Did You Know Aircraft are Made Tougher than Necessary?

The concern for structural integrity is enormous, and its maintenance is another.

An aircraft parking at the gate preparing for the next flight. Image by Author.

Flying in general means getting from point A to point B quickly and safely. To provide a safe and reliable medium of transportation, designing a fast and pressurize flying tube full of passengers and cargo means a lot of challenges that need to tackle before it is approved as a mode of transportation.

Long-range communication, controlling and stabilizing mechanism, fire protection system, the material used ( which need to be as light as possible but strong enough ), reliable sensors and indicators, approved maintenance program, studies of the human factor and so much more need to be considered before an aircraft is certified to fly safely and reliably.

The structure of an aircraft is the biggest concern of all. Countless researches are necessary to prove how it can cope with the extreme temperature changes, the differential pressure at the inside and outside of the vessel, heavy loads of fuel while holding onto all of these ultimate physics for 25+ years of average in operation.

In this article, we will look into the aircraft structure limitation and how it plays into making aviation the safest mode of transportation.

Engineers Nightmares

Structural integrity. That’s it. That is what off the top of the engineers’ heads when making the design for the fuselage and wings structures. There are many things to consider for such a massive engineering feat that takes multiple loads simultaneously. One of them is integrity.

Can you imagine the stress that aircraft should face when dealing with huge temperature changes in a short period? Or the pressure differential? Or the extreme weight itself plus another hundred tons of fuel and passengers?

For instance, an aircraft took off from a scorching hot 40 degrees Celsius Dubai to -40 degrees Celsius at 38000 feet in 30 minutes. Multiply all these cycles three times a day for 20 years. Yup, all of these are quite a headache for engineers to solve without making the structures strong enough.

Along the lifetime of an aircraft, the manufacturers need to consider many possibilities and scenarios that could arise such as lightning strikes, turbulence, hard landing, and dozens more. These events will be experienced by the aircraft over the years.

Minor occurrences such as bird strikes, accumulation of dirt, oil, and stain from the industrialized-polluted area also contribute for the structure’s integrity to degrade over time.

Furthermore, protection from corrosion, cracks, and metal fatigue should also include in the never-ending list as they could cause catastrophic failure if not detected early and mitigate properly. The structure should be strong and tough enough to contain all these forces acting on it.

Structural Limitation

In engineering, engineers typically set limit load and ultimate load to determine how strong the material is and to understand how efficient the design behind it. These two are crucial in fabricating the primary structures on an aircraft. They are the one that mostly takes the ground and flight load all the times and may lead to a fatal accident if degraded and remains unnoticed.

The limit load is the expected maximum load that the structure directly takes throughout the aircraft lifetime that includes emergency braking at high speed, heavy landing, rough turbulence, and other cases.

Ultimate loads on the other hand are limit loads multiplied by the factor of safety of 1.5. In simple words, it is where the loads are 1.5 times stronger than the limit loads. It is the requirement issued by both the Federal Aviation Administration (FAA, a department within the US DOT) and European Aviation Safety Agency (EASA).

What this means is that aircraft structures are built stronger than required for a better safety margin. Several Gs are needed for aircraft to encounter that particular ultimate load. It could be from sudden manoeuver at high speed or extreme hard landing. These two do not occur during a routine flight.

Although passenger aircraft are unlikely to experience the ultimate loads, it is still mandatory for the manufacturer to design and build the structure based on the requirement otherwise, it cannot be certified.

A Boeing 787 during the ultimate load test on the wings as it flexing upward in which a situation that it will never encounter during its lifetime. Credit to Boeing.

On a side note, both the FAA and EASA regulations specify that the aircraft structure must contain and distribute the loads evenly within itself. On top of that, it also must support the limit loads so that no permanent deformation takes place hence, it could not hampering the safety operation of the aircraft.

