B206 Free Wheel unit failure

[PerAsperaAdAstra]

Recently re-read a Canadian ATSB investigation about a year old, on a B206 Free Wheel fail, during a 180 auto, on opening throttle to full, the F/W did not engage, the RRPM decayed in the bottom flare despite full throttle. The F/W then did engage suddenly, but the slowed rotor caused the engagement with the full RPM engine, to be so violent it sheared off the rotor mast just below the main rotor hub, like a sort of torque overload induced mast bump...same effect, whole rotor blades and hub break away! The crew were close to landing in the flare, so very fortunately walked away! Probably on pretty wobbly legs! I heard another one of these occurred recently? Finland? The F/W unit was found to be corroded internally by water contamination, exactly how, was not certain. But: To me, a significant factor was that a restrictor orifice in the MGB oil system blocked up with debris at some earlier stage, due the corrosion contamination apparently, preventing F/W lubrication. I feel this is arguably the straw which broke the camels back. The interesting thing is this. Bell Helicopters put out a NON MANDATORY modification, to replace the restrictor orifice with a small filter which would still catch any debris, but still allow oil through to lubricate the F/W. Seems to me, that's a pretty essential modification?? Now I need to try find out if the two B206 BIII's we use for our training still have the restrictor fitted, or the filter ....Autorotation's anyone? I have to fly some with students tomorrow! Comments?

[Falcon Al]

The restrictor worked for a long time without any problems, maybe some other external influence was the issue; ie how regular maintenance was performed?
Provided your 206 is 'maintained' I dont think you will ever have an issue.

[vaqueroaero]

If I remember correctly it had been left sitting for months without being flown which contributed to the corrosion issue.
If your machines are flown regularly I wouldn't be overly concerned.

[Saint Jack]

I must agree with 'Falcon Al', the freewheel unit on the 206 series helicopters is one of the most reliable with only a (very) occasional forward oil seal change required. For example, in all my years maintaining JetRangers and Longrangers I have never experienced a freewheel chip light illuminating. Provided the initial shimming is done correctly these units will generally go to their TBO without any fuss.


However, I disagree (slightly) with the comment by 'vaqueroaero' i.e. "..If your machines are flown regularly I wouldn't be overly concerned..." Generally there is no need to fly a helicopter regularly to keep it in good shape. A ground-run every 7-10 days will usually suffice - during the run, which should include a minimum 5 minute period at 100% Nr, turn on ALL electrical and avionic systems and to the maximum extent possible ensure they perform properly.

[Ascend Charlie]

In flying the 206 since 1982, I have never seen a freewheel chip detector that is wired to the system. There is not even a checklist item on the preflight that says "Pull it out and look at it", which is an indication of how rare it would be to have anything go wrong with it.

However, I would ALWAYS pull the plug and wipe it on a white cloth and look at the residue - very rarely would there even be a whisker, usually just a grey sludge.

[Saint Jack]

Early model JetRangers did have a chip detector in the freewheel unit housing that was wired to the system. If this chip detector activated, it illuminated the 'TRANS CHIP' light in the cockpit and it was then up to the mechanic/engineer to determine by the process of elimination which specific chip detector was shorted (mast bearing, sump, oil monitor or freewheel). Later model JetRangers retained the freewheel chip detector but it was not wired into the system - possibly a result of the historical reliability of the freewheel unit.


The practice of pilots removing, examining, cleaning and replacing magnetic plugs has long been somewhat contentious, should they or shouldn't they. As 'Ascend Charlie' says, it's not a checklist item but how can you deter a safety-conscious pilot from doing it? Those against it say it isn't necessary and the practice carries the risk of a pilot incorrectly replacing the magnetic plug resulting in a possible oil loss (yes, I know they're supposed to be self-sealing) or the magnetic tip positioned where it cannot pick-up any debris.


There is one point on which all mechanics/engineers (should) agree and that it is the highly undesirable practice of pilots cleaning magnetic plugs without notifying maintenance. It must be remembered that in addition to catching the chips/flakes/slivers of a rapid component failure, the chip detector also monitors the gradual degradation of a component by the steady build-up of sludge/paste over an extended period of time. All conscientious mechanics/engineers keep a mental note of this. Therefore it follows that if anyone is cleaning the magnetic plug on every pre-flight then this very important information is lost. So if you feel you must look at it then go ahead, but please don't clean it, and ensure you put it back correctly.

[ericferret]

Saint Jacks post brought back a memory and a smile.

Me, What are you doing?

Pilot, Cleaning the chip detector (engine).

Me, Why?

Pilot, Well if we don't clean it every day the warning light comes on.

Me, How long has this been going on for?

Pilot, A couple of weeks.

Me, !!!!!!!!!!!!!!!!!!!!!

Some investigation and an engine change followed.

[fokkerpilot]

I just had a transmission chip light in a B206.
Found debries in the chip detector for the freewheel.

I have been told a bearing failure with several parts damaged including the shaft.

[vaqueroaero]

Here's mine from last year.......we noticed that on shutdown the Nr and Np were stuck together all the way to zero. Always look for a needle split when shutting the engine off.

[S92mech]

I've seen the same type of failure that Vaqueroaero's photo shows. The pilot reported the same, needles matched and noise on coast down. The 206B aircraft I maintained did not have a freewheel chip detector wired into the trans chip light, but 206L aircraft do.

[Falcon Al]

As PerAsperaAdAstra has not followed up on this thread, I can only assume the last 3 posters were on the right track, although there have been no reports in the news.

