Over Torquing

[handbag]

If a single engine turbine is over torqued, where is the damage in the drive train likely to occur and which components most likely effected first ? Also, which maneuvers are most likely to lead to OT. Not a type specific question but more of a trawling for info one.

Thanks.

[the coyote]

Come on, spill the beans, how high did it go? :E

[heli heney]

I reakon!! You sound guilty as! Let me guess you have a ''friend'' who may or may not have pull a tad hard? :O

[Peter-RB]

If you didn't win the lottery at weekend, then change the subject! :ooh:

PeterR-B

[idle stop]

Gearbox (es), rotor mast, drive shafts, Thomas couplings, blades....need I go on? And was the engine overtemped in the process?
Any suspected overtorque should be reported to the approved maintnenance organisation and investigated iaw the type's Maintenance Manual.

[EMS R22]

Come on, spill the beans, how high did it go?

My thoughts indeed....:}

[Shawn Coyle]

What gets changed depends on the amount and duration of the over-torque. And what type of recording device, if any, was fitted.
Is the overtorque supported by corresponding engine information, or was it only registered on the torque meter?
Fitting something like the Intellistart system to a helicopter is going to save you from just a hot start event, it's going to record all the engine and transmission information, and possibly save you from replacing components due to a very minor overtorque.

[FH1100 Pilot]

To answer your specific questions: If a single engine turbine is over torqued, where is the damage in the drive train likely to occur and which components most likely effected first ?

I would look at the tail rotor blades and associated drivetrain first. The maintenance manual will/should tell you where to look based on the severity of the event. The main rotor stuff (blades, gearbox, etc.) is MUCH tougher.

Also, which maneuvers are most likely to lead to OT.

And by "OT" I assume you mean "overtorque." Obviously, takeoffs and landings. Probably a landing gone wrong - oops! (No need for that much power in translated flight.) Left pedal turn in a very heavy hover, perhaps? Couple of different scenarios I can think of, each as "likely" as the next.

[Two's in]

There's no generic answer for this because design details in the transmission and power assembly dictate the load path. Most transmissions use flexible and rigid couplings where excess torque will displace or distort the disks in the coupling pack or the fasteners holding the coupling to the drive shaft which may give a visual indication of damage.

But make no mistake, over torque checks are for qualified engineers using the approved maintenance documents. As a pilot, your only indication is the torque indicating system and if that is ever exceeded, it must be reported to a maintenance person.

Your question is hopefully hypothetical, but the acid test of a professional pilot is reporting all and any damage they may have caused before it kills the next occupant.

[Gomer Pylot]

From what I've seen, takeoffs and landings are where overtorques usually happen, more often landing than taking off. I've known of a few that happened during a start. You can also just wind up the throttle too fast, but I think the most common scenario is a hot approach, and having to pull too much collective to stop. In a 206, it's very easy to overtorque on takeoff with a heavy load, by not being directly into the wind, having the nose come around, and having to shove in a lot of left pedal to prevent LTE, when you're at about 99.9% torque already. Large pedal inputs produce large torque spikes. My 206 takeoff technique was always to leave just a little in reserve, and most important, never let the nose move at all. If you keep the nose absolutely straight, you don't need to stomp on the pedal, and you can control the torque. Let it start to turn, and you're in trouble. I treat other models pretty much the same, because things are always better when you have a smooth control touch.

[spinwing]

Mmmm ...

And the good old 412EP will bite you if'n your not really careful too !! :=

[havick]

keep in mind that cyclic inputs do cause changes in tq..

also another point if you're taking off from an elevated helipad and you're just below your tq or even ITT limit, then keep in mind the attitude change that is about to happen to forward flight.

[nuthin]

chip lights will tend to follow an overtorque also. I always check em out when I get a different machine at handover.

[Epiphany]

Spingwing - been there done that and you can't reset the indicator.:sad:

[piggybank]

'If a single engine turbine is over torqued, where is the damage in the drive train likely to occur and which components most likely effected first ? Also, which maneuvers are most likely to lead to OT. Not a type specific question but more of a trawling for info one.'

