Flap Cable ACS

this is the broken cable. NOTE it is 1/8 (big cable)

[Kevin B]

FYIWed, Feb 26 2003

AD (Australia): Cessna
AWB 27-3 Issue 1, Cessna 336/337 Flap Cable Wear
Applicability: All Cessna 336/337 series aircraft.

Purpose: The purpose of this bulletin is to advise operators and maintenance organisations that excessive wear of the flap cable may exist in an area that is difficult to inspect during the periodic check of the cable in accordance with the maintenance manual. Excessive wear, corrosion or broken wires may result in failure of the flap cable, which can result in asymmetric flap deployment.

Background: This model aircraft has suffered a number of asymmetric flap extensions as the result of failed flap cables. The failure of the flap cable is caused by excessive wear at the inboard bellcrank, P/N 1462020-27. The wear location is at the tight radius bend on the bellcrank prior to the termination point of the cable. The cable wear is on the inside radius of the bend and is not noticeable without removal of the cable.

During one incident in the USA, the pilot lost control of the Cessna Model 336 on landing, but was able to recover control and successfully land the aircraft without sustaining damage or injuries. In a separate accident, again in the USA, the pilot of a Cessna Model 337 lost control of the aircraft on landing, after suspected asymmetric flap deployment caused by a failed flap cable.

In addition to the above two incidents, an incident occurred in Australia on 1 February 2003 where, on approach to land, the left flap completely retracted. Upon investigation it was found that the left flap operating cable P/N 1460100-7 had failed approx 1" from the left flap inboard bellcrank P/N 1462020-27. Further investigation revealed the right hand flap cable had broken internal strands. These were not visible until the cable was removed from the aircraft and carefully inspected. The cable has a severe bend by design at the inboard bellcrank location, which seems to exacerbate the wear and fatigue of the cable in this location.

The Model 336/337 Maintenance Manual requires a 100 hour inspection of the flap cable system. It is recommended that maintenance personnel treat the left and right hand inboard bellcranks (P/N 1462020-27) and all others in the flap system as "critical fatigue areas". A critical fatigue area is defined by FAA AC 43-13-1B as the working length of a cable where the cable runs over, under, or around a pulley, sleeve, or through a fair lead; or any section where the cable is flexed, rubbed, or worked in any manner. It also includes cable within 1 foot of a swaged fitting. As such, these areas require close visual inspection, which, in this case, can only be accomplished by complete removal of the flap cable so that hidden areas of the cable can be inspected for excessive wear.

In addition to the requirements of the Cessna 336/337 Maintenance Manual and the requirements of FAA AC 43-13-1B, operators maintaining the aircraft in accordance with CASA Schedule 5, must, according to Section 1 - The Airframe Part (2) (f);

"inspect the flight control system bellcranks, push pull rods, torque tubes, cables, fairleads, turnbarrels and pulleys".

A thorough inspection of the flap cable at the inboard bellcrank location will require complete removal of the cable in order to detect hidden damage.

Recommendations: Because of the criticality of the flap cables, and the difficulty of inspecting them in situ, the left and right flap cables should be removed and inspected for wear, broken wires and corrosion in accordance with FAA AC 43-13-1B, chapter 7, Section 8, paragraph 7-149(d) and the Maintenance Manual at the100 hourly or annual inspection, whichever first.
Reporting Action: If any excessive wear, corrosion or broken wires are detected, report it to CASA via a Major Defect Report (MDR) form (as required by CAR 52). In the MDR form, specify the exact location of the cable damage and include a sketch if possible showing the location of damage in relation to pulleys, bellcranks and other hardware. This additional information will allow CASA to assess the fleet-wide condition of the flap cable system and develop possible future action.

FMI: www.casa.gov.au/avreg/aircraft/AWB/27/003.htm

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[kevin]

I have been sick for a couple of days, and not monitoring the board. I just merged the three flap cable threads into one, to keep the information together.

I want to thank Kevin Swash for posting his pictures. Would it be alright if I posted those pictures on our tech data page Kevin?

I have some thoughts for you all on this subject.

I talked to several AI and A&P folks on this subject. The opinon was unanimous, and I promise I did not lead them on. Here is my best attempt to distill what I heard.