[normalbloke]

When I was a lowly pipeline observer, the stick wagglers I used to fly with always used to roll the throttle back to ground idle to check for a needle split before coming back to 100% again. Is this a mandatory part of the preflight on a Jetranger or something these guys chose to do just out of good practice? Were these checks done before the flights where the above documented failures have occurred? Would it have picked up on it or has the failure occurred between the needle split check and the incident?
Scary stuff indeed!

[vaqueroaero]

It's not mandatory practice to roll the throttle back, I would guess they chose to do it and it's not something that I do. However what I do make a practice of on preflight and post flight is to turn the blades backwards and listen very carefully to what's going on.
When mine failed it only had about 1500 hours on it. It's a 3000 hour component. It was sent off to Bell who determined that the failure was caused by the fact that it is in a 206 used for spraying and that it was not designed for the number of takeoffs and landings that we were subjecting it to.
I'll let others decide whether they agree with that or not. We had to buy the new one at full price.........

[mdovey]

Checking the needles split seems to be a standard item on engine start-up checklists for R22, 300CB and G2. How come this isn't standard on a 206?

Given, the three types above are pistons - is it some distinction between piston and turbine?

thanks,

Matthew

[Ascend Charlie]

I think if you read the B206 RFM it will be there. All checks done at full noise, roll back to idle, check the time for decel, should be around 4-5 sec. If too slow, the freewheel hasn't disengaged, and the rotor is holding it up there.

In pistons, the split is far more observable, the engine drops from 2300 to 800 RPM in a second, while the rotor, with some inertia, holds its revs for longer. But a turbine takes 4-5 sec to go from 58,000 to 40,000 RPM.

[whoknows idont]

The ones I flew had it in their engine shutdown checklist (roll throttle to idle, check engine deceleration time, 100% to 65% N1 should take 3-5s). While the pistons have it as part of the start-up checklist.
Now it does seem like a good idea to do it before the flight as well...

[FH1100 Pilot]

Not to be a tw*at, but...


A pilot in a 206B who rolls the throttle off before takeoff to “check the freewheeling unit” is showing evidence of a poor or nonexistent aircraft checkout. That’s not how it’s done in turbines. The RFM does not require it. And depending on the particular 206, it may either tell you nothing or give you a false indication that your freewheeling unit is not working properly.


All helicopters with reciprocating engines have clutches of some sort. With the engine off the clutch is disengaged. Turning the rotor does nothing to the engine.

In a helicopter powered by a free-turbine engine such as the 206 there is no clutch. The freewheeling unit is all you have. Thus, with the engine off turning the rotor backwards turns the engine. If the engine does not turn when turning the rotor forward you have just proven that the FWU is working properly. Doing this when you take the MR tiedown off is the best way.


So move the rotor backward, and check that the turbine wheels are turning but not rubbing (or locked up). Turn the rotor forward again and make sure the FWU disengages and the engine does not turn. Pretty simple. Get in and go flying.


The thing we do at the end of the flight/day is called a “Decel Check” and has nothing to do with the freewheeling unit. It is merely a check of the scheduling of the Fuel Control Unit (FCU). And it’s weird because we do it from full throttle (100% N2) and count down to IDLE N1. The time cannot be less than X (I don’t remember the exact figure; at PHI I think we used two seconds). All engines were different; some came down to idle quickly - others took their sweet time. RR-Allison gave no *maximum* time for the decel.

[vfr440]

F'wheel

That's EXACTLY the way we used to do it, M'East, Africa, Canada etc. Pretty much spot-on I'd say - VFR

[mdovey]

So, if I understand Ascend Charlie and FH1100 Pilot correctly, the splitting needles approach used in pistons, doesn't work for turbines (or at least doesn't give a reliable indication) due to the slower RPM decay of a turbine engine.

The way to check the freewheel unit on a turbine is to rotate the blades backwards (engine off - obviously!) which should rotate the engine, and then forward which shouldn't rotate the engine. This approach can't be used in pistons because they have a clutch which would have disengaged the engine at this point.

It'll be a while before I'll need to know this in practice but it is good to know.

thanks,

Matthew

[PerAsperaAdAstra]

Sorry for late follow up as such, some interesting comments, much appreciated. In the company aircraft we lease, I've had a "F/W Chip" light come on. Landed and student used his all singing and dancing "I" phone thingimy (I'm a total pre computer ager, when I joined the Air Force the most advanced piece of office kit at 17 Sqn was a hand cranked copier...) to send a photo to the blackhanders of some very fine metal schwarf on the F/W unit magnetic plug. Was told looks ok, good to continue. If it came on again however within 30 min or so, would have insisted they come and have a look. Yes the incident aircraft discussed earlier had stood for a while, but it had flown a number of auto's before the incident, and somehow water had contaminated the free wheel unit possibly through condensation (Canada - winter). I agree aircraft procedure does not call for throttle close check N2 split, but when I shut down, I do always snap the throttle closed to idle to watch the rate of N1 decay, but again that is to check for turbine rub. Moving the rotor during pre flight is good, always do it to check for any strange noises from engine (t-rub) and transmission and drives, but not sure about watching the power N2 turbines...as far as I know you can't see them, even looking down the exhausts? I still think the sieve is a better option then the restrictor in the F/W oil system. Basically it is a bit worrying that if it does go wrong, it is pretty catastrophic. But then again, that is the nature of the beast, I was discussing loss of the tail rotor emergency procedures with students, but didn't have the heart to tell them that the loss or partial loss/failure of a T/R blade would probably cause such bad imbalance that it would probably throw the tail rotor gearbox, leading to a massive CofG forward shift, making the loss of tail rotor the least of your problems...I think I will go off to the fridge to get a beer.