Loss of Tail Rotor Effectiveness, recovery etc
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My previous post in this thread on the effect of the governor on the whole issue of LFE / LTE is more magnified than usual in the Gazelle, thanks to the wonderful governor.
My experience has been that the tighter the RPM control that the governor has, the more pronounced the effect that yawing has on rotor RPM (and height control in a hover) - a governor that keeps the rotor RPM very close to the datum will react more strongly than one that sort of keeps the rotor RPM more or less perhaps maybe constant.
One manufacturer had to get a modification to their FADEC to overcome the problem of tight rotor (or N2 if you're really anal) control when they went through translational lift and the sudden change in drag on the rotor due to the change inflow made the governor decrease fuel flow quite markedly. Change in governor gain stopped them from plummeting to the ground.
The Gazelle has a very good governor- uses engine oil pressure as the 'medium' to sense changes, and reacts quickly to changes in power to keep the single-spool engine at the datum speed. If you start yawing quickly to the left, the governor senses the rotor speed is too high, and cuts back on the fuel. 60 degrees per second is not unusual, and that relates to 10RPM. Can't remember the Gazelles rotor RPM in actual RPM, but 10RPM is significant enough that the governor would sense it. So, without doing anything to the collective, you'd descend.
But one of the problems is that few people own up to having had a loss of tail rotor effectiveness and fewer bother to report it through official channels (in the civilian world, you'd not be looked on with any favor in your company if you started doing this, for example).
And as previously stated, accident investigators often don't think of this as a possible cause of accidents.
I know that the H-500 early series are notoriously lacking in the T/R department (we have one...)
My experience has been that the tighter the RPM control that the governor has, the more pronounced the effect that yawing has on rotor RPM (and height control in a hover) - a governor that keeps the rotor RPM very close to the datum will react more strongly than one that sort of keeps the rotor RPM more or less perhaps maybe constant.
One manufacturer had to get a modification to their FADEC to overcome the problem of tight rotor (or N2 if you're really anal) control when they went through translational lift and the sudden change in drag on the rotor due to the change inflow made the governor decrease fuel flow quite markedly. Change in governor gain stopped them from plummeting to the ground.
The Gazelle has a very good governor- uses engine oil pressure as the 'medium' to sense changes, and reacts quickly to changes in power to keep the single-spool engine at the datum speed. If you start yawing quickly to the left, the governor senses the rotor speed is too high, and cuts back on the fuel. 60 degrees per second is not unusual, and that relates to 10RPM. Can't remember the Gazelles rotor RPM in actual RPM, but 10RPM is significant enough that the governor would sense it. So, without doing anything to the collective, you'd descend.
But one of the problems is that few people own up to having had a loss of tail rotor effectiveness and fewer bother to report it through official channels (in the civilian world, you'd not be looked on with any favor in your company if you started doing this, for example).
And as previously stated, accident investigators often don't think of this as a possible cause of accidents.
I know that the H-500 early series are notoriously lacking in the T/R department (we have one...)
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LTE Recovery ?
After watching the 2 LTE videos posted recently, I pondered on how to recover form such incidents.
The only definite I know is that at the onset of LTE - (Being High Power setting and Full pedal), that you need to lower collective and forwrd cyclic out of it.
I would imagine the Skycrane would kind of cyclic out of the situation it found itself in.
But the Jet / Longranger incident...has me pondering.
Regards
Lotsahueys
The only definite I know is that at the onset of LTE - (Being High Power setting and Full pedal), that you need to lower collective and forwrd cyclic out of it.
I would imagine the Skycrane would kind of cyclic out of the situation it found itself in.
But the Jet / Longranger incident...has me pondering.
Regards
Lotsahueys
Last edited by lotsahueys; 1st Sep 2004 at 22:45.
First of all, you gotta get your definitions right.
LTE= loss of tail rotor effectiveness
Running out of pedal= demanding more from the system than it can produce.
There is some doubt about whether LTE really exists or is a figment of the B206's imagination.
If you are just running out of pedal, reduce power, reduce weight (pickle the load) and gain forward airspeed.
Know your aircraft, get a feel for it, and when you know your left leg is nearly straight out, poke the nose over and get some speed - try to keep some reserve up your sleeve - or trouser leg.
LTE= loss of tail rotor effectiveness
Running out of pedal= demanding more from the system than it can produce.
There is some doubt about whether LTE really exists or is a figment of the B206's imagination.
