The cable saga

4919
image: Civil Aviation Safety Authority
image: Civil Aviation Safety Authority

In the five years since a very near miss put the issue of flight control deterioration into clear focus, sixty-one unserviceable and potentially dangerous cables have been discovered on aircraft, and four cable failures.

It was to be a charity flight, giving a special needs child and their carer the chance to experience the wonder of aviation. Just before take-off, the pilot of the Beechcraft B33 Debonair did the final full-and-free movement control check.

The elevator controls felt odd, and the pilot decided to return and have them checked out. The inspection of the pilot’s uneasy hunch discovered the up elevator cable had failed where it ran between (over and under) two forward pulleys in the elevator control circuit, ahead of the instrument panel, where the cable changes direction from horizontal to vertical.

There was another Beechcraft, an A36 Bonanza, at the aerodrome, which was also due to do children’s joy flights that day. The owner of this aeroplane had its elevator cables inspected as soon as he heard of the B33’s failure. A quick blind feel behind the instrument panel about where the B33 cable had failed found an extensively frayed control cable in the same place on the A36.

Because the Beechcraft ‘single-pole’ control system has a downward spring load on the elevators to provide stability in cruise, had the cable broken in flight, the spring-loaded elevator would have immediately put the aircraft into a dive.

Soon after hearing of the incident CASA sent an email to Hawker Beechcraft, (now Textron), Beechcraft type clubs and major maintenance facilities. The email strongly suggested all flight control cables that have been installed for 15 years or more should be replaced and all other cables be inspected over their full run for wear and fatigue.

About the same time, CASA published Airworthiness Bulletin 27-001, issue 2, of October 2011. It applied to, ‘All aircraft flight control cable terminal fittings over 15 years old made from stainless steel specification SAE-AISI 303 Se, including, but not limited to, standard terminal part numbers AN669 and MS21260.’

The issue was internal failure of the stainless steel cable terminals.

The only solution for terminal failure of these types of cable is to replace the entire cable assembly. Internal corrosion in cable terminals cannot be detected by eye.

AWB 27-001 says: ‘An inspection for pitting on the terminal surface is not considered adequate to determine the extent of the intergranular corrosion that may exist beneath the surface of the terminal, because with this form of corrosion, in this material, the terminal may be close to failure and may even fail, with no visible pitting on the surface.’

The cables involved in the stainless steel terminal failures had been in place for 15 years or longer.

In January 2012 CASA issued a series of urgent airworthiness directives (AD/BEECH 33/48, AD/BEECH 35/74, and AD/BEECH 50/34) requiring owners of affected Beech types to immediately inspect the forward elevator cable and replace elevator cables over 15 years old. It also required continuing inspections of the entire elevator control system.

The issue was real: within days of the directive’s issue five more cables were found to be unserviceable.

After the directive, Beechcraft issued an improved inspection process and CASA amended its directives to remove the need for all cables to be replaced after 15 years in service.

Meanwhile, the parallel issue of cable terminal failures smouldered on.

In May 2012 CASA received a report of the control cable terminal separating in a Piper Cherokee Six. While turning from a holding position to the runway, during final control checks, the pilot reported remembering that the elevator control ‘felt unusual’ as he began the take-off run. Seconds later, he felt a change in elevator feel through the yoke and rejected the take-off. Investigation found one elevator cable terminal had separated. Only lock-wire kept the controls from complete failure. The aircraft was only 22 hours out of a 100-hourly inspection where the terminals had appeared to be quite serviceable.

CASA updated AWB 27-001 to Issue: 3 in June 2012:

‘Reports of flight control cable terminal fitting separation failures continue to be received in Australia, New Zealand and the United States,’ it said.

‘Investigations have revealed that the failed terminals had been in service for approximately 15 years or more.

Reports of terminal failures continued over the next several years and in February 2015 CASA published Airworthiness Directive AD/GENERAL/87.

It applies to any aircraft fitted with primary flight control cable assemblies using terminals constructed of SAE-AISI 303 Se or SAE-AISI 304 stainless steel, which have total time in service of, or exceeding, 15 years.

Its requirements are brief, and in the minds of some aircraft owners, brutal. Cable replacement is not cheap, it is true. But the alternative—failure in flight—is very likely to cause a crash, and has done so overseas. Indications suggest the problem of cable failure, through fraying or terminal separation is becoming acute. Of the 69 cable unserviceabilities recorded on the CASA Service Difficulty Report database, 61 have occurred since 2012, as have four of the 11 cable separations. The most recent involved a Cessna 150 in June 2016 which was undergoing its Supplementary Inspection Documents (SIDs) inspection. Its right aileron control cable terminal was found to be completely severed. Only lock wire held it together. The aircraft was 43 years old, but had a relatively low 3000 hours.

AD/GENERAL/87 requires control cables more than 15 years old, or of unverifiable age in an aircraft more than 15 years old, to be removed and then ‘mutilated in a manner that ensures the cable assembly can no longer be used in an aircraft.’

