Cable chafing through a fairlead.

[foglights]

I cannot find any reference to allowable contact of a 1/16" 7X7 steel cable running through a vertical ring lightening hole on a composite fairlead designed to be installed around the lightening hole as protection for the cable.

I realize that in most situations the cable runs fore to aft with no contact to surrounding structures. In this situation the cable runs through 99% of its length with no contact on any structures it passes through or by. However, in the area in question the cable passes through and contacts the composite fairlead by 'resting lightly' on it. It is the only spot on the length of the cable where the composite block is installed which makes me believe its actually serving its purpose. But inspectors insist that no contact should be made anywhere.....but I am having trouble finding any reference on these inspection specifications.

 

[berkshire]

You are Military,
So I do not know if this will apply.

Part 23 of the civil code says: 23.685 Control system details.
(a) Each detail of each control system must be designed and installed to prevent jamming, chafing, and interference from cargo, passengers, loose objects, or the freezing of moisture.

(b) There must be means in the cockpit to prevent the entry of foreign objects into places where they would jam the system.

(c) There must be means to prevent the slapping of cables or tubes against other parts.

(d) Each element of the flight control system must have design features, or must be distinctively and permanently marked, to minimize the possibility of incorrect assembly that could result in malfunctioning of the control system.

[Doc. No. 4080, 29 FR 17955, Dec. 18, 1964, as amended by Amdt. 23–17, 41 FR 55464, Dec. 20, 1976]

On light aircraft in a situation such as yours it is most common to put a fairlead that completely encloses the cable, provided there is no change in direction. What does the aircraft manufacturer say about this?
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[thruthefence]

Is the cable tension adjusted to spec?

1/16"?

Sounds like a trim or autopilot cable.

 

[foglights]

Its for a helicopter...its for a tailwheel lock mechanism.
Yes, its adjusted to specs.

Thank you for your input. I believe the term chafing refers to objects not involved with the cable installation and as such, the contact with the fairlead is permissible.

 

[kontiki99]

Before I launched into something complicated on this, I'd go see how it looks on other similar aircraft.

Is this a new aircraft on the production line or a maintenence inspection issue?

If it's new, anything could be mis-aligned, mis drilled etc.

On an old aircraft maybe a pulley brackets bent or there's some bent structure somewhere.

 

[rb1957]

if you took the fairlead off, does the cable move ? (ie is the fairlead out-of-true with the cable run)

 

[WKTaylor]

Foglights...

First..

I suspect what You call mean by "... composite fairlead designed to be installed around the lightening hole as protection for the cable..." is actually composite 'catapillar grommet' M22529/2-X. This edging material is strictly intended for steady state contact and/or incidental tapping contact to prevent damage to the edges of pass-thru parts. Rubbing across or along it will dislodge the edging and/or will wear Your cable/edging until the structure and/or the cable is damaged.

NOTE.
A true failead is a different animal [see below].

Second

Your acft FCS was probably designed to meet MIL-F-9490D "FLIGHT CONTROL SYSTSMS - DESIGN, INSTALLATION AND TEST OF PILOTED AIRCRAFT, GENERAL SPECIFICATION FOR" [superseded by the current DoD spec for flight control system design SAE AS94900].

For a copy of MIL-F-9490D, go to...

https://assist.daps.dla.mil/quicksearch/

3.2.3.1.1 Control element routing. Within the restrictions and requirements contained elsewhere in this specification,al1 portions of signal transmission subsystems,including cables, push-pull rods, torque tubes, and electrical wiring shall be routed through the airplane in the most direct manner over
the shortest practical distances between points being connected. Protection from use as steps or handholds shall be provided.

3.2.3.1.2 System separation,protection, and clearance. Where redundant cable, pushrod, or electrical wiring are provided, they shall be separated as required to meet the invulnerability requirementsof 3.1.9. Advantage shall be taken Of the shielding afforded by heavy structural members, existing armor plate, or
other equipment for the protection of important components of the control systems. Clearance between f1ight control system components and structure or other components shal1 be provided as necessary to insure that no probable combination of temperature effects, air loads, structural deflections, vibrations, buildup
of manufacturing tolerances, or wear, can cause binding or jamming of any portion of the control system. In locally congested areas only, the following minimum clearances may be used after all adverse effects are accounted for:

a. l/8-inch between static elements except those within an LRU where closer clearances can be maintained or where contact cannot be detrimental.

b. l/8-inch between elements which move in relation.to each other and which are connected to or are guided by the same structural or equipment element(s) except those within an LRU where closer clearances can be maintained or where contact cannot be detrimental.

c. l/4-inch between elements which move in relation to each other and which are connected to or are guided by different str~ctural or equipment elements.

d. 1/2-inch between elements and aircraft structure and equipment to which the elements are not attached.

