Bell 206 Jet Ranger Take-Off Power Range
 

The Flight Manual of the Bell 206 (Allison 250-C20R/4 Turbine engine) says:

TORQUE LIMITS
Takeoff Power Range 85% to 100%
(Do not exceed 5 minutes)

I do not want to open a discussion at what sections of flight this operating range is permitted. It says "Take-off", but in reality it can also be entered during hovering OGE, conducting power-ON autorotations or cargo operations..however this is not my question.

I understand that Torque over 85% at speeds more than 80 kts and Torque over 100% regardless the speed has to do with transmission limitations and mast bending.


My question is this.
Which system is an actual limitation here?

Considering it is a 5 minute limit, in my opinion the system which is affected, is N2 blades because of high power setting and thus high temperatures, which cause blade droop on N2 blades.


I would like to know what is your opinion on this and if someone has any document or maybe manual which explains this? Thanks

Torque limits are generally imposed by the transmission, not the engine. The torque limits of a 206 have to do with how much power the transmission can absorb.

I still ponder this to this day, i once had a pilot (ex military) who believed it was ok to pull the 5 minute limit...drop the collective...then pull 5 more minutes...over and over again.
Bad training or bad advice, either way thats not how it works.

Quite simple really; you take-off once therefore you can only use the 5-minute take-off rating once. If you want to use it again then you have to land. Simples.

Pulling it, then reducing, then pulling it, then reducing will only trash the transmission but hey if Uncle Sam or Auntie Betty are paying, who cares right?

gasperh90 Torque limitations nothing to do with the engine. The C20R produces
450HP, the 206 (Jetranger) transmission is limited to 317HP (100%) for 5 mins and 269HP continuous ie.85%. The engine is never working hard,.even with the C20B.
A further transient limit applies because of bending moments in the M/R mast,that
is speed above 85% torque is limited to i think - 80 knots. Its 15 years since i owned
and flew Jetrangers so memory fading a little !!
Claudia

You CAN fly it for 5 minutes in that power band, then lower the lever, then go back into it.

BUT! You will not make it to the engine overhaul on the suggested times - the engineers will pull it well before then - maybe TXMSN making metal, maybe turbine creep, maybe a lot of things.

The TBO is worked out on 5 mins of takeoff power per 1.3 hr flight, or thereabouts. Thus they expect you to pull the power maybe 1500 times during 2000 hrs of flight. If you do it 1500 times in (1500 x 5min) it is all used up and you have some expensive servicing due.

gas. Note 110% torque can be pulled for 5 seconds. This can be used for a
certain number of times in any one gearbox overhaul. Cant remember how
many times (as said 15 years dulls the memory!)

Ascend C. Your point 1-Yes you can but below 80 knots.
2 Engine nothing to do with it. please refer to my previous post.
3 Dont recognise (or believe) any of that and i have overhauled many
jetranger M/R gearboxes. Records of how many times 100% has been pulled
are not even logged or kept. Yes excessive use of the 100% limit could lead
to premature overhaul.

It is specifically a transmission limitation, nothing to do with the engine. The part that wears out is the sun gear. Repeated 'hard' use will make sure it doesn't get anywhere close to TBO.

I prefix this with I stand to be corrected by engineers and design bureaus from OEMs, but this is my understanding and have seen the visual evidence of such.....

As it was explained to me once upon a time, (and I hold this explanation true for many, if not all helicopter transmissions), the primary reason for the 85%, (other than the mast bending moments with ->IAS), is due to increased potential for damage to sun/planet/ring gears within the Tx.

As we all know, the Tx in most helos consist of a mix of sun/planet/ring gears conducting a massive rpm reduction and power transfer function. As >85% tq is delivered to the Tx to transfer to the Rotor system, the sun/planet gears rotate furiously to absorb, reduce, and transfer this power to the mast/rotor drive.

In doing this, the gears spin at a higher rpm and increased Tq. This creates physical movement (small, but significant) of the gear teeth intermeshing contact areas. Normal gearing and load transfer is calculated on a certain percentage of teeth contact area under normal operating power ranges. As RPM increases, this percentage meshing or contact area can significantly reduce. This results in an increasingly smaller contact area having to transfer a much greater load to the next reduction gear via the teeth.

If this goes on for any extended time (5min in this case), the teeth will become very hot. Particularly at the teeth tips or reduced contact area. The evidence of this can be seen by visual inspection and a localised burnishing will be seen. You may get away with this for one, two maybe 3 or 5 times....who knows...before the metal changes its characteristics, becomes more brittle (due excessive localised heating) and the tips of the teeth or the whole tooth/teeth could develop fatigue cracking and ....hey presto, chips if you are lucky, Tx failure if you are not.

I believe the REASON behind limits should be taught to every pilot so that they can better understand ....and abide by the intent of the limit rather than simply deciding to blow it off by making up an arbitrary fairy tale about their own private ratioanle why they thin the limit exits. Or as is often the case, thinking "got away with it once before without any oil leaks", without understanding the potential problem they could be passing on to the next poor shmuck who has fly his abused machinery.

It is another reason I am STRONGLY in favour of retro fitting monitoring systems such as ALtair/Shaddin etc to record EXACTLY what that poor little abused Tx has seen. If manufacturers will accept the reports from these exceedance monitors and give "maintenance credits" back to the operators with the balls to invest in such equipment especially if they can call the OEM and say, "hey we just have an exceedance of 5min 10 sec recorded on electronic monitoring with no other history of exceedances....if the OEM were able to say....check chip plugs, drain and change oil...maybe send sample for full wear debris analysis and if all OK...return to service...maybe with another chip check in 25 hrs.....saves the operator money and validates his investment in electronic monitoring,....makes us all a whole lot safer and affordable

I'll standby for incoming on this one.....

Ring, the RPM at the transmission is constant, but the power absorbed changes up and down.

Ring, apart from anything else, the faster the rpm in a gearbox actually the lower the torque, for a given power. But agree engine/transmission monitoring is very advantageous.

And can save dosh, had both an overspeed and an engine failure at different times in a twin and the first question asked was what did the engine get to. Blowed if I know, was sort of busy at the time given the circumstances in which they occurred.

As far as I know, the torque limitations for the BIII are the same as the L series, "If overtorque above 100% up to 110% should occur, no inspection is required". There is no mention of an amount of times this can occur, or a requirement to log the events.

LRP You are indeed correct on that as long as the 110% is less than
5 seconds. As i said 15 years since i was involve with those great old
ladies!-- but fond memories!

But does it say anywhere how frequently that 110% for less than 5 seconds can be used before there is an engineering penalty?

Common sense would say that a single event should be fine in the life of the gearbox but multiple events must have an effect on the wear and life of the gears.

Leaving it open to pilot's interpretation/guesswork is surely not what the manufacturers had in mind.

I'm not going to paste the page because Bell owns the rights to the manual, however the current L series (L, L-1, L-3, L-4) maintenance manuals have the same note in the special inspection portion of the maintenance manuals for the overtorque inspection:

"If overtorque above 100% up to 110% should occur, no inspection is required"

There is no mention of the 5 second time limit (RFM), or the necessity to track the occurrence. I do not have access to the 206B manuals but if my memory is correct it is the same.

I'm not endorsing the practice, don't shoot the messenger.

Aaaargh what’s a torque limit........never that lucky operating above 10,000’ :eek: