S61 Accident

[Tynecastle]

Any info out there on the S61 accident in Kerteh, Malaysia last week.
Everyone being very tight lipped about it, rumours abound about a tail rotor problem, hopefully the authorities will be straight up about it, whenever an older machine is involved in an accident, then the other companies who operate them get a bit nervous.

[212man]

A source with close contacts (like he spoke to the captain) tells me it was a TR control problem, possibly a rigging problem.

[NorthSeaTiger]

Sad to hear about this accident, hear is a link to the relating article, good to hear everybody made it out ok.


http://thestar.com.my/news/story.asp?file=/2005/4/17/nation/10712988&sec=nation

[simfly]

Great news everyone survived! Something in the back of my mind says that if it does turn out to be a rigging problem, hasn't this happened a few times before with the S-61? Not through design fault of the helicopter, but incorrectly rigged T/R control cables during maintenance... There was a similar incident here in Aberdeen in the late 90's (G-BCBJ?) where the T/R went as the aircraft was on final approach, thankfully on return from offshore! The cables were the problem there.

[NorthSeaTiger]

Yes G-ATBJ Incorrectly routed cables. around the guide pins, I beleive the guides were changed to nylon (previously steel) so they would break instead of the cable.

[toolguy]

The S61 still has metal pins guarding the pulleys for the tail rotor cables. The S76 went to nylon pins. The S61 pulley fairlead is too wide for plastic and there is a risk that that if the misrouted cable wears through the pin, which it will do in about two seconds, it will fall off the pulley and jam and break. The controversy is really about misrouting, steel or nylon, does not matter, just do not misroute the cable. The S76 has a tough spot near the transmission with two pulleys making an 'S' shape cable bend, tough to get at, but easier to see. The S61 has two pulleys buried in the flight control 'closet' behind the aux servo and mixing unit. Very tough to see and reach, even with an inspection mirror. Many mechs will attach safety wire to the ends of an old cable when removing it, which aids installation of a new cable, but you still have to make sure the new cable is inside the pulley groove and not outside the pulley, trapped between the pulley and guard. The S76 has self centering tail rotor, where the S61 does not. Either way, a broken cable is bad news.

Bottom line, visually check to make sure cable is routed correctly, then run through pedals on the ground, making sure you get the right tail rotor response. Running the pedals back and forth twenty times will cut a misrouted cable, so it would not hurt to do this on the ground, checking for full travel.

[NickLappos]

Misrouting the cable is a hazard because the keeper pin can cut the cable very quickly, sometimes in a few minutes of flight, due to the sharp radius it makes for the cable. The fraying is almost immediate, and as wires are cut, the fraying rate increases very quickly.

For the S-76, an astute pilot can sense that the cable is not routed properly, because the yaw forces are higher when the cable is not riding on a smooth pulley. If the cable is misrouted, the moving the pedals will cause the collective to nod up and down, because the extra yaw forces finds an easier path back to the collective through the yaw to collective mixing. While a slight amount of this collective nod is normal, if it gets big, and the TR has recently been maintained, have an inspection done.

The 76 has more than a centering spring, it has a mechanism that allows almost full pedal range with a broken cable. The device centers the TR pitch if both cables are cut. A similar device is on Black Hawk and S-92, as well.

[Tynecastle]

I feel the safest way when changing cables on either aircraft is to remove all the keepers untill after the cables have been tensioned and the range of movement checks carried out, only way to go.

Nick,
Nowadays on the 76 I would imagine a lot of companies do the Pedals / Coll check on the first start up, even if a pulley bearing is starting to get ratchety, the pilot will probably notice it during this test.
TC.

[Nigel Osborn]

I believe the first time this cable problem occured in the 76 was in Thailand. Quite a few of us had reported that that particular 76 had stiff rudder pedals. Finally one night, as we flew all day & the engineers engineered all night, the cables were undone & reassembled. The base manager, who also happened to be a licensed engineer, did an early morning ground run, said they were still a bit stiff & that he would do the 0600 take off & then get the machine grounded for a more comprehansive check in daylight.
Tragically the cable gave way at 6000 ft & the 76 spun all the way down to the sea killing all 14 on board. Since then the steel pins were replaced with nylon (or whatever), inspection holes made so that you can see the cables & the centering spring fitted.
Having flown over 30 different 76s, not too many have identical pedal pressure, so having a slightly stiff pedal doesn't always raise alarm bells.:(