Monday, May 1, 2006
Aircraft Flight Control Cable Systems - Maintenance Work with Zero Tolerance for Error
Maintenance work on flight control systems involving inspection, measurement, replacement and/or adjusting flexible cables is challenging in the least due to the physical and skill difficulties involved, besides the grave safety consequence of any errors.
Probably the most-feared situation a pilot can possibly imagine would be the traumatic, eye-popping event of a disconnected, cross-connected, mis-rigged or jammed flight control system.
All of these potentially tragic and often fatal happenings have and continue to occur regularly in aviation through human performance error, inadequate maintenance manuals, lack of experience/training/supervision, or poor hardware/system design. But the most regrettable of all reasons is the sometimes collective complacency of maintenance management and the workforce in not giving the highest priority to the details of flight control systems maintenance work.
Aircraft systems activated or controlled by cables, in particular flight control surfaces, have been used since the very early days of aviation. Typically, a modern flight control system input starts in the cockpit with some kind of action of a pilot. Then the movement is transferred through various combinations of linkage, i.e. arms, rods, tubes, horns, cranks, quadrants, pulleys, fairleads, temperature compensators, dampers, equalizers, adjustable stops, jackscrew drums and multiple lengths of cable finally ending with a terminal connection to a control surface. These terminal connections are further made up of a combination of sockets, balls, clevises, pins, keys, turnbuckles, and different ways of safetying.
Thorough inspection of cables can be tedious but it is essential, as corrosion can be hidden at the inside crevasses between strands. Wearing to the cables is more obvious and usually shows up on the outside surfaces of the cable.
And if all this wasn't already a confusing assemblage, no two manufacturers design their system the same way or always use the same terms for like components or parts. Aileron, elevator and rudder surfaces are the most common of aircraft flight systems actuated or trimmed by the use of control cables.
Powerplant controls, landing gear emergency systems and door activating systems also make use of control cables.
Control cables 101
Control cables are made up of several single corrosion-resistant steel wires wound around a single wire thus forming a strand. Then several strands are wound around a single strand.
Control cable nomenclature is designated first by the outside diameter in inches, then the number of strands, and lastly by the number of wires in each strand. For example, a cable identified as 3/8 7 x 19 would be a cable 3/8 inches in diameter, of 7 strands, and 19 wires in each strand. Inspecting cables for wear and corrosion is a boring and slow task of passing sections of a few inches of cable between a worker's hands and at the same time twisting the cable to expose all the surfaces of the strands. Wear, if present, will usually show up on the outside surfaces but corrosion can be hidden at the inside crevasses between the strands.
Simple cable control systems used in general aviation or older aircraft would involve only a few of the earlier mentioned components. Currently designed general aviation, corporate or commuter aircraft still use cable systems whereas modern transport category aircraft favor more electrically and hydraulically actuated systems.
Since a single control cable can only transmit a tension (pulling) force in one direction, it has to have a "follow up cable" in the loop that allows a counter pulling in the opposite direction, therefore completing the loop circuit.
Aircraft manufacturers' maintenance manuals present descriptions of flight control systems, including diagrams and sometimes pictorial representations of the various components and their locations. At best, for more complicated systems, this gives an airframe mechanic/technician a rough idea of how the system works.
Harder to explain or show is the task of adjusting or replacing a control cable itself for reasons of, say, wear or corrosion or when necessitated by replacement or trouble shooting of another component in the control system.
Manufactures' instructions regarding maintaining these systems often fall short of doing an adequate job. Federal Aviation Administration (FAA) Advisory Circulars (AC) 65-9A, "A & P Mechanics General Handbook," and AC- 65-15A, "A & P Mechanics Airframe Handbook," and the Standard Aircraft Handbook, TAB Aero, provide good basic information but not information specific to any particular aircraft.
Since the cable portion of the control system must be kept at a certain tension and compensated for the effects of temperature change or airframe movements, generally any disruption or breaking into of the system requires retesting and/or readjusting the tension. The various designated locations in the system for measuring or adjusting the tension can vary according to where the system had been disturbed.
When measuring the tension or adjusting the cable system tension the ambient temperature must be factored into the process. Manufacturers' maintenance manuals often will provide charts or graphs for making required conversions.
The manuals don't usually tell how to use a tension meter, where to place it in the length of cable to be measured/tested, or what standards to use for adjusting or safetying turnbuckles or other hardware devises in the system. This information would have to be found in various industry sources or civil aviation authority guidance materials mentioned earlier. In some cases, after adjusting the cable tension, the entire system may have to be rigged in order to reestablish the systems integrity.
Much, if not all, of the actual maintenance work is done in limited quarters, usually involving a single worker lying on his back or physically contorted in a narrow wing or empennage space with airframe structural members trying to penetrate his body.
Glaring floodlight or flashlight illumination/reflections and changing shadows adds to the conflicting mirage of trying to visualize what view one is suppose to be seeing in orienting which way to perform a task most likely never done before by the worker. The opportunity for human performance error is no greater nor devastating than when essentially working alone, in an inhospitable place, doing an unfamiliar job, and involving a directly related safety of flight maintenance task--all common to flight control system cable work.
The chance for human maintenance error is further heightened by foreign object damage (FOD) occurring while performing flight control system work. Besides tools, parts or hardware being left or lost in the work areas, even discarded pieces of safety wire, cotter pins, etc., could possibly jam the movement or operation of a flight control circuit.
