cut this situation was turnning throtles to idle and try to pass to flight gently aplly a lot of collective.
Excuse my english and thanks to all for your forum
Juank,
I might be out of order here, but you may be getting bouncing because your collective friction is too light and you maybe a litttle bit too gentle with the collective lever. I had a similar case with the Bell 205A-1, the pilot's seat is flexible and if you are holding the collective, as you probably are, any bounce in your seat is transmitted to the collective. You say that your maintenance crew cannot find anything wrong, please ask them if the static friction clamp on the left side of the collective torque tube is correctly set? There is a panel just below the left hand side crew door which gives access to the clamp. I apologise if this has been checked before.
I might be out of order here, but you may be getting bouncing because your collective friction is too light and you maybe a litttle bit too gentle with the collective lever. I had a similar case with the Bell 205A-1, the pilot's seat is flexible and if you are holding the collective, as you probably are, any bounce in your seat is transmitted to the collective. You say that your maintenance crew cannot find anything wrong, please ask them if the static friction clamp on the left side of the collective torque tube is correctly set? There is a panel just below the left hand side crew door which gives access to the clamp. I apologise if this has been checked before.
Oldlae is on the money.
JUANK
The dreaded "collective bounce". I have taken 2 aircraft straight to maintenance because of low minimum collective friction.
Afterwards when other pilots return to fly the aircraft the "whining" about the collective friction is louder than the engines.
The "chorizo's" in the pilot seat are too SOFT!
It is not "optional" - get used to it.
This issue is not limited to Bell medium products, it affects the Super Puma as well.
It has destroyed a few aircraft - if all else fails RTFM!!!!!!!!!!!!!!
JUANK
The dreaded "collective bounce". I have taken 2 aircraft straight to maintenance because of low minimum collective friction.
Afterwards when other pilots return to fly the aircraft the "whining" about the collective friction is louder than the engines.
The "chorizo's" in the pilot seat are too SOFT!
It is not "optional" - get used to it.
This issue is not limited to Bell medium products, it affects the Super Puma as well.
It has destroyed a few aircraft - if all else fails RTFM!!!!!!!!!!!!!!
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Ground Resonance/Bouncing in the 412.
Hi Gents,
Whilst slack collective friction MAY often lead to PIO's (Pilot Induced Oscillations) of the vertical kind (especially when the pilot's seat IS loose vertically, or the pilot sits very high on the seat and then his overly stiff collective arm magnifies any existing aircraft vertical 'bounce' quite quickly), the 412 is subject to other factors which can lead to considerable and quite scary Ground Resonance or Low Hover Resonance with a severe vertical vibration.
Two cases in point:
1. The Main Rotor blades are not well statically or dynamically balanced, nor weight-matched and the MR tuning has placed them in an 'out-of phase-harmonic' with the skid gear. This is accentuated when such poor blade dynamics and mix are combined with worn cross-tubes. I have known one such particular aircraft that would enter Ground Resonance at the drop of a hat and frequently did so, with scary results, usually during OEI run-on landings onto a tarmac or concrete surface - at which time it would enter a SEVERE vertical oscillation, which had it reached the third such oscillation would have been impossible to contain/recover from. The situation was only remedied by swiftly re-engaging the second engine at the onset of the oscillation and quickly lifting the aircraft smartly up to a ten feet hover. (- Change of underwear subsequently required -THAT bad!)
