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NWS keeps shearing towbar


Kal Quaife
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RNZAF C130H keeps shearing towbar pins. We found that the NWS steering indicator feed back drive chain in upper indicator dial had stiff links and stuck during travel 

and we thought that this was not allowing steering valve teeter totter arm to cycle whilst towing and causing a lock thus shearing towbar pins.

We lubed chain and it appeared to fix problem but now the shearing has reoccured.

Has anyone had this problem. We have not changed valve. It naturally does not do it with arm disconnected.

NWS rigging i.e cable tension correct and even and teeter totter arm correct. The NWS operates as required when on skid plates.

Cheers Kal

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The RNZAF at 40 SQN have changed two steering valves and still a fault. All linkages checked and within limits. Cables at correct tension for temp. Feedback chain in indicator below C/wheel had some stiff links which were lubed, fault still there. Nil binding in strut (grease plates used and NWS operated plus tow bar operated with scissors disconnected and nil binding. We did however change the felt seal PN 340340 that goes stits around steering rack tube that goes thru pressure skin. Therein it may be binding due to compression of upper cap tightening leading to momentary restriction whilst towing and causing the tension on cables to be different enough to actuate teeter totter. I will test control wheel torque and compare to other aircraft if possible. I will then take off top cap PN 340339 and lift seal up and recheck. Will let y'all know findings.

Meanwhile naturally recommended they tow aircraft with scissors disconnected.

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When you tow, you have to disconnect the NLG scissor.  The top and bottom are not linked.

Are you towing with the scissors connected?  You should never have any reason to even talk bout the steering control valve because it's not in play when the upper scissor is disconnected from the lower scissor.   

Im very confident when I say you might be towing incorrectly,

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I would like to think that I am proficient in towing aircraft. The C130 can be towed with scissors connected as long at turning limits are not exceeded (black lines on nose sides) and if required to exceed the scissors are disconnected. We at Woodbourne New Zealand do what ever is required. The towbar only shears when connected.

As stated prior I have pretty much nailed it to the felt/teflon seal at bottom of tiller ashaft.

Thanks for your reply.

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2 Page 2−3 l. Never tow the aircraft at an angle which will cause the main gear tyres to slide. Be careful when towing at sharp angles; tow very slowly. When towing from the nose gear, do not exceed the turning angle indicated by the black line painted on the fuselage, unless the upper and lower torque arms of the nose gear are disconnected and the distance between the upper and lower torque arm attach bolts does not exceed 26 inches.

Just a short excerpt from our Ground Handling manual NZAP6211.006-2-2. 

We tow i.e general flight line manoeuvring with scissors connected. I can see your point with having scissors disconnected at all times to stop numpties damaging aircraft.

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We had a hard rule that once a tow bar went on the scissors where disconnected, I'm told that early on the whole tow fitting was sheared off. I was not a Hydraulics guy so unfortunately I have nothing to offer as far as strut issues.

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I've always 100% disconnected scissors when towing as well.  There is zero gain by towing with them connected.

You might be hunting a self induced problem by towing with scissors connected.   If you really want to go down the rabbit hole though, I would 100% ensure your strut is serviced properly (meets and exceeds minimum X dimensions) and you can pull the flow reducers out of the steering actuators to ensure they are in good shape.  There are plugs on the aft side of the steering actuators, under that plug there is a flow reducer with a spring.  That limits how fast the actuators can move fluid through them.  The steering control valve doesn't do much when it's not being powered, the reduction in flow really resides in the actuator end caps.  You could also check the bearings on the forward steering actuator trunnions, those bearings are finicky and bind up quite often if not properly maintained and lubricated often.    Again, you are probably chasing a problem that is self induced though, no need to tow with scissors connected, it's all risk with no reward.

Chris

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Agree to all. I cannot find when the RNZAF ceased towing without disconnecting links. A bit of extra background to fault. Aircraft flew in for DLM and NLG removed.

Aircraft flew out same NLG and all its other paraphernalia the same except the felt pressure seal thru the NLG upper bulkhead where tiller arm goes thru.

This aircraft had no issues prior to DLM. The drawing we have for the seal 340340 is at rev M which say made from teflon per AMS3651 yet we recieved a felt seal (same as was removed). Naturally we went thru due approval process to get the felt seal installed as it differed from our drawing revision (personally I prefer felt).

As an aside our situation is unique as our hangars cannot take a Herc being towed in. Firstly we remove rudder and then vert stab then fully jack and remove wheels, brakes and torque struts and then lower MLG onto what we call the skateboard and NLG is left installed. 

The aircraft is now low enough for us to use a schoff to tow aircraft sideways into hangar. We have a smaller tractor on NLG. Small amout of coordination required to get aircraft centered and on correct spots for jacking. We then do the DLM and associated MLG etc functionals in hangar the we pull it all to bits to put on skateboard to get the thing outside only to redo the majority of functionals again i.e main and emergency functionals and brakes etc. A bit quicker as we have done timings and clearances in hangar prior.

Thought you would find this amusing.

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That is a lot of work to get it in the barn! One of my units we had to run about 150 feet of cable to the tow bar that ran through a block and tackle to get it into the barn, To pull it out we used a cable bridle attached to the main landing gear. There was a cut out above the door for the fin that closed with a garage type door on it.

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I do believe there was a Lockheed newsletter suggesting that the scissors should be disconnected for all towing.  You would have to do some research on that.  It takes all of 2 minutes to disconnect the scissors, don't understand why you don't.  I wonder what would happen if the tow bar was connected to the aircraft, but not the tug, ground test valve tied, aux pump turned on and nose wheel turned in one direction or the other, would the nose wheel steering center and some poor individual get knocked on his butt by the tow bar?

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  • 3 weeks later...

Never tow with scissors connected (beaten into us as apprentices). There is no requirement in towing job guide. However, there are several cautions and warnings to deplete system pressures, and ensure ground checkout valve is in normal (up) position before towing ....

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