Bell 206 BIII MAX NR failure

[spitfiremk9]

I have a question reg Max Nr on the 206 BIII.

What can happen to the rotor system if one would allow the NR to go beyond the limits
lets say up to 130%.
Have anybody done this, and came out of this situation. And what would be the damage to the machine if this would be aloud to occur?
Would shearing of the swashplate be one of the results, or separation of the blades , gearbox, drivetrain. It would be nice to have some expert opinions of this issue.

[Freewheel]

I know of somebody who did this and survived. Symptoms did not include separation of blades, gearboxes or drivetrain but he only went to 120% +- a few %.

I suggest taking a look at the maintenance manual for procedures to be followed in the event of a rotor system over speed. The components to be checked should give you guidance as to the likely failure points.

Please be seated when you open it up......:eek:


Edited to add, this was a governor runaway, not something that was "allowed" to happen.

[Ascend Charlie]

The stresses on the rotor head and blade attachment points, TT straps, would be enormous, and the overspeed also carries on to the tail rotor, the driveshaft and bearings, the N2 turbine, hydraulic pump, blah blah.

On a Robinson, any overspeed requires a "run-out" test where the T/R driveshaft is carefully measured with a micrometer to see if it has whipped around and gone out of limits. B206 has a lot more supporting bearings, but they still would have copped it.

Hope your mate put his hand up and confessed, and didn't just hope nobody would notice...

[OvertHawk]

If this question is being asked because you're aware that it's happened and are wondering about whether to report it...

Then please, PLEASE, do. If it's been oversped to anywhere near that degree then there is a huge chance that significant damage has occurred which could easily lead to a catastrophic failure in the future.

It might be you flying, it might be someone you care about, it might just be some other poor sod. It MUST be reported. Even if it costs you your job - otherwise it could easily cost someone their life!

OH

[Flying Bull]

OvertHawk is absolutely right.
And always remember forces rise with v square - so only "another few percent rpm" means a lot lot more stress within the T - bars and the whole system!!!!

[unstable load]

As an Engineer, I'd say even if you SUSPECT it happened you need to let the owner/Boss know what happened because the bits that got oversped were subjected to massive stresses that are not part of their normal operating regime and will require inspection and possible replacement.

[spitfiremk9]

Thanks for all reply`s

Just to make this clear. This is not anything I had part in, but something reportedly and most likely has happened to a machine in a company. Procedures where obviously followed, but only to the extent of and as far as the tech log is concerned. Nothing else where done accept maybe some inspections.
The result was a fatal accident , and two person`s lost their life, most likely because of this.
This helicopter accident happened back in 1992. And The main rotor of this helicopter was found some distance away from crash site, and looked like a complete unit, accept from some bends and damage on one of the blades, otherwise intact, and half of the mast with the pitch links where also on but had signes as they where twisted and had cut of and sheared of due to something that seems like a sheared swash plate.
The rest of the wreckage has signs of hitting the ground vertical on the nose.
No swash plate was ever found. The pilot was very experienced over 10 K hours. There was talk about mast bumping but, my source says this is not the case. Due to how the main rotor and mast with pitch links looked like. This aircraft had been trough a test flight earlier, and this is where things start to get strange.
The question is, can too high NR during an autorotation or governor failure up to 130 % NR on the 206 make damage to a swash plate, and make it shear?
again, expert opinions are welcome.

[longbox]

Firstly, if you suspect any over speed, or any number, forget 130% do not fly it. There are power on and power off limits for the 206 in the RFM. If the aircraft is set up correctly, the auto revs will be set as party of a maintenance function, if you find they are wrong and want to go too high in auto you can control this will lever, your ear will soon tell you before the gauge does if the RPM climbs. If it is power on then roll the throttle off (i.e governor failure) to control rpm. If you let it go to 130% power on, everything is up there including the engine, transmission, head, tail rotor and the remainder of the drive train. It is all scrap post incident and in theory any part could fail when those abnormal loads are applied not just the swashplate.

[FH1100 Pilot]

Spitfire, I see where you're going - trying to narrow the blame for the fatal accident on a swashplate that failed due to an overspeed. And maybe that did happen.

The Bell Maintenance Manual for the 206B does specify certain things that must be done when the main rotor goes above 114%. The gauge however only goes up to 120% so I doubt that anyone can say for sure what the value is once 120% is exceeded. And it doesn't really matter. Once you go above 114% you're "done." The mechanics get to take the machine apart.

But here's the deal: Above 114% the TT straps, fittings, pins and bolts all have to be scrapped. The blades must be inspected, and the damage tolerances are so strict (e.g. elongation of the bushing hole by .0015") that in any serious overspeed (like the one you describe) the blades would surely have to be replaced.

The manual then goes on to tell us about the tail rotor, and also the oil cooler both of which are susceptible to overspeed damage. But curiously it says nothing about the main transmission, swashplate or even tail rotor gearbox. (The engine is a different story and one must consult the appropriate engine manual for overspeed conditions/inspections.)

So the question becomes: Were these inspections properly complied with after the overspeed event?

You seem focused on the swashplate. The swashplate is "driven" by the drive link (that beefy hinged bracket that clamps to the bottom of the mast and connects to one side of the rotating swashplate). If the drive link were to fail then most certainly the aircraft would come apart. But, would the drive link become compromised in an overspeed? Bell does not seem too worried about it. Drive links *do* fail from time to time - perhaps this was the case. But there is no hourly inspection criteria nor life-limit for the link itself (at least, none that I could find in the 206 MM).

You mentioned that the pilot was very experienced. I know it sounds callous, but whenever I hear that, I think to myself, "So what?" Please do not think he was immune to pilot-error. High-time pilots crash too. I also have 10K+ hours, and I am just as capable of crashing a helicopter due to my own stupidity as the next guy. Nobody is immune from making mistakes.

The sad thing about helicopter crashes is that there is often too little wreckage left to make a definitive declaration of what caused the crash. It's even harder when you know the crew. So in this case, while the drive link/swashplate *could* have failed, it would be very hard to make that connection to the fatal accident without knowing exactly what was done to the ship after the first overspeed.

[spitfiremk9]

Thanks again for taking the time to answer me reg this issue.

FH1100 Pilot. That was one great answer and it basically sums it up.

You are right reg pilot experience.

Yes I am focusing on the swash plate, but the cause of the accident can be more extensive than that off course. Its just one persons opinion I am investigating.

It could also be that they did follow the MM and did all the inspections after, but my reliable source tells me they did not.