3.2.3.1.3 Fouling prevention. All elements of the flight control system shall be designed and suitably protected to resist jamming by foreign objects.
....
3.2.3.2.4.12 Fairleads and rubbing strips. Fairleads shal1 not cause any angular change greater than 3 degrees in the direction of the cable underall conditions including those due to structural deflections in flight. Fairleads shall be split to permit easy removal unless the size of the hole is sufficient to permit the cable with swage terminals to be threaded through.

This spec has lots of other good info You should be aware of.

OBVIOUSLY rubbing contact between a cable [wire-rope] and a sof element not intended for rubbing by a wire-rope is a set-up for damage.

Along with the inspection comments provided by the "other guys", suggest You check the pulley axis-bolts and bracket holes for wear/elongation/distortion.... and ensure that the pulley-cable alignment is within -9490 spec allowables, as follows...

3.2.3.2.4.6 Cable and pulley alignment. Fixed-mounted pulleys shal1 be aligned with their cables within 2 degrees as specified in AFSC Design Handbook DH 2-1, I DN 3B1, Subnote 1.1.3(1), Cable Pull. Where a control cable has an angular motion with respect to the plane of the pulleys, the maximum misalignment resulting from this motion must not exceed 2 degrees, and the cable shall not
contact the pulley (or quadrant) flange for the total cable travel.

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[graemew]

G'day Foglights and Wil
I'll add my little bit if I may.
Will you are quite correct in what you say and if Foglight looks in FAR27.685 he will see the following (extract)
(a) Each detail of each control system must be designed to prevent jamming, chafing, and interference from cargo, passengers, loose objects or the freezing of moisture.

(c) There must be means to prevent the slapping of cables or tubes against other parts.

(d) Cable systems must be designed as follows:
(2) The design of the cable systems must prevent any hazardous change in cable tension throughout the range of travel under any operating conditions and temperature variations.
(5) Pulleys must have close fitting guards to prevent the cables from being displaced or fouled.
(6) Pulleys must lie close enough to the plane passing through the cable to prevent the cable from rubbing against the pulley flange.
(7) No fairlead may cause a change in cable direction of more than 3°.
(10) There must be means for visual inspection at each fairlead, pulley, terminal, and turnbuckle.

There is an expectation that a genuine fairlead will fully capture the cable and that it may be used to effect a small change in direction (3°).
If Wil is correct and you are dealing with a piece of grommet around a hole then contact is not permitted in the static state and the clearances indicated in Wil's peresentation should be applied. If the clearance requirements cannot be met then you will need to:
1. Have a conformity check performed to verify that the installation is correct. You might like to witness that.
2. Reposition a cable fairlead to provide the necessary clearance (and adjust the drawing accordingly). Within the cert requirements of course.
3. Reposition a cable pulley to provide the necessary clearance (and adjust the drawing accordingly). Within the cert requirements of course.
4. Add a cable fairlead and adjust the drawing accordingly, and also verify that the IPB and any maintenance instructions adequately reflect the change in configuration.
5. If this is not the first one reworked, and this one was found to conform, you might consider a formal inspection of the previous ones to verify freedom from the problem. If they are free of the problem and the current one is free from nonconformities (except clearance) you might consider a conformity check of the previous aircraft. (sorry to get complicated, but logic demands it)
Remember:
"the sky is not a limit, it is a place to visit"
Regards
GraemeW

 

[thruthefence]

If this thing locks a swiveling tailwheel, is it covered under "flight controls"?

And while anything chaffing on an aircraft is to be avoided, this may be just as designed & certified.

 

[graemew]

G'day thruthefence,
The requirement in the FAR's is for Controls, and a system that locks/unlocks a tailwheel is a control.
If it was certified with a cable rubbing where it shouldn't, it points to an inadequate certification process.
The first person to touch it will be duty bound to fix it and to show the the final result is compliant.
Unfortunately this kind of thing happens more often in the real world than is generally recognised. The certification process is good, but far from perfect.
"the sky is not a limit, it is a place to visit"
Regards
GraemeW

 

[thruthefence]

It would be interesting to search the SDR's by aircraft, system, and part number, and see how many problems regarding this installation have been reported since the helicopters certification.

While not stated in the current thread, the poster has had other questions regarding the S-61N helicopter, which came into service in the late 1950's, (the -N model entering service in 1962) and served in the military as the SH3-A "Sea King". So
let's assume for the moment it is indeed the same aircraft.

"If it was certified with a cable rubbing where it shouldn't, it points to an inadequate certification process."

I find it extremely unlikely that this aircraft could be flying in commercial and military service for 50 some years, and chaffing of the tailwheel control at this particular fuselage station not come up as a oem service bulletin, or advisory as some sort.

"When in Texas, and you hear the sound of thundering hoofbeats, do not first think of Zebras or Wildebeests"

 

[foglights]

I would never classify a tailwheel lock control a flight control in the ordinary sense. Afterall, no need to control the lock while in flight.
And yes, I had the same feeling regarding the acceptability of the condition given the previous flight status of the aircraft, however I was having trouble finding mention of criteria or, rather parameters of such contact conditions, acceptable or otherwise.

Your links were greatly helpful.