If you are just running out of pedal, reduce power, reduce weight (pickle the load) and gain forward airspeed.
Know your aircraft, get a feel for it, and when you know your left leg is nearly straight out, poke the nose over and get some speed - try to keep some reserve up your sleeve - or trouser leg.
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Ascend Charlie,
Thank you! You are so very spot on. The problem we have is that the folks who make one helicopter that gets itself into NETR (Not Enough Tail Rotor) calls the condition LTE, as if a very adequate tail rotor suddenly enters the Bermuda Triangle and quits helping. This is to obscure the fact that it has too little tail rotor authority to begin with, so some slightly abusive flying causes it to get the spins.
Some facts:
95% of all LTE mishaps are suffered in ONE helicopter model (guess which one!) and 100% occur with one Brand of helicopter.
The term LTE was devised to help that company protect itself from suits from pilots around the world, and also to be sure people think all single rotor helis can get LTE.
If the pilot droops the main rotor rpm by a lot and hits the pedal stops and turns around, it is NOT LTE.
If a pilot pulls 120% main rotor torque and spins around it is NOT LTE.
Very few heli models have ever experienced LTE, most models will never experience it, some models get it way too often. That is because the lucky winners of this smallest tail rotor contest are blessed with too little tail rotor thrust, and actually suffer from NETR.
The real cure for LTE is to avoid those models that have experienced 95% of all true LTE events.
I would also say if your left leg is getting too much unbent, lower the power somehow, as the fastest way to get some antitorque back.
Thank you! You are so very spot on. The problem we have is that the folks who make one helicopter that gets itself into NETR (Not Enough Tail Rotor) calls the condition LTE, as if a very adequate tail rotor suddenly enters the Bermuda Triangle and quits helping. This is to obscure the fact that it has too little tail rotor authority to begin with, so some slightly abusive flying causes it to get the spins.
Some facts:
95% of all LTE mishaps are suffered in ONE helicopter model (guess which one!) and 100% occur with one Brand of helicopter.
The term LTE was devised to help that company protect itself from suits from pilots around the world, and also to be sure people think all single rotor helis can get LTE.
If the pilot droops the main rotor rpm by a lot and hits the pedal stops and turns around, it is NOT LTE.
If a pilot pulls 120% main rotor torque and spins around it is NOT LTE.
Very few heli models have ever experienced LTE, most models will never experience it, some models get it way too often. That is because the lucky winners of this smallest tail rotor contest are blessed with too little tail rotor thrust, and actually suffer from NETR.
The real cure for LTE is to avoid those models that have experienced 95% of all true LTE events.
I would also say if your left leg is getting too much unbent, lower the power somehow, as the fastest way to get some antitorque back.
rjsquirrel:
...And which brand would that be, Sikorsky Skycrane?
If the tail rotor is not supplying enough thrust to counteract the MR torque, thus allowing an uncommanded yaw, then I would accept that condition as one in which the tail rotor is not being effective (i.e. has lost it's effect). If we want to strictly and narrowly define LTE as...well, I'm not sure what the strict definition of LTE is, so I cannot finish that sentence.
We know and even Bell admits that during textbook LTE the tail rotor never stops producing thrust - that it is never "stalled" as an airplane's wing can stall. So for whatever reason, sometimes a yaw rate builds and the maximum thrust provided by the t/r is not sufficient to stop it. Sounds like LTE to me, why quibble?
Uncommanded yaw rates are bad. Pilots must be conditioned if it happens to stick in and hold *full* countertorque pedal for as long as it takes and to reduce the MR torque if and when possible. Neutralizing the controls or putting in anything less than full pedal is not going to help and maybe only going to make things worse.
Some facts:
95% of all LTE mishaps are suffered in ONE helicopter model (guess which one!) and 100% occur with one Brand of helicopter.
95% of all LTE mishaps are suffered in ONE helicopter model (guess which one!) and 100% occur with one Brand of helicopter.
If the tail rotor is not supplying enough thrust to counteract the MR torque, thus allowing an uncommanded yaw, then I would accept that condition as one in which the tail rotor is not being effective (i.e. has lost it's effect). If we want to strictly and narrowly define LTE as...well, I'm not sure what the strict definition of LTE is, so I cannot finish that sentence.
We know and even Bell admits that during textbook LTE the tail rotor never stops producing thrust - that it is never "stalled" as an airplane's wing can stall. So for whatever reason, sometimes a yaw rate builds and the maximum thrust provided by the t/r is not sufficient to stop it. Sounds like LTE to me, why quibble?
