Tail Rotor Control Failure

[MightyGem]

A helicopter with blades going the US way has a tail rotor failure. On setting up for a normal approach, as speed is reduced the nose yaws to the left. This indicates a failure in a high power setting and the nose will come back to the right as power is applied at the bottom, allowing a fairly slow run on, or perhaps a hover.

Flight Manual advice advocates a wind from the right to assist. This is the bit that I can't get my head around. I would have thought a wind from the left would be better in helping to prevent the nose from going too far to the right.

Comments please.. . <img src="confused.gif" border="0">

[ShyTorque]

MG,

I agree with you regarding the side for the wind. Which RFM is it?

Having taught on a simulator and watched many experienced pilots get themselves in a tizz, and me seemingly forever personally destined to move between left to right and right to left MR blade directions, I wanted a foolproof method of remembering.

The simplest method I have come up with is this:

Lucky side = retreating blade side. It works for all conventional helis (main + tail rotor) that I have flown. Get the nose out to the retreating blade side on the approach and get any crosswind also on the retreating blade side.Lucky side = Retreating blade side.

That way, as more collective power is brought in close to the ground with speed reduction, the nose comes round to straight ahead through increasing Tq reaction. Try a dummy approach at height to find a speed & power combination that you are happy with for the approach. Avoid soft surfaces if possible, to lessen the risk of a skid or wheel digging in with yaw applied, which might result in a rollover.

There was a previous thread on this same forum few months back, some very useful discussion on it.

[ 23 January 2002: Message edited by: ShyTorque ]</p>

[helmet fire]

MightyGem:

The situation you have described MAY be too much left pedal, ie left pedal stuck too far forward, but you never know unless you do much more diagnosis than you have explained - or until you get to the bottom and apply power. So I am assuming you have a left pedal forward case.

Cross wind causes displacement of pedals in the hover & slow forward flight. Therefore, if you have a right crosswind, the nose will want to turn right unless you apply more left pedal (than a nil wind hover). Thus, in your situation of too much left pedal, a wind from the right would aid in you getting the aircraft skids straight.

I take my hat off to anyone who can think about all this in the air though. I am more likely to scream some sort of panic driven obscenity over the intercom, punch out a really cool & calm sounding mayday, and then try to react to the situation I see as I apply power at the bottom! Please let me never have to do one for real...

:)

[VLift]

More tail behind the mast than nose in front, e.g. weathervane, weathervaning (sp) assisted by wind from right, me thinks.

[Mark Six]

MG, presumably you are talking about a tail rotor control failure (stuck at a fixed pitch), not drive failure (not turning). As you reduce speed the aircraft will yaw RIGHT (but the nose may still be left of your direction of travel), as you lose the streamline effect which helps to counter torque. If it yawed left it is because of the power (torque) reduction, not speed reduction. You are correct in that as you apply power in the hover the nose will want to turn to the right, however if you have a "high power" failure (your terminology), presumably you are describing a scenario where the tail rotor control failed ("stuck") with more pitch than what was required to counter the amount of torque you will pull in the hover, so the nose will still go left, and wind from the right will assist to counter it. Incidentally direction of yaw as you approach does not necessarily tell you which direction the aircraft will turn in the hover as yaw at any stage of your analysis will be affected to varying degrees by your airspeed and torque (and rotor rpm if you can adjust it). Confused?????

[ 23 January 2002: Message edited by: Mark Six ]</p>

[ShyTorque]

A tail rotor stuck at high power will allow a slower speed approach and possibly a hover, but it is possibly unsafe to try to achieve it. Once the aircraft begins to rotate in the hover, the only way out is to chop the power, very difficult if single pilot and the ECLs are in the roof panel. It is likely to end in a heavy landing at least.

Which is why a pilot shouldn't try to come to the hover in this situation. A gentle approach, with the aircraft nose just out to the retreating blade side to a running landing at low groundspeed is what should be aimed for.

If the nose comes round towards 12 o'clock on the approach, the cyclic should be checked forward to prevent any further speed reduction. If the nose is kept offset a little, the final collective movement to cushion the touchdown will also bring the nose round to straight ahead for a safe touchdown at low speed with nil yaw.

However, all aircraft have different characteristics so the top priority must be to confirm the manufacturer's advice!

[oryxs]

The wind thing has to do with the tailrotor and which side it's of the tailboom eg effectiveness of the t/r and how you can get maxi use out of the wind. Did this with a student on the sim and the stronger the wind was from the right(french machine), the easier the exercise became. Keeping the nose to the retreating side is real good advice because it gives you more options at the bottom.

[MightyGem]

Thanks guys. . .ST, it came from the Lynx FRCs, which says the opposite that the EC 135 recommends. I've successful landings from both high and low power settings in the sim.

HF and MS both make sense for your scenarios, obviously everything depends on the TR setting when the controls fail.

[Nick Lappos]

With the symptoms as described, the tail rotor controls are producing too much thrust in the part power descent, so the nose swings left. For such a rare situation (often found in simulators, rarely anywhere else) the idea is to make an approach so that the speed is slowly reduced and power is slowly brought up until the speed/power combination for a nose straight ahead is found. If timed well, the touchdown occurs then. If the pilot allows the nose to get to the right because the power was allowed to get too high (and the speed was allowed to get too low) a gentle nose down will increase the descent, but will reaccelerate the helicopter until the nose swings back straight ahead. If you mess up and the nose begins to spin to the right, don't fully cut the throttle, but pull it back slowly a bit at a time while increasing the collective to arrest the descent. This will allow you to get more main rotor torque and less tail rotor thrust by reducing the rpm, and thus reducing the tail thrust and stopping the spin.