Bending these cables 90 degrees on the flap bellcrank (and other bellcranks) is done in many places, on 337s and other aircraft. 43-13 would seem to indicate that bending a 1/8" cable 90 degrees is not allowed, but the aircraft have been certified this way. All of the folks I talked to were sure that a very high percentage of cables that have been in service longer than 10 hours or so, and then are removed and bent in the opposite direction, will break as described in Kevin Swash's post and the FAA maintenance alert. Also, they believe that 100% of cables bent 90 degrees like this will not be serviceable for reinstallation once they are removed because they will be flattened excessively, and unserviceable per 43-13.

So, from what I learned talking to these folks, the following is my opinon, and my opinion only, offered for whatever it is worth to all of you. I am not a licensed aircraft mechanic. Here goes:

1.) If you remove the cable for inspection, you will probably end up replacing it. It will be unserviceable due to broken strands and/or flattening, even if it has been on the aircraft a small number of hours.

2.) If you do not remove it, you should inspect it carefully, using a tissue to detect broken strands. If you find even one broken strand in this area, you should R&R the cable.

3.) If an AD is issued relating to the problem, it should not be for removal and inspection of the cable, but rather to add a time change requirement for this cable. General opinion seems to be 5000 hours in service is a good maximum life. I have no expertise in that area, don't know what the number should be.

Comments?

Kevin (Mackenzie)

[kevin]

Also, to Kevin Swash, if you have time it would be nice if you could explain why you think Fred's cable is defective from the picture... I don't have an opinion either way, I am just curious to understand yours better.

Thanks,

Kevin Mackenzie

Kevin to Kevin!

Pics no problem, use them where you please.

Your comments and questions
Bad design yes I agree but it works and has for a long time.
With regard to looking at the cable in situ if it has broken strands you want to know about it. It seems that the heart strands go first and these are not able to be inspected in situ. The cable in my pic looked like Freds in the mirror. This aircraft was 8 hours out of annual by a company that looks aftere several 337's.
1/ Broken strands. Just loosen the turnbuckle and undo the nut on the ends and dont bend it 90 if you are of the opinion that the cable will suffer damage from opening it up. Wiping a tissue on the outside of the cable in situ will not reveal the wear on the inside or heart of the cable. Wipe the inside to be sure.
2/ Must be removed to inspect.
3/ Nothing lasts forever and 5000 would seem a realistic life at the moment. Lets see what the lower time airplanes look like before we fix a life.
Why do I think Freds is defective?
Well look at the cable in the same location as ours broke and you will see that the cable has unwound slightly. This I would be concerned is due to wear and broken strands against the bellcrank and or in the heart strands.
Best to be safe than sorry!
Hope this answers all your questions.
Best regards
Kev

[kevin]

Kevin,

Thanks for taking the time to reply. My trouble, as an aircraft owner, is that if you ask a question of three aircraft mechanics, you end up with five opinions. I took you comments back to the folks I had talked to before. Some opinions were "that sounds right", others were that once the cable has been bent 90 degrees and stressed, the strands on the inside of the cable will be work hardened, and will break just from the act of removing the cable. These folks felt that removing and reinstalling the cable, especially repetitively each year, would do more damage than good, and they stuck with their opinion that if you remove it, you should replace the cable.

BTW, for anyone that isn't aware, the cables are relatively inexpensive, a couple of hundred dollars, but rerigging the flaps on both sides is many hours (perhaps as much as 20) of labor on a Robertson equipped P337 like mine. I don't know what the labor might be on a standard 337.

The exact science of aircraft maintenance...

