Understanding Pressurization...with a balloon!

While on the topic of Aircraft structures, and having used the term ‘pressurization cycle’ not too long ago, it would be practical to cover the topic of pressurization.
First off, I think we understand the need for pressurization of aircraft’s flying at high altitudes. In plain terms, pressurization helps maintain a positive pressure in the passenger cabin, one that is comfortable for passengers to breathe normally in.
What is slightly less known, is that the cabin pressure is not maintained at sea level pressure, that is, the pressure your body might experience at sea level.

So why take all the pains to pressurize the cabin at all, if we cannot keep it at a pressure which is most favorable for the human body?
The answer lies in the structural ability of the aircraft cabin (fuselage) to withstand differential pressures. Ok, now that deserves an explanation!
As we move upwards from the surface of the earth, atmospheric pressure decreases as the air becomes rarer. As mentioned, to overcome the effects of this on the human body, the aircraft is pressurized. So, while the atmospheric pressure (pressure outside the aircraft fuselage) is very low, pressure within the fuselage is quite a bit higher.

Imagine a balloon being inflated, Yes an ordinary birthday balloon! Now, if the pressure within the balloon becomes critically high, in relation to the atmospheric pressure surrounding the balloon, it will…well, burst! This example simply helps appreciate that by pumping air into a balloon we are increasing it’s internal pressure, and when it(differential pressure between internal and external of the balloon) gets beyond that which the balloon is capable of holding, it bursts.

Similarly, an aircraft structure can only withstand a limited amount of differential pressure between the (internal) cabin and the (external) atmosphere….before it, well,… I think we know now!

If you read between the lines, there are 2 factors at play here, between which we must run a compromise. First, the human requirement for cabin/fuselage pressure to be at a level where human physiological functions (mainly breathing) can occur normally. And second, a differential pressure (between cabin/fuselage pressure and external atmospheric pressure) that the aircraft structure can withstand.

Strange dichotomy! Well, if you are an aviation professional, you are probably already quite accustomed to such compromises, technically speaking that is!! Ok, Lets proceed.

So, to meet both these requirements, the cabin pressure is basically set at what you might experience on earth at about 6000 to 8000feet above sea level. This pressure seems quite comfortable for a wide range of people. At the same time, this seems reasonably comfortable for the aircraft structure to withstand without…. The problem is actually compounded the higher the aircraft flies; but, let’s put that away for the moment, lest we put your brain under a cornucopia of information, compelling it to…!

The next time your doctor advises you against flying due to a medical condition, the consequences of this reduced pressure on your body, is probably what he is highlighting.

Today's Airliners

A question that then comes to mind, what exactly was the structure like in the case of the comet aircraft in the early 1950’s and what is it like in today’s airliners?
The Comets structure was called the stressed-skin fuselage structure because the Aircraft skin took the primary structural loads in flight. This type of structure is technically called the monocoque fuselage construction. We saw how this created fatigue for an aircraft fuselage that was constantly flexing under pressurization and depressurization loads.

To overcome this problem, designers came up with a new structural design which distributed the flight loads to stronger lateral(vertical structures such as bulkheads) and longitudinal (horizontal structures such as stringers and longerons) members that ran all along the length of the fuselage. The picture above shows an intersection of the inner wing with the fuselage. Notice how the fuselage is made of frames (vertical members)and horizontal reinforcing members - apologies for the quality of the picture. More pictures here..

So how did these structures help?
Well, essentially, flight loads were systematically distributed amongst vertical and horizontal structural members, easing the overall load on any one single structure. This dramatically reduced the fatigue factor on any single structure of the fuselage adding longevity as well as ensuring that if one, or a set/series of structural members were damaged (due to fatigue or even overloading in flight due to turbulent weather, aerodynamic overloading, foreign object damage etc.), the other members would be able to carry the loads atleast until the aircraft was able to descend and land safely. That is, in the event of damage to part of the aircraft structure, the overall structural integrity of the aircraft would not be compromised. This is what designers and engineers refer to as Failsafe construction, sometimes even referred to as Damage Tolerant structures.