Performing this kind of work on the maintenance line or at an airport passenger/cargo terminal is rarely attempted because, in reality, it takes personnel who have previously accomplished the specific job. Otherwise, it turns out to be a learning experience for those involved and would probably result in a delayed or canceled flight.
Airlines normally would only send out experienced personnel from the main base to accomplish this kind of work. No doubt, this work would also be classified by an airline as a required inspection item (RII) or double inspection and necessitate a second inspection (it is inherent and understood that the mechanic doing the work inspects his own work first) of the work accomplished by a qualified RII inspector or senior qualified licensed worker.
If performed outside the main base without their assistance, this work very likely could be a case of neither the worker nor the RII inspector or licensed worker ever having performed the actual job before.
The writer has a special insight and appreciation of the subject, having early in his career worked as a certificated lead airframe & powerplant mechanic of an airline "hangar hot shot" team assigned to flight control system work. No matter how much time was spent in the manuals, it took at least a few times actually doing a particular job under close supervision before one was confident of doing it alone.
Back then, a foreman would never assign a flight control job involving cable work unless the mechanic selected was known to be competent at completing the task correctly. Too much was at stake, i.e. safety of flight and a potential flight delay coming out of the hangar.
Likewise, savvy mechanics knew the potential exposure to fouling up doing such work was very high; besides, it was a demanding, tough job and most mechanics would avoid being assigned this kind of work at all costs.
So usually a shift foreman or manager was smart enough to single out only those mechanics who liked challenges, could `cut the mustard,' and expected small favors slipped to them as part of the deal. In those days, upper management kept close track of and favored supervisors who could turn out the work with the minimum number of aircraft delays/cancellations. These recollections are all part of the reality of how things on the hangar floor work.
Not that shoddy work or maintenance error can ever be excused or fully mitigated in aviation, but when it comes to the area of flight control system work, the potential for catastrophic results exceeds all other aircraft systems combined. We just have to think back to Greek mythology and consider the story of the first fatal aerial accident when Icarus' wing feathers separated during flight.
Any malfunction of a "flight control" results in the loss of "control of flight"; it's as simple as that reversal of word order. In these situations, there is no other place for a bird of flight to go but down in a crash with the feathers of forgetfulness or complacency trailing behind.
Bart Crotty is an airworthiness/maintenance/safety consultant and the maintenance human factors chairman for the International Society of Air Safety Investigators. He is a former FAA airworthiness inspector, trainer and designated airworthiness representative. Crotty has worked for repair stations, airlines, aircraft manufacturers, safety organizations and several foreign civil aviation authorities. His career spans over 40 years. He has an FAA airframe and powerplant mechanic certificate and a bachelor of science degree in aeronautical engineering.
Mis-Rigged Control Cables: A Pernicious Problem
From a Transportation Safety Board (TSB) of Canada report of a takeoff incident and immediate return to field involving a Convair 340/580 freighter:
"The task of hooking up the control cables [for the elevator] is, in itself, very basic. There are only two cables and it does not require training to expert levels to understand the system and to recognize that the consequences of hooking the cables up backwards can be disastrous. This analysis will focus on how five aircraft maintenance professionals (with different levels of experience) worked on this system and allowed the aircraft to be dispatched with the elevator trim operating backwards. ...
"First, the crew had been working the night shift and ... they had experienced five changes in sleep patterns in the past five weeks, from daytime to nighttime.
"This was an older generation aircraft for which the company had not yet developed a complete set of work or task cards. ...
"Thus, a significant amount of interpretation is required. When he connected the control cables, the crew chief had no objective cues to help him decide which cable ends should be attached together. ...
"The apprentices who carried out much of the work in the installation had never accomplished this task before, a fact that the crew chief was not aware of. ...
"The crew chief, who was required to supervise and train the apprentices, had himself never received supervisory training. ...
"The CARs [Canadian Aviation Regulations] require that two individuals inspect the control system for correct assembly, locking, and sense of operation, and that both individuals record their signatures in the technical record. There is no requirement that either individual be independent of the work being done. The AME who accomplished the `independent inspection' did not notice that the controls were incorrectly connected. ... Each AME simply signed one of the two blanks in the logbook, regardless of who had done the work and who had inspected the work." Source: www.tsb.gc.ca/en/reports/air/1997/a97o0077/a97o0077.asp
From a Transport Canada airworthiness notice of 10 Oct. 2001 dealing with inspection of control systems:
"Despite widespread educational efforts and the introduction of special inspection requirements, incidents of faulty control rigging continue to occur." Source: www.tc.gc.ca/civilaviation/maintenance/AARPC/Ans/C010.htm
Regarding the crash on takeoff Jan. 8, 2003, of a Beech 1900D operated by Air Midwest:
"In the chain of events leading to the fatal crash ... the pilots' inability to prevent the nose from pitching up excessively into a stall was the result of improper maintenance performed on the elevator two days before. ...
"Poor maintenance came under intense scrutiny at the NTSB's [National Transportation Safety Board] final hearing ... If the maintenance manual, since revised extensively, had been available for the elevator control cable rigging work done on the airplane during maintenance two days before, the cables more likely would have been rigged properly. For want of a good manual, the work was done wrong. For want of properly rigged elevator cables, pitch control authority was restricted. For want of 1� of elevator motion, the airplane and all 21 passengers and crew aboard were lost." Source: Air Safety Week, March 1, 2004, p. 7.