2. Many pilots fail to notice when one (or more) Main Rotor Droop Restrainers fail to disengage properly from their static position during run-up. Because we are all so used to having 412's shaking us to death on the ground anyway with bad tracking, and the fact that our backsides are also definitely 'attuned' to a certain basic level of 'shake, rattle and roll', the failure of a droop-stop to come out properly and cleanly during engine/rotor run-up is therefore often masked. The vibration this causes is, however, quite a marked, constant, equal and very regular WHUMPER (One-Per-Rev) vertical vibe whilst on the ground. If this problem is not noticed or correctly diagnosed and addressed whilst on the ground [by winding the engines back to idle, giving the stick a gentle 'stir' and then winding the rotor back up to 100% again slightly faster (normally this disengages the sticky restrainer, sometimes with a slight 'clunk' or control jump)] then any subsequent attempt to lift the aircraft into a hover with the droop stop(s) still engaged will demonstrate a serious level of One-Per vertical vibration, very similar to that described above. The 'stuck' droop restrainer cannot normally be cleared in flight, but only on the ground. Continued flight will just damage the stuck restraining mechanism even more and scare the pilots markedly. Usually, a total shut-down and maintenance attention will be necessitated, as worn parts or stuck bearing on the mechanism often cause this problem in the first place. The more polluted your operating environment, the more likely this is to occur unless specific attention is paid to cleaning and properly maintaining the blade droop restrainer mechanisms. The RFM DOES warn about this occurrence but this is often forgotten on a daily basis, as a droop-stop sticking is usually a quite rare occurrence.
The RFM States (Pre-Flight): "DURING LIFT-OFF TO HOVER, ANY ABNORMAL INCREASE IN ONE PER REV VIBRATION MAY INDICATE ONE OR MORE DROOP-STOP RESTRAINERS HAVE FAILED TO DISENGAGE FROM THE STATIC POSITION. VERIFY PROPER OPERATION PRIOR TO FLIGHT."
Obviously these two forms of induced vibration are just two causes of nasty vertical vibrations, but I would suggest that our Spanish pal gets his engineers to check his droop stop restrainer mechanisms for worn or stuck parts or bearings.'' Fire pollution gums things up!
If you have one of those rare beasts that just likes shaking itself to death as in (1) above, then the only answer is often to deliberately 'detune' the main rotor (all blades equally adjusted) within the available limits of the Auto RRPM, such that it no longer sings in harmony with the cross-tubes. Changing the cross-tubes is another option if they exceed lateral max spread.
Those other old 205 & 212 drivers out there may well remember the wonderful vertical bounce encountered whilst flying underslung loads with a two-bladed system, when the load and rotor got into 'synch' and tried to vertically vibrate themselves to equality! With the 412's unforgiving (and gutless) main rotor blades, it is also prone to induced vertical bounce with an underslung load, particularly with a long line. The only answer here is to change the length of the line/sling to change the induced frequency.
Hope this helps our Fire-Fighter buddies in Espana!
Cheers,
O
Whilst slack collective friction MAY often lead to PIO's (Pilot Induced Oscillations) of the vertical kind (especially when the pilot's seat IS loose vertically, or the pilot sits very high on the seat and then his overly stiff collective arm magnifies any existing aircraft vertical 'bounce' quite quickly), the 412 is subject to other factors which can lead to considerable and quite scary Ground Resonance or Low Hover Resonance with a severe vertical vibration.
Two cases in point:
1. The Main Rotor blades are not well statically or dynamically balanced, nor weight-matched and the MR tuning has placed them in an 'out-of phase-harmonic' with the skid gear. This is accentuated when such poor blade dynamics and mix are combined with worn cross-tubes. I have known one such particular aircraft that would enter Ground Resonance at the drop of a hat and frequently did so, with scary results, usually during OEI run-on landings onto a tarmac or concrete surface - at which time it would enter a SEVERE vertical oscillation, which had it reached the third such oscillation would have been impossible to contain/recover from. The situation was only remedied by swiftly re-engaging the second engine at the onset of the oscillation and quickly lifting the aircraft smartly up to a ten feet hover. (- Change of underwear subsequently required -THAT bad!)