Uncommanded yaw rates are bad. Pilots must be conditioned if it happens to stick in and hold *full* countertorque pedal for as long as it takes and to reduce the MR torque if and when possible. Neutralizing the controls or putting in anything less than full pedal is not going to help and maybe only going to make things worse.
OK, Fan, so if a pilot allows his RRPM to bleed by pulling too much power, and the aircraft settles or even crashes, is that a case of LEE (Loss of Engine Effectiveness)?
Of course not.
It is a pilot error, by demanding more than the system can produce. Mind you, under some conditions it is pretty hard to avoid going for that extra pull, but if it is outside the aircraft abilities, it is not LTE.
LTE would be defined as a situation where the tail rotor should be producing sufficient thrust, with plenty of pedal travel available, but suddenly decides not to.
A bit like a car going around a corner - it should be within its capabilities, but in the case of one car (I think it was a rear-engined Ford) it decided instead to roll over. Did Ford commission an investigation into bad roads? No, they removed the car after Ralph Nader showed the public that it was unsafe at any speed.
Even Mercedes Benz redesigned their baby car after it failed the moose swerve test.
I have over 6000 hrs on the 206 and ran out of pedal, knowingly, many times. Mostly on powerline inspections, or else film jobs. I always knew when i was approaching the limit, and just flew out of it. Whether the relative wind was from 330, or 080, or 160 or straight up the choof, it never behaved in an unpredictable way.
But I cursed it soundly for not being able to hang in there.
Of course not.
It is a pilot error, by demanding more than the system can produce. Mind you, under some conditions it is pretty hard to avoid going for that extra pull, but if it is outside the aircraft abilities, it is not LTE.
LTE would be defined as a situation where the tail rotor should be producing sufficient thrust, with plenty of pedal travel available, but suddenly decides not to.
A bit like a car going around a corner - it should be within its capabilities, but in the case of one car (I think it was a rear-engined Ford) it decided instead to roll over. Did Ford commission an investigation into bad roads? No, they removed the car after Ralph Nader showed the public that it was unsafe at any speed.
Even Mercedes Benz redesigned their baby car after it failed the moose swerve test.
I have over 6000 hrs on the 206 and ran out of pedal, knowingly, many times. Mostly on powerline inspections, or else film jobs. I always knew when i was approaching the limit, and just flew out of it. Whether the relative wind was from 330, or 080, or 160 or straight up the choof, it never behaved in an unpredictable way.
But I cursed it soundly for not being able to hang in there.
lotsahueys - going back to your original question - in the skycrane incident the sudden change in wind strength and direction was the culprit taking the aircraft from ETR (enough TR) to NETR quite quickly. In situations like this, if height and obstacles permit, allowing the ac to yaw into wind can often sort the problem, accompanied with a slight lowering of the lever. If all that doesn't work then you are into the process described above - lose weight (pickle load) and try to get forward speed -ETL over the TR.
I have suffered NETR in the Wessex (Sikorsky) on many occasions and the old Mk 1 Lynx (Westlands) was renowned for it - manufacturers who don't put a big/powerful enough TR on the back of their machines shouldn't hide behind the label of LTE.
I have suffered NETR in the Wessex (Sikorsky) on many occasions and the old Mk 1 Lynx (Westlands) was renowned for it - manufacturers who don't put a big/powerful enough TR on the back of their machines shouldn't hide behind the label of LTE.
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...Ascend Charlie:
???Umm I beg to Differ!
I have never yet flown any machine that has a mind of its own and can decide to not produce sufficient T/R Thrust to over come Torqe!!
I always thought it was a series of Conditions that the pilot has found himself in...
LTE would be defined as a situation where the tail rotor should be producing sufficient thrust, with plenty of pedal travel available, but suddenly decides not to.
I have never yet flown any machine that has a mind of its own and can decide to not produce sufficient T/R Thrust to over come Torqe!!
I always thought it was a series of Conditions that the pilot has found himself in...
Unfortunately there is no avatar smilie for showing tongue-in-cheek irony.
Of course no helicopter ever does it.
That's why LTE is horsefeathers. Simply, Not Enuf Tail Rotor.
Sorry, that tongue is right outside the mouth, but you get my drift.
Of course no helicopter ever does it.