This type of fuselage design came to be called the Semi-monocoque construction design. Today, every pressurized aircraft involved in commercial operations, is built with a failsafe design philosophy.

The Coke-Can Theory.

If you’re looking to find this theory (as named) in a text book, or the internet, it’s likely you won’t find it. Simply put, it’s ‘made-up’!...the name, that is..!!

Let’s take a look at that can again. Assume it’s an empty can with the outer skin smooth, seamless and unbroken through it’s entire length. Now, I make a little dent on the surface, with a finger nail, at any point along it’s length. Next I place my palms on the two ends of the can, and compress….
At some point, the pressure is sufficient to crush the can; and, it’s very likely the ‘fault’ line, along which it caves in, lies on the point at which the can was dented.

It’s logical to assume that when pressure was applied at the two ends, all that pressure was concentrated along that fault line - localized stress concentration. When the pressure exceeded the threshold, the can gave in along that fault line and crumpled. If you try the same experiment without denting the can outer surface, the compression required to crush the can will be greater, and, if you keep the pressure applied at the two ends ‘square’, (i.e., perpendicular to the length of the can) the can will likely crush ‘straight-in’.

Based on this simple experiment, one can imagine the effect of a fatigue induced ‘fault line’ in a pressurized fuselage of an aircraft.
In a stressed skin fuselage construction, fatigue, due to constant flexing of the fuselage structure at every pressurization cycle (As mentioned in the previous post) is the major factor that induces a fault line on the aircraft skin; usually at the point of maximum load bearing, or at points of changes in fuselage cross-section etc. A fatigue induced fault line could start out at the microscopic level, and with continuous load reversal (flexing), grow until it causes the structure to fail entirely.

Why didn’t they think about such a seemingly simple phenomenon before they put people in them? To be fair to the designers of the De Havilland comet aircraft, they did conduct tests for structural integrity. However, fatigue related failures are those that occur over a period of time. Also, speculatively speaking, simulating the effects of fatigue in the hostile environment that an aircraft flies in (at altitudes of 35000ft above sea level), may not have been appropriate. Maybe due to a lack of understanding of conditions at that level, maybe due to the lack of availability of appropriate testing equipment…?

Irrespective, the fatigue factor, and the ‘coke-can theory’ got the stressed skin construction fuselage beat a hasty retreat. Empty cans…they made their noise!

“It’s a bird, it’s a plane …it’s a Coke Can!” - Airframes

I wonder how many people think of airliners they fly in, very much like the can in the picture above! In fact it started out much like that in the early 1950’s with the De Havilland Comet aircraft. The Comet became quite (in)famous for tragic in-flight structural failures, almost complete and rapid decompression at 35000ft above sea level.

There were a total of 3 such disasters with the Comet (and a loss of many lives), all of them within a short time after entry with the airlines at the time. This indicated to the designers that there was a flaw in the basic structural design of the aircraft-A ‘stressed skin’ fuselage construction design that imposed or transferred, almost the entire structural load on the aircraft skin. That is, the aircraft skin became the primary load carrying structure. Though that might sound absurd, it wasn’t really a case of structural overloading on a relatively slight aircraft skin that caused those catastrophes. In fact, we may be quite tempted to ask “What were the designers thinking?! How can the aircraft skin be asked to withstand structural loads of the magnitude relevant to an airliner??”

To be sure, the aircraft skin was built many times stronger, heavier, and thicker than today’s airliners. The flaw lay in the fact that repeated pressurization and depressurization (called pressurization cycles*) of the fuselage, induced fatigue in the structure (in this case, the outer skin) of the aircraft. This in turn created a mini fatigue fracture or a fault line on the aircraft skin resulting in localized stress concentration. The rest of the account lies in the coke can theory! A further diagnosis of this theory in future posts.

*Pressurization cycles- One pressurization cycle consists of one complete pressurization and one complete de-pressurization of the fuselage. In plain terms that means one complete sequence of the events involving,
1. A Take-off
2. Climb to cruise altitude
3. Descent from cruise altitude, and
4. Landing
Pressurization and Air conditioning

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.