On experimental helos, I had the chance to practice these to touchdown on many occasions, and it is not too difficult. It is even possible to make a go-around, if you gently raise the collective to get a small but controllable left yaw, and concurrently lower the nose to begin a gentle climb.

About the wind, it depends on where the controls are stuck. If the tail thrust is more than that needed for hover, the right wind is helpful. If the tail thrust is less than hover thrust, a left wind is helpful. In both cases, your use of collective and speed dominate, and the wind contribution is small, unless the wind is hellacious, in which case you should land into the wind! Generally, controls that stuck do so where they are placed, and not beyond, so it is unlikely that the tail controls will stick to a position of hover thrust or greater (since one assumes that they stick in a forward flight setting). I believe the flight manual case cited assumes the controls stick to very high thrust.

For a tail-control-stuck-in-cruise case, if the runway has a wind from the left, this will add some anti-torque to the situation, and will allow the touchdown to be made slower (picture that a healthy helicopter will need some extra right pedal in the hover i.e. a little less tail rotor thrust, which is where the pedals are stuck).

After touchdown, lowering the collective will cause a sharp left yaw, and a possible ground-loop. One way to avoid it is to leave the collective pitch at the touchdown setting and while rolling out, pull back the throttles slowly, while using brakes to hold things straight. The throttle roll-back will reduce the main and tail rotor RPM exactly together (of course) and so will reduce their thrust concurrently, thus maintaining the anti-torque balance.

[ 24 January 2002: Message edited by: Nick Lappos ]</p>

[before landing check list]

To make a long story short, regardless on the technique, or what the book says...nothings says you have to be in a hurry. Unless you are out of gas and or on fire..TAKE YOUR TIME. Do numerous aproaches with different speeds, change direction to take advantage of prevailing winds. Hell, even do this at altitude to feel things out use the radio.. No hurry.. . j

[ 26 January 2002: Message edited by: before landing check list ]</p>

[greenarrow]

With the lynx sim, I tried No1 in ac drive to power the hyd system. Then fly the aircraft on the No2 as long as the power is available.. .This would allow a greater pitch to power and would result at times in a lower speed touch down with a bit of an overpitch. Also with a slower torque reaction in the yawing plane, therefore easier to keep straight at the bottom. Just a thought!

[Chinook]

Had the misfortune of jammed pedals on a UH-1H a few years ago flying number three of three at low level (by day thankfully).

All the drills of control, analysis of the skid ball and crew coord were applied ... NOT!

Sadly the co-pilot asked if I would like him to get out the checklist and go through the actions, to which I replied 'F@#$ the checklist, were getting this b$%^& on the deck!!' and then I realised I had transmitted my curt, impolite and poorly phrased response over the UHF radio.

The craft was flying straight and true at 40-50 knots and a reasonable power setting for a running landing and the pedals would move occasionally of their own accord, and we did not have time to muck about.

I am always aware of the shabby CRM I displayed that day but the crew and aircraft were unharmed and I got to write an incident report instead of letters to families.

My advice when faced with a TR malfuction of any kind ...... look out the window make the world look normal if you can and land the beast the most expeditious way you can.

Walking away is a win in that case.

[helmet fire]

Chook: You almost exactly followed the procedures I thought I would most likely follow (see last para of my post above)!! :) :)

I am very intrested in what the cause of the problem was, do you know?

Well done for getting it on the ground. <img src="cool.gif" border="0">

[scrubba]

chinook,

apart from the p*ss poor CRM that you apparently recognise but also appear to quite comfortable with (otherwise we wouldn't have had the joy of reading about it), your message appears to be, regardless of your training, abandon the emergency checklist at the first hint of trouble and make it up as you go along.

and you would be a qfi/standards person as well, i presume...

[Grey Area]

Mighty Gem, my old diamond, I'm not sure if you are talking about Army or RN Lynx FRCs. The old RN cards were not the original advise but a set of checks put together by a certain Lynx Sim instructor and tested in the Lynx sim. Frankly they were dangerous, the sim does not have a valid flight model for the TR failure. If you compare the sim to the real thing with left pedal stuck in the 120 kt cruise position you will find completely different behavior so you quite patently can't research new FRC actions (especially if your objective is promotion!).

[Roofus]

Can I throw my pennies worth in?

Most of this is excellent advice if you have or can acheive forward flight!

Most helicopters, particularly military & emergency services, spend large chunks of their life in the hover. Where you're pulling the most power & as such putting the most strain on the machine.

I've had a tail rotor drive failure at 40' just prior to landing & a rather too well publised tail rotor control failure at 380' in the hover.

I can assure you thinking time was at a premium, ie there wasn't any.

By 'eck what I'd do to have one fail in the cruise! <img src="rolleyes.gif" border="0">

The wind issue is to do with the tail & the old weather cock effect, as previously mentioned by those far more astute (is that a word??) than I.

On the subject of simulators...they cannot show you exactly what will happen & perhaps aren't entirely realistic BUT they do place that seed of thought in your mind. I felt the sim time I've had helped alot. If nothing else it allows your brain to make the connection quicker.....'hmmm we're turning.....aaah.....b*gger'.

Cracking replies though....aaah we never stop learning! <img src="smile.gif" border="0">