Control Cable Tension - Flight Controls IV

This one is perhaps one of those items a maintenance person must pay particular attention to. Incorrect cable tension can quite easily go unnoticed. The results of this can lead to failures such as cable slippage which may result in cable runs or twisting which could further lead to the cable jamming and subsequently lead to system component failures such as pulley system wear, or internal cable wear, broken wires etc.

Cable tension varies with temperature, and to understand the principle behind that you will need to read up on Thermal Conductivity and Coefficient of Thermal Expansion.
The way I understand this is by the analogy of electricity/telephone lines between poles. Why do these cables always give the impression that they are ‘hanging’ or rather slack between poles. The reason for providing that slack is to give allowance for expansion and contraction of the cables with changes in outside air (ambient) temperature.

Just like that, control cables in aircrafts must be set with the correct tension based on the ambient temperature. In aircraft design, every component has it’s own coefficient of expansion. Therefore, it becomes all the more important to rig cables for the correct tension even based on the expansion coefficients of components they’re attached between.

As part of the Aircraft rigging procedure cable tension is set with reference to charts that compare rigging loads to be applied the cable for a given ambient air temperature.

As mentioned in the section on cable components, Cable temperature compensators and turnbuckles are used to fine tune the cable tension after the application of rigging loads.

Cable System Components - Flight Controls III

The question that might then come to mind is that HOW is the pulling action of a control cable converted into a pushing one. The answer as was supplied above is via a ‘looped system’. However, there are other components within that loop that result in actuation of a device/control surface etc. Each of these components have a particular function such as:

1 . Serving to change the direction of application of force(Pulleys, Bellcranks, quadrants, jackscrew drums),
2. Transferring the motion of the cables into push-pull ( As above, and Rods, tubes, horns)
3. Acting as a support for the cable (Pulleys, Fairleads)
4. Forming a terminal connection between cable and actuator (sockets, clevises, pins, rod-ends),
5. Forming a connector between repaired sections of cables (cable splices, turnbuckles),
6. Serving as point of adjusting cable tension (temperature compensators, turnbuckles)

For a more vivid description of each component and it’s function, you could refer your Airframe and Poweplant Mechanics Handbook.

Brief Outline- Cable Control Systems - Flight Controls II

Cables
We know control cables are made up of corrosion resistant steel wires, but how exactly are they ‘wired’ up? I have a very simple way of keeping tabs on this:
Wires around a single central wire = Strand.
Strands around a single central strand = Cable !
Just how many wires, and how many strands? That’s where cable classification comes in. Different applications require cables of different diameters and that is based on 2 aspects:
Cable loading – or the amount of load the cable application will require;
Cable Flexibility – based on the route and distance the cable must travel.
The classification is therefore given in diameter (in inches) and the number of strands/wires in the cable. So ¼ 7x19 would imply 1/4 inch diameter, 7 strands in each cable, 19 wires in each strand.

A very important aspect in understanding how cables take loads is knowing that a cable can transmit a force that is unidirectional only; that is, via Tension or a pulling force. That however does not restrict it only to pulling applications. It can exert a pushing force, or compression, as well. In order to do this however, it has to form a ‘loop’ that allows counter pulling, in the opposite direction.

EXAMPLE: In regards to the primary flight controls in the smaller training aircrafts today, an action to lower an aileron on one wing, causes an opposite aileron movement on the other wing. With the rudders and elevators this may not seem apparent, but if you examine at how pulling and pushing the control column in the cockpit operates the control surface, you will realize that the cable control applies the pulling and pushing motion of the control column to the actual control surface, via a pulling action (Tension load).

Cable Control Systems- Flight Controls I

Cable systems have several applications in an aircraft. Flight controls systems is the first that comes to mind and quite so. No pilot wants to have a situation on his/her hands (literally!) that you gently pull back on the controls and it’s either got completely jammed (!), or will move freely enough to indicate it’s ‘come loose’!, and sometimes, even cause the aircraft to react in the opposite sense!
Ok, now that might be a very traumatic experience, therefore highlighting the importance of the flight control systems. But almost equally important are other applications of control cable systems. Take for example the engine controls, or the nose wheel steering systems, or the landing gear systems and even braking systems. These are the more general applications and it is a given fact that each aircraft type will have it’s very own applications.
Following posts will indicate points to consider when dealing with control cable systems.

The License to learn…?

Most people are quite divided over this topic. The idea of starting this is not to make a statement or even my opinion. Please go ahead and use the ‘comments’ link at the bottom of this post to make your opinions known.
The license that’s issued to us by the DGCA after all that fanfare of attending AME school, going through apprenticeships and on-job-training, and amidst all that finding the time to study, even burning the midnight oil, and then finally passing the DGCA exams, the license you’re issued has just GOT to be it right ?

Well...., maybe not!
Here is where the argument starts. “Obviously! The DGCA itself has given you written authorization to perform maintenance activities within the scope of the authorization/license”
Quite true! This is evidenced by the fact the DGCA issues the BAMEL (Basic Aircraft Maintenance License) on applying for it, after clearing your exams on Regulations (C.A.R), Paper II or general engineering and workshop practices, and Paper III in the appropriate category. Additionally, the license to certify aircraft comes only after Paper IV or the ‘specific’ paper that covers Aircraft or engine type.

In short, the DGCA has put you through stringent checks and put in place several procedures that ensure the tasks you’re authorized to complete are well known to you and that you are competent in that respect.

The other school of thought is of the opinion that the license is one that actually,... starts the learning process! That, even though the license is issued, you can’t really claim “oh yeah! I can do it all! I have the authority to do so!”

The argument clearly surrounds the complacency factor. However, there’s one more thought to this. Do we really look at obtaining our AME license and the responsibility accompanying it, as a ‘destination’? Or, would we rather want to account for it as a constant journey,…one that is often referred to as the ‘the learning process’!

There certainly is more to it…! Read on and do make your opinions known.

What is...

FBO – Fixed Based operator.
The FBO is an aircraft service center at an airport that typically provides a host of services including fuels, lubricants, de-icing fluids, hydraulic fluids, maintenance, aircraft hangar services, and ground handling support.
The FBO is quite similar, so-to-speak, to a highway stop-by! One best suited to the weary traveler!
Besides the basic services mentioned above, FBO’s also provide weather and communication facilities, including internet access for customers. Now, you’re looking at travelers, so the next thing would be food! So, there would ideally be vending machines and refrigerators stocked with snacks.
Additionally some FBO’s may also provide car rental services, hotel reservations and lounge facilities, for example, to the overnight travelers.
Some of the amenities and conveniences that could also be provided by an FBO include:
Special Flight planning rooms equipped with updated maps, access to weather services;
Executive conferencing areas;
Private showers/restrooms;
Catering and ground transportation for crews;
Free hi-speed internet connections.

So, an FBO is quite the “customer centric” aircraft service center, and though they do provide extensive maintenance support, the focus is on the “overall customer experience” at the airport, a bit different from the MRO focus on total maintenance support.