2. Many pilots fail to notice when one (or more) Main Rotor Droop Restrainers fail to disengage properly from their static position during run-up. Because we are all so used to having 412's shaking us to death on the ground anyway with bad tracking, and the fact that our backsides are also definitely 'attuned' to a certain basic level of 'shake, rattle and roll', the failure of a droop-stop to come out properly and cleanly during engine/rotor run-up is therefore often masked. The vibration this causes is, however, quite a marked, constant, equal and very regular WHUMPER (One-Per-Rev) vertical vibe whilst on the ground. If this problem is not noticed or correctly diagnosed and addressed whilst on the ground [by winding the engines back to idle, giving the stick a gentle 'stir' and then winding the rotor back up to 100% again slightly faster (normally this disengages the sticky restrainer, sometimes with a slight 'clunk' or control jump)] then any subsequent attempt to lift the aircraft into a hover with the droop stop(s) still engaged will demonstrate a serious level of One-Per vertical vibration, very similar to that described above. The 'stuck' droop restrainer cannot normally be cleared in flight, but only on the ground. Continued flight will just damage the stuck restraining mechanism even more and scare the pilots markedly. Usually, a total shut-down and maintenance attention will be necessitated, as worn parts or stuck bearing on the mechanism often cause this problem in the first place. The more polluted your operating environment, the more likely this is to occur unless specific attention is paid to cleaning and properly maintaining the blade droop restrainer mechanisms. The RFM DOES warn about this occurrence but this is often forgotten on a daily basis, as a droop-stop sticking is usually a quite rare occurrence.
The RFM States (Pre-Flight): "DURING LIFT-OFF TO HOVER, ANY ABNORMAL INCREASE IN ONE PER REV VIBRATION MAY INDICATE ONE OR MORE DROOP-STOP RESTRAINERS HAVE FAILED TO DISENGAGE FROM THE STATIC POSITION. VERIFY PROPER OPERATION PRIOR TO FLIGHT."
Obviously these two forms of induced vibration are just two causes of nasty vertical vibrations, but I would suggest that our Spanish pal gets his engineers to check his droop stop restrainer mechanisms for worn or stuck parts or bearings.'' Fire pollution gums things up!
If you have one of those rare beasts that just likes shaking itself to death as in (1) above, then the only answer is often to deliberately 'detune' the main rotor (all blades equally adjusted) within the available limits of the Auto RRPM, such that it no longer sings in harmony with the cross-tubes. Changing the cross-tubes is another option if they exceed lateral max spread.
Those other old 205 & 212 drivers out there may well remember the wonderful vertical bounce encountered whilst flying underslung loads with a two-bladed system, when the load and rotor got into 'synch' and tried to vertically vibrate themselves to equality! With the 412's unforgiving (and gutless) main rotor blades, it is also prone to induced vertical bounce with an underslung load, particularly with a long line. The only answer here is to change the length of the line/sling to change the induced frequency.
Hope this helps our Fire-Fighter buddies in Espana!
Cheers,
O
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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.
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.
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thanks to all for your answers.
thanks oracle, i think it will be caused by the cross-tube but there were one think that i can´t understand. i don´t understand why i continued bouncing flying (bigs bounces one-per-rev of SEVERE vertical oscillations)?.
the droop stop restriners i think there were right because i was flying three hours in fire-fighting when happened the first time (two hours of flyght, 40 minutes of rest and one hour of flyght in fire-fighting until i descend to recover the firefighters). i was flying only whith one hp engage in SAS mode (BJC). the landig zone was ten degrees of slope (left side), the collective was not full down because the slope, i need to apply collective to avoid slipping and the ciclyc styc was to the left to maintain the slope.but the helicopter was stable and without vibrations. When the firefighters open the basket to save the bambi bucket the helicopter started to severe verticals ascillations. i maintain in that position to
allow firefighters to derogate, and when i was close to dump i take of but the helicopter continued whit verticals oscilaciones and we lose the tail rotor control during 30 or 40 seconds. when i regain the tail rotor control the verticals continiun in all the time and i could disconnect the hp1 and all verticals disappeared. i go to my base and change hp1 box control, and we make a one hour test flight testing hp1.
maintenance not found anything abnormal and they thought that was due to a wrong behavior of hp1.
The next flight was a cruise flight and it was fine.
