|
Bell 212/412 Throttle question
Question for the 212 gurus.
When the throttles are full open, and you roll the twist grips past open, into the spring cushion range, should the engine surge? I've been told by very experienced and knowledgable mechanics that they should not if properly rigged, however it's been my experience that many do. Any thoughts or references? Thanks |
Originally Posted by GS Pilot
(Post 11704066)
Question for the 212 gurus.
When the throttles are full open, and you roll the twist grips past open, into the spring cushion range, should the engine surge? I've been told by very experienced and knowledgable mechanics that they should not if properly rigged, however it's been my experience that many do. Any thoughts or references? Thanks |
Originally Posted by GS Pilot
(Post 11704066)
Question for the 212 gurus.
When the throttles are full open, and you roll the twist grips past open, into the spring cushion range, should the engine surge? I've been told by very experienced and knowledgable mechanics that they should not if properly rigged, however it's been my experience that many do. Any thoughts or references? Thanks Cushion at idle and top end can if not rigged properly can effect the rest of the set up. You answered your own question really with due respect. Also consider rigging as per MM procedures assumes perfect control runs, bellcramks etc, without bearing wear amongst those linkages. I would suggest check the top end cushion and bottom cushion and work back form there. What maintenance has been carried out recently? Forums are not really a good idea to ask for trouble shooting advice. |
Salusa, I’m wracking my brain trying to recall the details. There was a time I could draw a passable schematic showing the P3, Px and Pg lines and the HMU and bob weights. But absence and 3 subsequent types has dimmed that. What is the mechanism that would cause surging from a minute throttle movement?
|
Originally Posted by GS Pilot
(Post 11704066)
When the throttles are full open, and you roll the twist grips past open, into the spring cushion range, should the engine surge?
Originally Posted by 212man
(Post 11704128)
What is ggd mechanism that would cause surging from a minute throttle movement?
|
As a very (very) basic point of order....
Gas turbines don't (can't) have throttles. A throttle is a mechansim that controls the volume of air entering an engine. Similar in concept to a 'choke', also logically refererred to way back when as a 'strangler'. This concept has no relevance or connection whatsoever with gas turbines. Throttles, by definition, only belong to hangmen, piston engines and I gather some sexual deviants largely of the younger generation... Thrust lever, power lever, speed select lever - call it whatever you like, but on a turbine it's no more a throttle than it's a clutch. |
Originally Posted by meleagertoo
(Post 11704173)
As a very (very) basic point of order....
Gas turbines don't (can't) have throttles. A throttle is a mechansim that controls the volume of air entering an engine. Similar in concept to a 'choke', also logically refererred to way back when as a 'strangler'. This concept has no relevance or connection whatsoever with gas turbines. Throttles, by definition, only belong to hangmen, piston engines and I gather some sexual deviants largely of the younger generation... Thrust lever, power lever, speed select lever - call it whatever you like, but on a turbine it's no more a throttle than it's a clutch. Bell Helicopter has recently investigated three incidents of cracked or fractured collective stick tubes on the model 212. In two cases the tube had fractured in overload at the slot for the throttle pin, while operating the collective on the ground without hydraulic assist. The other collective stick was found cracked at the throttle pin cutout during inspection. Total time on the collective sticks was unknown. Further investigation into the use of the collective has revealed that, in some cases, improper application methods of the collective can induce higher strains to the throttle grip slot area. Collective application should be from the throttle grip position. It should never be accomplished from the end of the collective stick at the collective head, particularly with hydraulics off. Operation from the collective head will induce higher strains on the collective stick that are concentrated at the slot for the throttle pin. Bell Helicopter previously released one-time inspection requirements for the collective stick in accordance with TB 212-87-98 and 412-87-60. The technical bulletin inspection requirements are specific for the throttle slot area. Bell will be introducing these same inspection requirements during a future revision of the 212 and 412 maintenance manual, chapter five, 5 year / 3000 hour inspection interval. Operators are encouraged to review the inspection requirements per the applicable technical bulletins and comply with the inspection requirements when the collective stick is removed for other maintenance. |
Originally Posted by Salusa
(Post 11704102)
Forums are not really a good idea to ask for trouble shooting advice. Thanks for the replies...it's enormously helpful. |
Originally Posted by wrench1
(Post 11704153)
No. The "spring cushion" is related to cable travel only and is to ensure the engine fuel controls are on their mechanical stops and not limited by the cable travel. There should be no engine speed change when moving the throttles into the "cushion".