That's why LTE is horsefeathers. Simply, Not Enuf Tail Rotor.
Sorry, that tongue is right outside the mouth, but you get my drift.
lotsahueys,
Back to your original question of how to recover.
If for whatever reason or definition you don't have enough anti torque (tail rotor thrust) and you have lost directional control (ie spinning), the only solution is to reduce torque. Whether it be LTE, or low RRPM, or no more pedal left, or a control failure or a severed drive shaft, the Torque is winning over the anti torque.
Hindsight is always easy, especially when you are sitting on the ground and not in the saddle at the time. But when I watch that video of the 206 I personally think the pilot was quite slow to take corrective action, ie it spun a few times prior to recovery.
In that particular situation I would have immediately lowered the collective fully and if that wasn't enough to stop the spin also rolled the throttle off. (ie removed ALL torque if I had to), then regained speed and brought the power back in. It looked like he had sufficient height to be able to do that.
None the less, full marks and well done to the pilot for recovering safely, it would have got the blood pumping no doubt!
Back to your original question of how to recover.
If for whatever reason or definition you don't have enough anti torque (tail rotor thrust) and you have lost directional control (ie spinning), the only solution is to reduce torque. Whether it be LTE, or low RRPM, or no more pedal left, or a control failure or a severed drive shaft, the Torque is winning over the anti torque.
Hindsight is always easy, especially when you are sitting on the ground and not in the saddle at the time. But when I watch that video of the 206 I personally think the pilot was quite slow to take corrective action, ie it spun a few times prior to recovery.
In that particular situation I would have immediately lowered the collective fully and if that wasn't enough to stop the spin also rolled the throttle off. (ie removed ALL torque if I had to), then regained speed and brought the power back in. It looked like he had sufficient height to be able to do that.
None the less, full marks and well done to the pilot for recovering safely, it would have got the blood pumping no doubt!
Last edited by the coyote; 2nd Sep 2004 at 12:31.
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"Just a pilot"
Emergencies are seldom identical to training scenarios, so it really doesn't matter what you call'em. What counts, is not putting yourself into an unsurvivable situation. In other words have a realistic plan, safe limits, and situational awareness.
Yaw is primarily related to torque at a hover. You can induce yaw by increasing TQ, and reduce it by decreasing TQ. If you hit the pedal stop, and the pedal applied isn't effective, and power variation isn't the extra minute bit of authority, you must make big changes- first power, then airspeed to get the vertical stab effective, and perhaps to execute an autorotational landing if nothing works.
Sometimes, you have to say "no" to a request.
Yaw is primarily related to torque at a hover. You can induce yaw by increasing TQ, and reduce it by decreasing TQ. If you hit the pedal stop, and the pedal applied isn't effective, and power variation isn't the extra minute bit of authority, you must make big changes- first power, then airspeed to get the vertical stab effective, and perhaps to execute an autorotational landing if nothing works.
Sometimes, you have to say "no" to a request.
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To revisit history - the LTE incidents typically happen when turning right at no airspeed, or low airspeed, when out of ground effect.
part of the problem is the engine governor will reduce rotor RPM with respect to the earth (not necessarily with respect to the airframe) and this will affect the tail rotor RPM, and hence tail rotor thrust. The faster the rate of rotation, the more the governor will back off the main rotor RPM (with respect to the earth, not the airframe- the governor is directly connected to the N2 turbine, and is basically thus counting 'blades passing tailboom' - and since you're rotating in the opposite direction to the rotor, the governor thinks the rotor is going too fast)
The tail rotor is still producing thrust, but just not enough to overcome the rate of yaw.
The tested and proven solution is to add full left pedal and forward cyclic and get some airspeed over the vertical stabilizer to get some directional stability.
But prevention is the better solution.
part of the problem is the engine governor will reduce rotor RPM with respect to the earth (not necessarily with respect to the airframe) and this will affect the tail rotor RPM, and hence tail rotor thrust. The faster the rate of rotation, the more the governor will back off the main rotor RPM (with respect to the earth, not the airframe- the governor is directly connected to the N2 turbine, and is basically thus counting 'blades passing tailboom' - and since you're rotating in the opposite direction to the rotor, the governor thinks the rotor is going too fast)
The tail rotor is still producing thrust, but just not enough to overcome the rate of yaw.
The tested and proven solution is to add full left pedal and forward cyclic and get some airspeed over the vertical stabilizer to get some directional stability.
But prevention is the better solution.