The next one was fine until i landed. helicopter at ground, collective full down, hp1 off (in caution to prevent a system fail), 100%NR, no vibration, one minute to descend all firefighters, and when they open the basket to draw the bambi bucket the helicopter sudenly become to jump, a big jumps. i take of fast, when the fire fighters broke the helicopter, to prevent ground resonance but it continued jumping, i had full control but the helicopter has several verticals vibrations one-per-rev. i try to take of but noises in the transmission were increasing (i think was the lift link) and when i was over 30 knots and one hundred feet the helicopter try to yaw 30º to the right in a jump, I fixed that position but it try to another jump to the right in 30º. i had to shutdown the engines and try to land in autorotation and crashed the helicopter.
two more helicopters in other company has similar problems and crashed the helicopters.(2005, 2006, 2006 and 2007).
I did not have communication maintenance.
i continue flying 412 with caution in ground.
i was sure that all verticals vibrations disappear with throttles in iddle posittion or shutdown engines. Now bell recommended 4º slopes.
thanks to all
thanks oracle, i think it will be caused by the cross-tube but there were one think that i can´t understand. i don´t understand why i continued bouncing flying (bigs bounces one-per-rev of SEVERE vertical oscillations)?.
the droop stop restriners i think there were right because i was flying three hours in fire-fighting when happened the first time (two hours of flyght, 40 minutes of rest and one hour of flyght in fire-fighting until i descend to recover the firefighters). i was flying only whith one hp engage in SAS mode (BJC). the landig zone was ten degrees of slope (left side), the collective was not full down because the slope, i need to apply collective to avoid slipping and the ciclyc styc was to the left to maintain the slope.but the helicopter was stable and without vibrations. When the firefighters open the basket to save the bambi bucket the helicopter started to severe verticals ascillations. i maintain in that position to
allow firefighters to derogate, and when i was close to dump i take of but the helicopter continued whit verticals oscilaciones and we lose the tail rotor control during 30 or 40 seconds. when i regain the tail rotor control the verticals continiun in all the time and i could disconnect the hp1 and all verticals disappeared. i go to my base and change hp1 box control, and we make a one hour test flight testing hp1.
maintenance not found anything abnormal and they thought that was due to a wrong behavior of hp1.
The next flight was a cruise flight and it was fine.
The next one was fine until i landed. helicopter at ground, collective full down, hp1 off (in caution to prevent a system fail), 100%NR, no vibration, one minute to descend all firefighters, and when they open the basket to draw the bambi bucket the helicopter sudenly become to jump, a big jumps. i take of fast, when the fire fighters broke the helicopter, to prevent ground resonance but it continued jumping, i had full control but the helicopter has several verticals vibrations one-per-rev. i try to take of but noises in the transmission were increasing (i think was the lift link) and when i was over 30 knots and one hundred feet the helicopter try to yaw 30º to the right in a jump, I fixed that position but it try to another jump to the right in 30º. i had to shutdown the engines and try to land in autorotation and crashed the helicopter.
two more helicopters in other company has similar problems and crashed the helicopters.(2005, 2006, 2006 and 2007).
I did not have communication maintenance.
i continue flying 412 with caution in ground.
i was sure that all verticals vibrations disappear with throttles in iddle posittion or shutdown engines. Now bell recommended 4º slopes.
thanks to all
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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!
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!
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Oracle:
Beg to differ with you. Flying a 412 in ATT mode is the only way you'll get any modes of the autopilot to work.
It works just fine in all modes of flight, including the hover. You just need to learn how to use it.
Beg to differ with you. Flying a 412 in ATT mode is the only way you'll get any modes of the autopilot to work.
It works just fine in all modes of flight, including the hover. You just need to learn how to use it.
SC
Haven't got a reference with me but I thought that prolonged hover operations in ATT mode were not recommended.
I have flown helicopters which stay in an ATT type of mode for hover ops and I certainly find it easier for some applications, winching, boat transfer etc - Does it actually say you shouldn't and why not ATT mode for the 412 in prolonged hover ops? or have I got this totally wrong?
