Bell 412 free-wheel unit fail

[alouette]

Just learned a new thing today which is not in the books i.e. flight manual... If the free-wheel unit on one engine fails then the first indication is a C-Box chip light, followed by a torque split and subsequent drop in NR... The scary thing on all this is that the NR will drop into the basement with no chance to recover from that. Thus, the ship will drop out of the sky like a rock if one waits too long to put the aircraft on the ground. :}

[hillberg]

Yep, 412 has too much added parts,Heavy :rolleyes: Get a 212SP better OEI :D

[GoodGrief]

You did the right thing or you wouldn't be able to tell the story.Aircraft or sim?

[Aesir]

?? Idle the failed power unit and fly home.

[alouette]

we did this procedure in the sim. But still scary enough if you wait too long, because that NR goes flat out bottom. As said, the procedure is not in the AFML.:}

[alouette]

@Aesir; you cannot idle the failed power unit as it drags down everything with it. it slows down the second engine, and subsequently slows the main rotor speed. If that happens you have to get your soggy bottom on the ground asap!

[Aesir]

.. are you talking about a binding free wheel unit?

I´m just trying to understand the problem. Not getting it yet :confused:

[Jet Ranger]

Me too...Alouette, can you give us a few more details?

Do you mean on such a situation - that in a case of OEI, free wheel unit stays blocked together with the bad engine (doesn´t make it free) ... and pull down NR together with the good engine ?

Hmm...possible...sounds not good...

JR

[Encyclo]

Sorry Hillberg,

What we are talking about here (Cbox clutch) is the same on 212/412, so no advantage on the 212 :8

As for the failure mode of a sprag clutch, it is non-engagement, not lock-up. If you have a clutch malfunction (don't like the word fail :=) it will allow the Pt to go past the rotor RPM and other power section and you will get a weird split. At this point, the worst that can happen is sudden engagement which could 'snap' something... not a good situation :eek:

Good news is that this is an extremely rare occurence and you're probably better off practicing stuff that is likely to happen.

[Aesir]

I´m still unclear about the problem.

However a clutch slip can happen on any helicopter type although extremely unlikely. I´ts important to shutdown or idle the affected engine immediately to avoid sudden reengagement.

http://www.tsb.gc.ca/eng/rapports-reports/aviation/1997/a97p0044/figure1.gif

On the 212/412 it´is always possible to enter autorotation and land as is in any helicopter with failed sprague clutch or decrease power demand so the other engine will keep up (which the 212 will do easily ;) )

Alouette, where you riding along in the sim?

[hillberg]

Screw the sim .A 212 is a lighter airframe, 212SP will bearly fly on one engine at high DA, Sprag slips and the affected power plant goes high as the Nr droops, Lower the collective & secure the engine, OEI Temp & torque limits apply. A 412 ? More installed trash.

[212man]

Sounds more like the scenario being highlighted is a seizure within the C-Box, which will really spoil your day. In the early 212s there was still the original freewheel at the MGB input from the 205, which would have handled this failure, but they were removed........

[alouette]

What happens is, the affected engine (broken free-wheel unit) will drag the good engine and the NR down. As mentioned, the first indication is a C-Box light, and then one has to monitor the torque and NR closely. If the torque split occurs then a rotor rpm drop will follow. From then on, the bad things will develop very fast.

One has to put the aircraft down immediately - not as soon as possible. We practiced this, and if one waits too long the aircraft comes out of the sky like a rock because the NR will continue to drop irregardless if the bad engine is shut down or not.

And yes, the C-Box will seize up as well!

[212man]

yes, the C-Box will seize up as well!

Exactly - that's what is dragging down the Nr and the good engine, it's trying to drive a box of seized gears. I'd be wary of believing the simulation too much, if you really do get big chunks of metal breaking up and jamming the gears, things may well stop pretty quickly.

[Brian Abraham]

What do you mean by the "free wheel unit fails"? What mode of failure are you referring to? Seized bearings?

[Jet Ranger]

Has that ever happened on 212/412 in the real situation?

JR

[alouette]

It has happened on a 212 some time ago. And yes, the free wheel unit bearings broke up.

The accident was fatal.

[flyer43]

Back in the mid eighties, shortly after Bell decided that the C-box was so reliable that they could dispense with the freewheel unit between it and the MGB to save weight and cost etc, I happened to be on a Bell 212 in Trinidad which was fitted with the new C-box unit.
Shortly after departure from base with an underslung spray bucket fitted to disperse an oil spill close to the shore, we experienced a double indication of C-box oil pressure failure!
Luckily, we were only a short distance from land as, in the few minutes it took to return and land, the C-box oil temperature was rocketing skywards!
As pointed out already by 212man, things could have become rather hairy had we been further from a suitable landing spot, other than the sea of course!!

[Brian Abraham]

It has happened on a 212 some time ago. And yes, the free wheel unit bearings broke up.

The accident was fatal. Could you point us to the accident report?