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Pprune fan,
Notwithstanding the video (which could have been experienced by any helicopter anywhere if the rotor rpm is pulled low enought) the vast vast vast majority of "LTE" accidents are in Bell 206, because that heli has a tail rotor that is just too small. Bell invented the term LTE as a clever way to make the problem universal, and therefore not theirs! It is far easer to take out the rubber stamp that says "Pilot Error" than it is to fix the helicopter.
Someone in pprune once posted an accident summary that showed that 95% of all LTE occurred in Jetrangers.
Note Shawn Coyle's comment that the Bell 206 hover weight is actually limited not by power, but by its tiny tail rotor!
LTE = NETR
Notwithstanding the video (which could have been experienced by any helicopter anywhere if the rotor rpm is pulled low enought) the vast vast vast majority of "LTE" accidents are in Bell 206, because that heli has a tail rotor that is just too small. Bell invented the term LTE as a clever way to make the problem universal, and therefore not theirs! It is far easer to take out the rubber stamp that says "Pilot Error" than it is to fix the helicopter.
Someone in pprune once posted an accident summary that showed that 95% of all LTE occurred in Jetrangers.
Note Shawn Coyle's comment that the Bell 206 hover weight is actually limited not by power, but by its tiny tail rotor!
LTE = NETR
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D
it, this subject keeps coming back at me all the time. And everytime I think that I finally understand what you wise guys are talking about and most important what I'm doing up there in the air you get me all confused again.
Could someone please clearly point out the difference between LTE, LTA, tail rotor stall etc. And please if it doesn't cause any inconvenience use plain English
Cheers,
A youngster listening to the old wise guys
it, this subject keeps coming back at me all the time. And everytime I think that I finally understand what you wise guys are talking about and most important what I'm doing up there in the air you get me all confused again. Could someone please clearly point out the difference between LTE, LTA, tail rotor stall etc. And please if it doesn't cause any inconvenience use plain English
Cheers,
A youngster listening to the old wise guys
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Plain English:
Any single rotor helicopter can run out of pedal if the main torque is driven high enough and the rpm is drooped low enough, but almost every helicopter has a hard time doing this within normal flight maneuvers because most tail rotors have enough margin.
Some helos with very marginal tail rotors are more prone to finding insufficient anti-torque within normal maneuvers, and for them, extra care is needed, and the term LTE was invented. LTE stands for "Loss of Tailrotor Effectiveness" which actually never happens, because the tail rotor never loses effectiveness, and actually produces its required thrust under all conditions within the normal envelope. When LTE rears its head for those helos that are prone to it, what really happens is that the small thrust margin is not enough to overcome the wind, or the extra main rotor torque that the pilot is pulling. In those cases, the pilot hits the pedal stops, the aircraft keeps spinning, and its Katy Bar the Door.
In the old days we thought the tail rotor was stalling or some such myth, but we now know that the paltry tail thrust margin of some helos is just not enough, and that the other forces (wind and main rotor torque, chiefly) can swamp the tail rotor. For those marginal helos, care must be taken with crosswinds and power applications at low speed to prevent this swamping of the tail thrust (LTE).
For most helos, the tail rotor is bigger, and has more pitch available so that it can overcome moderatly abusive flight conditions, and not make the helo swap ends. These helos do not ever experience true LTE.
Recovery? Reduce main torque, and also reduce the crosswind. Not easy when doing 100 degrees per second to the right! The old AH-1G cobra gunships that I flew were so prone to this (due to pitifully small tail rotor margins) that we used to practice recovery techniques in checkout.
Any single rotor helicopter can run out of pedal if the main torque is driven high enough and the rpm is drooped low enough, but almost every helicopter has a hard time doing this within normal flight maneuvers because most tail rotors have enough margin.
Some helos with very marginal tail rotors are more prone to finding insufficient anti-torque within normal maneuvers, and for them, extra care is needed, and the term LTE was invented. LTE stands for "Loss of Tailrotor Effectiveness" which actually never happens, because the tail rotor never loses effectiveness, and actually produces its required thrust under all conditions within the normal envelope. When LTE rears its head for those helos that are prone to it, what really happens is that the small thrust margin is not enough to overcome the wind, or the extra main rotor torque that the pilot is pulling. In those cases, the pilot hits the pedal stops, the aircraft keeps spinning, and its Katy Bar the Door.