Thanks
Turkey
Haven't got a reference with me but I thought that prolonged hover operations in ATT mode were not recommended.
I have flown helicopters which stay in an ATT type of mode for hover ops and I certainly find it easier for some applications, winching, boat transfer etc - Does it actually say you shouldn't and why not ATT mode for the 412 in prolonged hover ops? or have I got this totally wrong?
Thanks
Turkey
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I think you'll find that prolonged ground operations in ATT mode are what you're thinking of. The actuators can motor the stick to try and make the helicopter maintain a constant attitude and with some drift on the gyros and sensors, can result in a lot more out of center than the hub and associated components would like.
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ATT Mode
Greetings, 412 Drivers All,
Thank you muchly, Bomber, for pointing out that I WAS indeed (quite clearly) talking about the inherent dangers of RUNNING LANDINGS in ATT Mode.
Hovering in ATT Mode is actually bloody daft if you are a Bell pilot. Precision maneuvering, winching by day, landings, take-offs, underslung and ops close to the ground are all recommended by Bell as being carried out in SAS MODE. Although some pilots insist on using ATT all the time, you will end up with thumb-strain and will never be as smooth as the 'sloppy-stick' pilot using SAS. This is the way Bells were designed to be flown - 'free and easy'!
'Clunking' the 412 Force-Trim in and out in ATT Mode does not make for smooth or comfortable flight and unnecessarily wears out the Trim Release Switch & your thumb! Must be a Sea King/MIL driver, perhaps? Do you like that huge lump of hard skin on your thumb?
Today's reading is from the first book of the Bell 412 RFM (Before Take-Off):
"AFCS - Select ATT or SAS mode as desired (ATT mode shall be used during IFR Flight; SAS mode recommended for ground operation, hover and take-off)."
If you MUST (dementedly) continue to fly in ATT Mode for landing, then I would recommend (as Bell themselves do) that you do NOT do OEI run-on landings in that mode or you will likely need:
a) New skid shoes/cross-tubes/aircraft/job &
b) new underwear.
In ATT Mode, if you do not keep the Stick Trim Released during the smoothest of greasy-ace landings, you WILL find the aircraft/AFCS inputting exactly equal and opposite roll/pitching movements to the normal 'wobbles, bumps and grinds' encountered during this landing procedure. It is quite easy to thereby enter Ground Resonance, or into a joint aircraft/Pilot Induced oscillation of the very worst sort. That's why Bell specifically warns in the Bell 412 RFM (Before Landing):
"RUN ON LANDINGS MAY RESULT IN ROLL OSCILLATIONS WHILE ON THE GROUND. IF THIS OCCURS, LOWERING COLLECTIVE FULL DOWN OR TURNING SCAS OFF WILL STOP THE OSCILLATIONS."
I rest my case and go back to playing with my sloppy stick..........
O
Thank you muchly, Bomber, for pointing out that I WAS indeed (quite clearly) talking about the inherent dangers of RUNNING LANDINGS in ATT Mode.
Hovering in ATT Mode is actually bloody daft if you are a Bell pilot. Precision maneuvering, winching by day, landings, take-offs, underslung and ops close to the ground are all recommended by Bell as being carried out in SAS MODE. Although some pilots insist on using ATT all the time, you will end up with thumb-strain and will never be as smooth as the 'sloppy-stick' pilot using SAS. This is the way Bells were designed to be flown - 'free and easy'!
'Clunking' the 412 Force-Trim in and out in ATT Mode does not make for smooth or comfortable flight and unnecessarily wears out the Trim Release Switch & your thumb! Must be a Sea King/MIL driver, perhaps? Do you like that huge lump of hard skin on your thumb?
Today's reading is from the first book of the Bell 412 RFM (Before Take-Off):
"AFCS - Select ATT or SAS mode as desired (ATT mode shall be used during IFR Flight; SAS mode recommended for ground operation, hover and take-off)."