The compliance date for AD/GENERAL/87 is 1 January 2018. Cables more than 15 years old, or of unverifiable age must be replaced by then. It will be money well spent.

Share this article
Print this pageEmail this to someoneShare on Facebook0Tweet about this on TwitterShare on LinkedIn43

3 COMMENTS

  1. Respectfully, I request clarification of the defect reports regarding the failure of cable terminals.

    Do they state the type of safety wire wrapping involved? Your illustrations (in this article and previous) of failed terminals seem only to show the rather odd way of multiple coiled lock wire rather than the NAS safety clips.There are many ways of locking turn barrels that only allow for one pass of wire through the terminal ends and having the wrap at the bronze barrel.

    Can corrosion caused by dissimilar metals be a factor in these failures.

    Regarding the frayed cables, is there an indication of stainless or carbon steel being prevalent?

  2. G’day All aircraft owners and operators.
    As a LAME of 38yrs in GA and a short time in the airline environment, I have experienced a vast array of cable un serviceabilities. Firstly, the cable failures as mentioned on the A33 and A36 should have been detected a long time before they got to the stage they were in at or near failure. Lazy and cheap maintenance practices are to blame and this practice is partly due to the constant push for cheaper maintenance by owners and operators. Not a good thing.
    The type of safetying of the turn barrels is irrelevant. However I would note that if a MS21256-* safety clip had been used in the Piper example the cable would have completely separated. What is critical is the time in years of service not hrs in service when talking about the cable terminal ends. It is all dissimilar metal corrosion because of the different types of metals used to make the cables and the terminals. In the joint there is an exclusion of O2 which is critical in the corrosion resistance in Nickle or Corrosion resistant steels suck as Stainless Steel, this allows and aids in the dissimilar metal corrosion taking place from the inside out which cannot be inspected.
    With regard to type of cable material, Carbon steel cables wear less than stainless steel cables which are more susceptible to wearing out on pulleys and fair leads. Partenavia P68’s have an AD pertaining to stainless steel cables and terminating action is to install Carbon steel cables.
    It is also worth noting that internal wear which can and does occur as cables run around 90 deg and greater turns and multiple wraps is very often not detected until the cable bursts open due to a general lack of knowledge of the tell tail signs of internal cable wear. Also rusty carbon steel cables are very often overlooked in the push to keep the cost of maintenance down and keep the customer happy.
    A constant contradiction for the industry which is supposed to put safety first.
    For your own and others safety in your aircraft, the 15 yr limitation on cables is a very low price to pay and if your aircraft is like most out there, a Cessna, Piper or Beechcraft etc of the 1970’s vintage or older you probably have the original cables still in service. They were never designed to stay in service that long.
    And don’t ignore the trim cables, they can easily fray and jam your tab in a very un helpfull position.
    Don’t wait to the last minute, get those cables replaced asap.

  3. In the article you state:

    “Because the Beechcraft ‘single-pole’ control system has a downward spring load on the elevators to provide stability in cruise, had the cable broken in flight, the spring-loaded elevator would have immediately put the aircraft into a dive.”

    I found this a little difficult to understand and suggest you further explain and perhaps clarify. In my experience of around 1,000 hours of Bonanza flying I have found the following to be true.

    The spring load is effective in damping the elevator feel during normal operations and it’s effect can be seen clearly when the aircraft is static or, in my experience at a IAS of around 40kts or less. Once the aircraft speed exceeds this figure the airflow has sufficient force that the controls “fly”. That is if the control wheel is released the controls will take up a position relative to the IAS and the trim setting. If the aircraft is trimmed and a steady IAS is maintained the aircraft is positively stable in pitch and little if any control load is evident.

    My understanding of the purpose of the spring loading is to supply sufficient “feel” to the system to prevent over control. Your suggestion would imply that the spring loading is of such magnitude as to be able to overcome the aerodynamic loading of say an cruise IAS of 145kts. That is, to quote your article, should the cables fail in cruise say, then the spring loading is of sufficient power that it will overcome all aerodynamic force.

    Why is aerodynamic force at say 40kts. then sufficient on take off and landing to overpower the spring effect of holding the elevator in a full nose down position?

    Assuming for this example that the primary flight control cable or cables for the elevator failed completely at a particular IAS given a constant C of G would the elevator not still be able to be repositioned using the elevator trim system?

    Peter Dwyer hit the nail on the head when he said, “Lazy and cheap maintenance practices are to blame”.

    I would think the regulator should be alarmed at the failure of aircraft maintenance standards in Australia of the magnitude you disclose. What have the FAA, the regulator of the country of origin of these aircraft, had to say on the subject.

    Time,perhaps, to get away from the office and start observing the real world maintenance practices to see where things have been going wrong!

    Regards John Raby

Leave a Reply