Duplicate Inspection – Flight Controls VIII

The Previous post would have highlighted that sheer negligence has the potential of creating disaster, of devastating proportions. So what can we do to prevent such a thing from happening? Besides stringent quality audits and the use of technology (again!), most civil aviation regulators have come up with a provision in their civil airworthiness requirements, to have flight control systems checked a second time (duplicate inspection) if they have been disturbed in any way during routine or non-routine maintenance. Essentially this means that once a task associated with flight controls is completed, a different engineer/mechanic/inspector will ensure the systems is rigged correctly, and certify the same in the maintenance log book with his/her signature. Kind of like taking-a-second-opinion.
This ensures that if indeed a person did somehow rig the system incorrectly, it can be caught out by a second (likely more qualified/experienced) ‘set-of-eyes’. As I’ve mentioned earlier, control system rigging is not an easy task. It requires a person working in a sometimes very physically constrained environment, with not the best lighting, not the greatest ventilation, and not the best angles of vision (sometimes inverted!). Add to it a whole bunch of cables with several other teeny-weeny bits of equipment, fluid hoses/tubing and wires surrounding the work area, and you have a readymade curry of errors waiting to happen! Not to mention the humongous degrees of patience required from the person working on the system. Speaking of patience, I have personally had the not-so-good experience of holding onto the lamp illuminating the work area of a cable tension setting job, and shifting a tiny bit to relieve a sore bum, only to have my engineer sit up, grab the lamp, and tell me to get the hell out of there!...and send someone else!!

So, it is a tough job, and I think I have successfully convinced you about that!

That also satisfies the claim that making error’s with the flight control system is not difficult.
Duplicate Inspection does alleviate this possibility to a large extent. But there have still been errors; just like the one pointed out earlier. So why does that happen?
Well, the answer to that may be a simple “complacency” issue to a slightly less-than-simple “complex/advanced/digital technology” which can confuse the daylights out of a slightly older generation person!
Having said that, things are being made “easier-to-grasp” nowadays and there is supplemental training all personnel are provided to cover the latest technologies. However, the complacency factor still exists. One thing, though, that can be completely avoided is inspectors “pencil-whipping” an item without a detailed examination!
More on complacency in the following posts.

“You have ...Power!” - Flight Controls VII

We spoke about human error last time around. Now with regard to human error, before I give the impression of finger-pointing the FBW, I want to clarify that human error is applicable equally to both the FBW and control cable systems. This means that it is as easy to make mistakes rigging flight control systems on both types. It’s a difficult task, maintenance-vise to rig and fine tune these systems. The FBW’s today have something called a Built In Test Equipment (BITE) that checks for system malfunctions and gives a reasonably accurate error indication and sometimes locations of malfunctions and equipment requiring attention. However, one still needs to get all the plugs in the right sockets! Figuratively speaking? Not really.

Here’s a story of how things nearly got out of hand.
"Two Maintenance Mistakes Reveal Similar Systemic Shortcomings"

It was a result of cross-wiring the Captains side-stick. That is, a right Aileron ‘up’ command actually led to the left aileron going up, and vice versa. Something that should have been caught out at some stage? Well, at the least during the Preflight? Well, this one escaped all safety nets, and brought a wing tip within 5 feet of the ground!
Again, I’d hasten to add that cable control systems have been cross rigged on many occasions as well. It’s just that in a FBW system, given the advanced nature of the systems, back-ups, error indications, and what not, we should be able to prevent something like this. Something as basic as this!
Maybe it’s easier to get complacent when you have technology of the type available today, to back you up? Maybe. My view of it is that technology can and should be used; but more so as an aid. If we fail to connect our plugs in the right sockets, and to read what technology is telling us, we are not only failing to make use of that technology, but we are arming it (in the true sense of the word) to take away from us, the power we gave it, ENTIRELY!
Until the next time you hear - “You have control” – Fly Safe

Advantages and Dis. - The FBW system - Flight Controls VI

The Fly-by-wire(FBW) concept we discussed earlier, has several advantages over the cable systems. Amongst a few of them, fewer movable components, lesser wear and tear, as a result lesser maintenance; greater precision in control surface movement (via digital input/output), and providing a better interface with other aircraft (and engine) systems, including the Automatic flight control/director systems (what we ordinarily call AutoPilot). The FBW also lends itself to incorporating backup systems and providing what is in technical parlance called, Failsafe systems. Example: If there is a malfunction(say in the hydraulic system), there will be an automatic shift to a standby system and an indication of this in the cockpit. Alternatively there could be a complete systems shutdown and a transfer of control to the pilot. There’s backup!
The primary advantages, though, are greater efficiency and weight saving. These feature most commonly in airline and aircraft manufacturers decisions to shift to the fly-by-wire system.
Now, there are disadvantages too. The main amongst them being those associated with any other electrical systems such as short circuits, system overload and some other more predictable and therefore more controllable factors such as heat buildup, electromagnetic interference etc.
These were some advantages and disadvantages.

In the next write up, we’ll talk about how human error can outdo the best of systems- in our case the FBW system.

Cable systems...redundant? - Flight Controls V

I came across an article the other day that highlighted just why control cable systems are so critical to flight safety. While it will be incorrect for me to quote what the article stated, I can give a general idea about it’s key contents. First, today, I’ll give you a brief idea about the state of the art technology called fly-by-wire; and a comparison with cable systems.
In the age of fly-by-wire, cable systems are fast becoming redundant. In a fly-by-wire system, a cable is basically substituted with electrical signals. In effect the job of a cable system is accomplished electronically using systems that measure the extent (and rate) of control input by the pilot in the cockpit, which is then translated into the actual control surface movement via electrical servo motors/valves and hydraulic pressure. The nagging thought of “well, how does the pilot get the ‘feel’ of the controls” is also resolved via an artificial feel system that takes it’s inputs from the Air Data computer to sense the speed at which the aircraft is traveling amongst other external factors (such as air temperature, density and pressure).
In a cable system, the cables (assisted by other components such as pulleys and turnbuckles and bell cranks) carry the mechanical input from the pilot in the cockpit to the control surface (elevator and rudder on the tail plane and ailerons on the wings). In fly-by-wire, these inputs are ‘transported’ via electrical wires. They connect to servo motors at the control surface. The servo motor determines the extent and rate of movement desired by the pilot, and transmit this to a set of valves also within the servo motor (Technically these valves are called kinetic valves). These valves basically function to direct hydraulic pressure (by opening and closing pressure and return ports in the hydraulic pressure system) to the appropriate site of the hydraulic actuator. The actuator is a simple piston inside a cylinder, with the piston end connected to the control surface via linkage/mechanical rods to ‘actuate’ the surfaces in the desired direction, to the desired extent, at the desired rate!
Now that doesn’t sound simple, and it isn’t. But one thing you would probably understand is that there are lots of things that can easily go wrong. Luckily, advanced systems also come with advanced backups built into the system, to prevent malfunction. They don’t, however, eliminate the ‘human’ touch. And where there’s a human touch, there’s bound to be a human error! This is what we shall talk about in our next write up.

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.