If the M/AFCU linkages are not on the stops or missed rigged, moving the throttle into the cushion could induce additional fuel flow on the manual side or reposition the schedule cam on the auto side. Both instances will cause a speed change. My concern is he notes this on "many" aircraft?? |
Originally Posted by GS Pilot
(Post 11704239)
...Over last 18 years I've flown about 10 individual 412 or 212 airframes and at least 1/3 of those exhibited this behavior..
|
Originally Posted by 212man
(Post 11704128)
Salusa, I’m wracking my brain trying to recall the details. There was a time I could draw a passable schematic showing the P3, Px and Pg lines and the HMU and bob weights. But absence and 3 subsequent types has dimmed that. What is the mechanism that would cause surging from a minute throttle movement?
Which engine is surging? Is it a continuous "up and down surge" or does it settle down? How much is it surging by? N1/TQ/ITT rate? What is the mod status of TCU/Beep Trim/ITT Trim? Did the snag just appear, what maintenance has been carried out recently? Does it happen in Manual Mode? Plenty more questions but won't bore anyone with the rest of them. That's what I meant about using forums for troubleshooting without knowing the full facts. I fully appreciate anyone looking for general technical advice and information, but to troubleshoot an operational snag without knowing the full situation and facts is just going to be general opinions on a forum. Edit : Properly rigged and with no other issues, engine(s) should not be surging when throttles opened to full and then into the cushion range as the stops will already be "engaged" and the additional cushion range movement won't have any effect. |
It’s interesting to note that we may also not even be talking about the same phenomenon. My understanding of ‘surge’ is not what Salusa is describing.
|
Originally Posted by 212man
(Post 11704389)
It’s interesting to note that we may also not even be talking about the same phenomenon. My understanding of ‘surge’ is not what Salusa is describing.
OP needs to define symptoms of "surge" and whether it's a 212/412. I presumed he meant that rolling the throttle(s) into cushion was causing a up and down hunting of N1 along with associated ITT and Torque. We all know the results of presumptions though... If 212 has it been modified to TCU torque limiting only with ITT Trim on #2 Engine for Torque/ITT matching ? Can't remember the specific Bell TB but it's an option... If 412, what series and engine model? If 412SP and above it will be TCU limiting only the with ITT trim on #2 Engine. The Bell definition of ITT trim can be confusing. Effectively on Bell 412SP and above and modified 212 the TCU will only Torque Limit not Torque Match. Torque matching is done done manually with beeping #2 engine to match #1. Four way China hat. Up and down twin beep. Left and Right beeps #2 engine only to match #1. Anyway, I'm digressing... To many open questions and blank spaces, to even make an opinion. Bit rusty myself on Bell 212 and 412. Do miss them though. Cheers. |
If 212 has it been modified to TCU torque limiting only with ITT Trim on #2 Engine for Torque/ITT matching ? Can't remember the specific Bell TB but it's an option... The Bell definition of ITT trim can be confusing |
Maybe the OP means,in other countries ,`hunting` of RRPM/TQ/ITT etc,about the `datum`,but is noticeable in flight.A`SURGE`,in the normal connotation,and my experience,on several helos,Scout/Gaz/WX/WW,means a breakdown of airflow through the compressor,leading to overfuelling ,and certainly on Gnome/T-58.``WHOOSHHH,/Bang,TIT,FLAMES,ETC..Scout/Wasp/Gaz,just a banging/chuffing for a couple of seconds,as the engines on those have a `centrifugal/agricultural compressor`,which is more benign..
|
Originally Posted by sycamore
(Post 11704436)
Maybe the OP means,in other countries ,`hunting` of RRPM/TQ/ITT etc,about the `datum`,but is noticeable in flight.A`SURGE`,in the normal connotation,and my experience,on several helos,Scout/Gaz/WX/WW,means a breakdown of airflow through the compressor,leading to overfuelling ,and certainly on Gnome/T-58.``WHOOSHHH,/Bang,TIT,FLAMES,ETC..Scout/Wasp/Gaz,just a banging/chuffing for a couple of seconds,as the engines on those have a `centrifugal/agricultural compressor`,which is more benign..