In the old days we thought the tail rotor was stalling or some such myth, but we now know that the paltry tail thrust margin of some helos is just not enough, and that the other forces (wind and main rotor torque, chiefly) can swamp the tail rotor. For those marginal helos, care must be taken with crosswinds and power applications at low speed to prevent this swamping of the tail thrust (LTE).
For most helos, the tail rotor is bigger, and has more pitch available so that it can overcome moderatly abusive flight conditions, and not make the helo swap ends. These helos do not ever experience true LTE.
Recovery? Reduce main torque, and also reduce the crosswind. Not easy when doing 100 degrees per second to the right! The old AH-1G cobra gunships that I flew were so prone to this (due to pitifully small tail rotor margins) that we used to practice recovery techniques in checkout.
Spunk - if the helicopter yaws when you don't want it to you apply pedal to stop the yaw. If you reach the pedal stop and the aircraft is still yawing you have reached the limit of your tail rotor authority (max pitch). In this condition the torque produced by the engine driving the rotor cannot be balanced by the anti-torque force from the TR.
One of 2 things can happen now - either a. the rate of yaw will gradually decrease and stop, possibly because you have yawed back into wind and weathercock effect on the fuselage/vertical satbiliser has produced sufficient anti-torque force to balance MR torque, or b, the rate of yaw will increase and your only course of action is to reduce the MR torque (lower the lever) and/or gain speed (the TR benefits massively from translational lift).
The question you have to ask now is why?
Have you put the aircraft in a position (high AUM, high DA) where you are at the limits of the RFM and been caught out by slightly different conditions ie wind direction and strength than you expected?
Have you flown the aircraft badly and failed to anticipate the requirement for extra pedal as you lose translational lift (requiring both more torque to hold height and more TR thrust to maintain heading)?
Have you allowed a rate of yaw to build up because you weren't paying attention and the available pedal isn't enough to stop it?
Have you pulled so much lever that the Nr is drooping so that the TR is slowing down as well and reducing the thrust it can produce?
All of these could be described as LTE because it isn't doing what you want it to and there is not enough TR thrust. BUT they are all equally a case of NETR for the same reasons.
Ideally a helicopter will always have a surplus of TR thrust available to cope with all situations but if the TR is too small then it is much easier to encounter uncorrectable yaw in all the above situations - THIS is what critics claim is wrong with the 206, it just doesn't have sufficient surplus TR thrust to make the pilot's job easier.
It is thoeretically possible to put a TR into vortex ring state but I think the rates of yaw/crosswind would be phenomenal unless the aircraft was hovering at very low power and subsequently TR pitch (therefore the flow through the TR would be minimal and small rates of yaw could give an opposing airflow) ie in a strong updraught.
Sorry Nick you must have posted while I was writing!
One of 2 things can happen now - either a. the rate of yaw will gradually decrease and stop, possibly because you have yawed back into wind and weathercock effect on the fuselage/vertical satbiliser has produced sufficient anti-torque force to balance MR torque, or b, the rate of yaw will increase and your only course of action is to reduce the MR torque (lower the lever) and/or gain speed (the TR benefits massively from translational lift).
The question you have to ask now is why?
Have you put the aircraft in a position (high AUM, high DA) where you are at the limits of the RFM and been caught out by slightly different conditions ie wind direction and strength than you expected?
Have you flown the aircraft badly and failed to anticipate the requirement for extra pedal as you lose translational lift (requiring both more torque to hold height and more TR thrust to maintain heading)?
Have you allowed a rate of yaw to build up because you weren't paying attention and the available pedal isn't enough to stop it?
Have you pulled so much lever that the Nr is drooping so that the TR is slowing down as well and reducing the thrust it can produce?
All of these could be described as LTE because it isn't doing what you want it to and there is not enough TR thrust. BUT they are all equally a case of NETR for the same reasons.
Ideally a helicopter will always have a surplus of TR thrust available to cope with all situations but if the TR is too small then it is much easier to encounter uncorrectable yaw in all the above situations - THIS is what critics claim is wrong with the 206, it just doesn't have sufficient surplus TR thrust to make the pilot's job easier.
It is thoeretically possible to put a TR into vortex ring state but I think the rates of yaw/crosswind would be phenomenal unless the aircraft was hovering at very low power and subsequently TR pitch (therefore the flow through the TR would be minimal and small rates of yaw could give an opposing airflow) ie in a strong updraught.
Sorry Nick you must have posted while I was writing!