Kevin

[Wayne Pearce]

Hi Kevin, I'm in Adelaide
I also got the AWB and went straight to the LAME and asked how seriously he checked the cables during last annual I think I got the "all care no responsibility answer"
So I'm trying to get a better understanding of the situation with a few questions:
1. Your example is a pretty old serial number did the design or cable part number change during production
2. Did your cable crimp ferrule show any signs of corrosion
3. Did the cable or other components show any sign of corrosion
4. Is there any wear on the cable or components that can be contributed to multiple parts abrading each other
5. Are there any other visible signs prior to removing cables
6. Do you think this failure is related to
Atmospheric corrosion
Stress corrosion
Tensile stress
Mechanical stress ie. routing, or
fatigue

I suppose what I'm asking is this related to the earlier models, high cycle, corrosion or some other factor which will help me build a picture and maybe believe what I am being told by the maintenance guys


Regards ............. Wayne

Kevin
I am no way suggesting these be pulled every 100 or annual that would be over the top. Pull it once now, fix it if they are bad then go fly a few thousand hours then take a look again.
If the heart strands have workhardened to the point that the cable will break when bent 90 degrees its probably time to change it anyway.
Cables are even cheaper if you find a shop that can manufacture them rather than buying genuine. We manufacture our own to sample and they sell for around $700 Australian dollars for the set of 6. (thats approx US$420) If any one local wants a set I can make from my samples here to save downtime.
twenty hours of labour sounds a little high cables only took us about 16 hours for the six. We took the oportunity to pull the flaps, grease the rollers, remove the bellcranks and grease the bearings, grease the rod ends and remove corrosion on the bellcranks and re etch prime. Total labour was approx 38 hours.
Wayne
1/My parts book shows the same cables all the way through 337 production without change.
2 & 3/There were no signs of corrosion in the cable or ferrule.
4/ No wear due to abrading, I believe it to be internal wear in the cable due to the angle.
5/ Slight visible signs in situ but very difficult to detect.
6/ Tensile stress perhaps. But good old fatigue definatly. At 7504 hours with an average flight time of 0.7 that would be roughly 22,000 cycles of the flap cable
Call me if you want 07 3275 3210 at work or 0419 182 206.

As I mentioned to a customer yeterday his 70 model plane with 7,300 hours at approx 150 knots has covered 1,400,000 miles. These planes are not getting any younger and need TLC!!

Reagrds
KS

[Don Nieser]

If anyone is interested, I have new stainless steel flap cables P/N 1460100-307 and P/N 1460100-308 and the retun cable P/N 1460100-334. Also, I have most other cables for 337 and 0-2 aircraft. My prices are lower than any others.
Don Nieser
Commodore Aerospace Corp
6221 Commodore Lane
Oklahoma City OK 73162
405-722-4079 phone and FAX
405-503-4686 hangar cell phone
nieser.02.337parts@juno.com

[Bob Cook]

Just went thru an annual. Cables were removed and inspected. Both in Good shape. Total hours 2725. "P" model.

They were reinstalled and we are ordering two new one to be replaced at the next chance.

Bob

[Don Nieser]

I have new Cessna stainless steel flap cables:
P/N 1460100-307 LH $60.00 each
P/N 1460100-308 RH $65.00 each
P/N 1460100-334 Return $65.00 each
Don Nieser
Commodore Aerospace Corp
6221 Commodore lane
Oklahoma City OK 73162
405-722-4079 phone and FAX
405-503-4686 hangar cell phone
nieser.02.337parts@juno.com

[Francisco]

This reply goes to the board afther looking at the pictures Fred-E the rods for the flaps on my 336 have also been damaged by screws the one on the left wing has a grove of about 1 inch and a pin hole which has gone all the way into the hallow tub.

Should I Replace them before I fly againg?

Thanks


Francisco

[WebMaster]

Reminds me of a movie about ATC.... Land a million planes safely, no one says a word. One little mid-air, and they're all over you.

We hope that the information is valid, but based on things that have happened, and the information that Keven has presented, I think his observations are 100% accurate.
Thanks

[Kevin McDole]

Tin Man was a movie about a deaf guy who creates a computer to hear & speak for him (Timothy Bottoms).

Tin Men was about aluminum siding salesman, Danny Devito & Richard Dreyfus.

Pushing Tin was about competitive air traffic controllers, John Cusack & Billy Bob Thornton.

My plane's been in the shop too much. Is it obvious that I have too much time on my hands?

[WebMaster]

I knew there was Tin in there, and have seen all of them.
Thanks, it was Pushing Tin, which was actually quite humorous. Especially the scene when they stand on the ruway with a 747 passing overhead.