On the off chance you mean a single engine on a twin engined helicopter, its possible to overtorque the one engine without overtorqing the transmission system, saying this with the Bell 212 in mind.

If my memory serves me right this is true to the first level of overtorque inspections required at 106% (without books at hand to refer to). At 112% overtorque a lot of bits will need sending away for overhaul and inspection.

A big problem is when someone sees what is happening, maybe with eyes locked on TOTs who can truly say how long it was on over torque and what the peak was, time seems to 'telescope' in times of instant anxiety.

All the info for a decision will be in the maintenance books for both airframe and engines inspections.

[Soave_Pilot]

From what I learned from A-stars.
If you pass a certain limit for a certain time, you have to check everything, turbine, drivetrains, transmission blah blah blah, only mecanics know where to look for damage. Some birds will let you go past the red for a very short period of time, without causing damage. But it will be registered.:=

overtorque x time in it = damage :}

[VeeAny]

Another thing to be aware of

Most machines have a published transient inside which there is no maintenance action required.

A lot of machines have a varying list of inspections dependent on, as has been said above how far over you were and for how long. A little bit and for not very long costs less and vice versa.

The newer Agusta 109s (E and S models) have a very specific bit in the maintenance (or perhaps overhaul) manual that says something like above the transient limits remove the gearbox for maintenance at manufacturer (not a direct quote but you get the idea). Could catch some people coming from a 206 background and before anyone says it I wasn't looking because I had done it, I was putting together some training materials.

[charlieDontSurf]

My experience is that overtorque happens most likely on delivery of a heavy load, often if you come in too fast, or with a tailwind. Too hard pedal-inputs will result in an overtorque if you are on the limit.

In the AS 350, the freewheeling-unit may need replacement after a high overtorque, or several minor ones.
We had to replace a freewheeling-unit last year after a 16-sec 108% overtorque...! (There was a guy that ran under the load, and the pilot HAD to hold it until he was safe!). That unit had a pricetag of about 18,000,-Euros.:)

[paco]

If it was a 206, take a quick look at the brown stuff sealing the top and bottom parts of the MR gearbox for cracks (the two parts twist against each other), and the mast for crazing. Look under the engine at the pan and also the fuselage just forward of where the tailboom joins it for wrinkles. If you see anything there it was definitely over 120%!

110% on a 206 is a quick inspection, and 120% not much more. But as Nick Lappos once said - that's one out of your jar of sweeties gone!

It's always best to tell maintenance - engineers are very interested in their machines and like to know stuff like that. Sh*t happens and most of us have overtorqued at some time or other. In my case it was a sling load in a couple of gusts that came together from different directions before I managed to pickle it.

As for manoeuvres, you can get a lovely torque spike doing a power on recovery from autorotation if you don't watch it.

Phil

[Heli+]

Let's not forget, that you can build helos that can not be over torqued.

Russian helicopters (it's always mentioned in general, I don't know which models it applies to or not) have no TQ gauge. The drivetrain is just built and rated so that it takes whatever the engine can deliver.
When the RRPM comes down you know you are over the TQ limit ;-)

[handbag]

well nice of the judge and jurors there to cast their sentence. Nothing got over torqued. Was just looking for an interesting conversation which some participated in. :D

[piggybank]

The same mentality was probably used when they designed the Puma 330J

[riff_raff]

handbag,

With 2 spool turboshafts that have the power turbine in the back of the engine (ie. the opposite end from the PTO), it's usually very easy to damage the power turbine shaft with torsional overload. The reason is that the power turbine shaft in these engines tends to be very slender due to space limitations, and thus usually has very small stress margins in torsion. To avoid this issue, small turboshaft engines sometimes put the power turbine at the front of the engine (like the Rolls/Allison 250).