If you MUST (dementedly) continue to fly in ATT Mode for landing, then I would recommend (as Bell themselves do) that you do NOT do OEI run-on landings in that mode or you will likely need:
a) New skid shoes/cross-tubes/aircraft/job &
b) new underwear.
In ATT Mode, if you do not keep the Stick Trim Released during the smoothest of greasy-ace landings, you WILL find the aircraft/AFCS inputting exactly equal and opposite roll/pitching movements to the normal 'wobbles, bumps and grinds' encountered during this landing procedure. It is quite easy to thereby enter Ground Resonance, or into a joint aircraft/Pilot Induced oscillation of the very worst sort. That's why Bell specifically warns in the Bell 412 RFM (Before Landing):
"RUN ON LANDINGS MAY RESULT IN ROLL OSCILLATIONS WHILE ON THE GROUND. IF THIS OCCURS, LOWERING COLLECTIVE FULL DOWN OR TURNING SCAS OFF WILL STOP THE OSCILLATIONS."
I rest my case and go back to playing with my sloppy stick..........
O
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Oracle:
Having flown the 412 in ATT mode in hover and some quite precise landings (but not running landings - you're right), I had no particular difficulty in controlling the helicopter smoothly. The secret is, once you've got the machine roughly trimmed - do not use the force trim - merely fly against the forces and hold the stick where you want it. You will probably have to change your grip from a 'fist around the grip' to using just fingers, but I've never seen anyone who wasn't shown how to do that revert to the other way of flying.
Works like a charm. But if you insist on using the force trim switch all the time, you'll not like the results.
Those who've flown a Hughes 500 or Enstrom where you can't turn the stick forces off and who have learned to fly holding a force for short durations where it didn't make sense to retrim learned how to do that.
The real problem is that the spring gradient and forces in the stick are just a bit too high. Ditto on the A109 series - those forces are even higher, and result in folks who were brought up without a trim system turning off the trim without realizing that the trim is necessary for any of the AFCS to work properly.
But ATT mode works quite well in the hover if you know how to use it.
Having flown the 412 in ATT mode in hover and some quite precise landings (but not running landings - you're right), I had no particular difficulty in controlling the helicopter smoothly. The secret is, once you've got the machine roughly trimmed - do not use the force trim - merely fly against the forces and hold the stick where you want it. You will probably have to change your grip from a 'fist around the grip' to using just fingers, but I've never seen anyone who wasn't shown how to do that revert to the other way of flying.
Works like a charm. But if you insist on using the force trim switch all the time, you'll not like the results.
Those who've flown a Hughes 500 or Enstrom where you can't turn the stick forces off and who have learned to fly holding a force for short durations where it didn't make sense to retrim learned how to do that.
The real problem is that the spring gradient and forces in the stick are just a bit too high. Ditto on the A109 series - those forces are even higher, and result in folks who were brought up without a trim system turning off the trim without realizing that the trim is necessary for any of the AFCS to work properly.
But ATT mode works quite well in the hover if you know how to use it.
Any B412 experts out there?
While comparing the Flight Manual to the numerous other bits of paper claiming to be checklists, I found an interesting bit which the engineers couldn't resolve:
After starting Engine 1, the RFM says to increase N2 to 77-85% before starting Eng 2.
BUT! It says that if Eng 2 is started first, the N2 MUST be 85% before engine 1 start.
Why the difference? The engineers say that a lower RPM, around 80%, for engagement is preferable to stay away from the band of vibrations.
Nick Lappos, you might be in the ideal position to answer this one?
After starting Engine 1, the RFM says to increase N2 to 77-85% before starting Eng 2.
BUT! It says that if Eng 2 is started first, the N2 MUST be 85% before engine 1 start.
Why the difference? The engineers say that a lower RPM, around 80%, for engagement is preferable to stay away from the band of vibrations.
Nick Lappos, you might be in the ideal position to answer this one?