Definition of Surge/Stall/Hunting? Also thinking about it why would you need to roll the throttles into top end cushion on a 212/412? I can sort of see it happening on a rapid roll up or roll down during acceleration and deceleration from ground idle to flight idle, but when in flight why even touch the throttles except in a situation that needs it? No disrespect to the OP at all, however personally I'm not sure what is being asked or the reason for the post in the first place? I got carried away assuming it was an engineering issue, but believe now not the case. |
when in flight why even touch the throttles except in a situation that needs it? |
Originally Posted by 212man
(Post 11704479)
Indeed! We had some pilots that did this every time they landed as some kind of safety check. My view was just open them fully on start then friction them properly!
I suspect they do it because it's what Bell says to do. https://cimg8.ibsrv.net/gimg/pprune....0f44f2b610.jpg The twist grips can get slightly backed off (can I say retarded?) in flight due to inadequate friction and or too much inadvertent twisting during collective manipulation...it seems appropriate that prior to landing, a pilot would want to double check that they are both full open. I don't know how else to determine "Full Open" then to rotate them slightly into the cushion, feeling that pressure, and allow that spring pressure to push them back slightly until pressure is zero, you are now out of the cushion range, throttle is set full open. It's during that travel into the cushion that I've experienced what I would call a surge, (sorry if that word had other connotations for some of you.) but let's call it an "observed increase in the affected engine parameters (N1, N2, NR, ITT,) which are characteristic of a positive application of fuel delivered to that engine". That increase is steady, proportional to the degrees of rotation applied to the twist grip, and correspondingly decreases in an inverse fashion as the throttle is allowed to rotate out of the spring cushion range, back to the full open position. If there was "hunting" or compressor stalls, recent maintenance, strange lights in the sky, unexplained erections, or anything else pertinent, it would have been mentioned. Thanks again for the helpful replies...I think I got what I needed....and then some. |
Check rigging, cushion makes feeling of end of running for throttle Links and contact with afcu full Max stop.
Check for first contact of AFCU prior MFCU.. Other factors, acceleration check and bleed valve. Of course be sure the governors function. A quick Check for correct rigging is to check full open throttle with contact in max AFCU Stop, gap for max MFCU stop aprox.0015 inches and dial aprox 97 degrees |
Sounds like you have a rigging problem.
When you move the "throttle" down to IDLE you are actually adjusting the Max N1 position of a spring in the mechanism versus the speeder spring in the N1 governor (FCU). i.e you have changed the "schedule" in the FCU and have a new Max N1 it is just now a lot lower. Conversely when you open the "throttle" you are adjusting max N1 up to the point where the PTG (N2) takes over. The PTG when in its range is pushing via regulated and governed pressure bias against the same spring to achieve the required N2 equilibrium "on speed" via changes in the N1. i.e the PTG changes N2 by resetting the speeder spring in the FCU N1 governor. Because your throttles are mis-rigged you suddenly change the equilibrium point and you will get an oscillation in that engine and of course more than likely the other one as well. Keep in mind if it is mis-rigged without actual preload against the N1 stop you may not get all the N1 you wish for when you need it! Hence - the check for throttles OPEN. It is imperative that there is pre-load. Simple check is on ground shutdown, check that there is preload against the N1 stop at frictioned WOT. You may still be getting springback but not from the right place due to mis-rig. Looks like Cabick beat me to it. |
| All times are GMT. The time now is 19:54. |
Copyright © 2026 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.