Gearboxes/sprag clutches/TR drives usually are designed more robustly than the engine. So these components are less likely to be damaged by transient overtorques. The only exception that comes to mind is if a helo has undergone a significant engine power upgrade with the existing gearbox, where the only thing preventing gearbox overtorque is the pilot and/or FADEC. This type of engine upgrade would be done to improve performance at hot/high conditions, but could also cause overtorque at SL conditions if not carefully controlled.

Hope that helps to answer your question.
riff_raff

[blackhand]

HANDBAG

As already alluded to, depends on amount of torque pulled.
Most airframes have at least three limitations.
Take off power - up to 5 minutes at approximately 5 - 10% over 100% power.
Max continuous.
And transient as stipulated in the POH.

The engine itself has torque limitations but I think you would have overspeed or overtemp problems prior to reaching it.

RIFFRAFF
Were did you learn your helicopter maintenance theory?

It all sounds the opposite to what I was taught.

Torque limits generally pertain to the airframe NOT the engine, it has to be a massive over-torque before engine maintenance has to be carried out.

Reverse flow engines are used as it makes the engine shorter in length.

Cheers
BH

[EBCAU]

Ah, but Blackhand I think you are wrong in this statement:

"Take off power - up to 5 minutes at approximately 5 - 10% over 100% power."

I don't know of helicopters can allow this period of time over 100%, but I stand to be corrected. I think you might be referring to transient limits that are generally around a few seconds duration, and usually not permitted for intentional use.

[Gomer Pylot]

In the models I've flown, torque is a transmission parameter, not an engine parameter. In most models, the engine(s) is capable of delivering much more torque than the transmission is rated for, at well below any engine limits, such as temp or N1, at least at or near sea level. Exceeding torque limits requires inspection of the transmission and drive train. Replacement of components may be mandatory, depending on the torque recorded. For most exceedances, it's just an inspection, and a few beans may be removed from the can.

[Shawn Coyle]

Ah - limitations!
They are complicated, and often not clearly explained. But you need to obey them, as they affect the airworthiness of the helicopter.
Transients are there only to keep you out of trouble. Using them on a regular basis is never a good idea - in fact in flight testing for certification, no credit can be given for transients.
Takeoff power is not always 100% - sometimes more, sometimes less. Whatever the number is, obey it.
And riff raff - I don't know any free turbine engine that has the power turbine ahead of the compressor turbine in the flow. The RR 250 series has the turbines arranged that way for reasons best known to the original Allison engineers - all long since retired.

And something for all to consider - if there is no equivalent engine indications of high power (such as N1 or TOT), is it really an over-torque? Remember that the torque meter is measuring drag on the blades, and a sharp increase in drag on the blades can cause a large change in torque - without the engine producing the equivalent power...
All the more reason to have a monitoring system like Intellistart if you don't have a FADEC / DEC.

[riff_raff]

Shawn Coyle-

"The RR 250 series has the turbines arranged that way for reasons best known to the original Allison engineers"

The torsional strength, stiffness and low critical speed of a long, small diameter driveshaft passing from an aft mounted power turbine to a front end PTO presents a very difficult design challenge. Take a look at the lengths Turbomeca went through to avoid the issue. They located the PTO gear drive where it is surrounded by hot exhaust duct:

http://www.turbomeca.com/public/turbomeca_v2/html/upload/img/4a9e3de9e87192s2.jpg

Pratt & Whitney took the same approach as "those long gone Allison engineers" with their PT6. They put the power turbine in front:

http://www.flightglobal.com/airspace/media/galleries/images/5626/500x400/pratt-whitney-pt6a-67-cutaway.jpg

I was working on the design of a turboshaft drivetrain a while back. The engineers at the engine company were very nervous about any torsional vibrations or dynamic loads in our gearbox because their drive shaft only had an ultimate torsional MoS of 50% at MRP.

Regards,
riff_raff

[blackhand]

Hi Shawne
was thinking of long ranger 5 min take off power.

Riff-Raff
depends how one looks at it. The power turbine on all the engines I know about are "after" the gas producer relative to airflow.
As far as reverse flow turboshaft engines are concerned, the power turbine may be in front of th GP turbine physically but not relative to airflow.

BH