The Code of Federal Regulations or 14 CFR Part 25, which is the federal regulation that prescribed the requirements and standards for manufacturers and OEMs in constructing a commercial aircraft, disclose that the structure needs to be 1.5 times stronger than the maximum predicted limit load. The statement in 14 CFR Part 25.303 regarding the mandatory design that needs to conform with is seen below;

“unless otherwise specified, a factor of safety of 1.5 must be applied to the prescribed limit load which are considered external loads on the structure”

This regulation code has to be fulfilled and proved to the authority before it can be certified. What it means is that all the primary structures, including the seat tracks beam that hold the seat in place, must follow this guideline. If not, then it shall not be certified.

Structural Maintenance

There is nothing perfect in this world. Nevertheless, we in the aviation industry think nothing beats the proverb ‘better safe than sorry’. Every thinkable aspect needs to be taken seriously and should never be compromised.

Come to think of it, the structural integrity itself plus the intensive maintenance program in place make a perfect combo to combat any flaw that could not be detected otherwise.

If you could imagine, the fuselage structure is actually a giant pressurized balloon up above the sky and will come back on the ground unpressurized. So, for multiple times a day for several years, it expands and contracts just like a balloon, although undetected with the naked eyes.

This cyclic phenomenon can create fatigue stress somewhere in the structure.

The aircraft maintenance program gives tremendous details on where and when to inspect these cracks based on extensive analysis from the manufacturer and other organizations. Even the slightest crack can elongate, thus weakening the metal.

A much more detailed inspection using eddy current or ultrasonic technique can detect subsurface cracks that are hidden underneath what is assumed to be a perfect surface.

An NDT inspector is checking for any crack in certain areas on the upper part of the horizontal stabilizer utilizing specialized equipment meant for an eddy current inspection. Image by Author.

Older aircraft need a lot of time and areas to examine as they reach the recommended interval, whether from flight hours or cycles ( just like your car, it is recommended for servicing when it reaches the stated kilometers or months ).

A flight cycle is the pressurization and depressurization of the fuselage, meaning the aircraft has taken off and landed. Of course, the area within the crack is then repaired or replaced depending on the severity.

For perspective, each rivet hole on the aircraft skin needs examination for cracks each time rivets are removed and again for the rivets themselves after fastened. Usually, damaged skin that needs to be patched with a new one is included for these procedures to totally eliminate any suspected cracks and as an added safety measure.

An example of an ongoing skin repair process on the fuselage. The Structural Repair Manual of Boeing instructs to inspect for each un-riveted hole for cracks before proceeds any further. Image by Author.

Only the approved license holder for the ultrasonic or eddy current inspection can sign off the task that utilizes their credibility and knowledge.

So, even when the small holes are concerned for crack, one can imagine the thoroughness for crack assessment on bolts, mountings, and other critical structures that constantly on dynamic and static load.

A screenshot of Boeing’s SRM indicates the step in C (2) to inspect for cracks after the installation of rivets. Image by Author.

It’s not just crack that engineers are concerned with, corrosion is also a big no-no for the structure. The environmental condition where the presence of higher humidity could lead to metal corrosion even when painted.

Not only limited to humid conditions, the excessive salt concentration in the air such that near the ocean also contribute for the electrochemical reaction in the metal to take place.

What is common for humans doesn’t mean it cannot do any harm to aircraft. Metallurgists have and are currently brainstorming to minimize the susceptibility of metal from corroded.

If the corrosion is not taken seriously, it can eventually thin out the alloy hence weakening the area. Moreover, it also can spread to the subsurface area and corroded much deeper into the metal if not mitigated properly.

Cracks will develop later because stresses accumulate on an area that has thickness irregularity and we know what that could bring us next.

Corrosion is the worst aircraft enemy that is not even allowed on aircraft. Fortunately, it is more visible compare to cracks during an inspection.

But no need to worry, with a better understanding of the properties of alloys nowadays, manufacturers and engineers can determine the best procedure and standard when handling these aluminum alloys to reduce the risk of corrosion.

Starting from the manufacturing, to the heat treatment process, up until the maintenance, each step is crucial to prevent corrosion from happening. Even a slight manufacturing flaw could trigger the corrosion process.

Preventative Action

Higher manufacturing precision and modern technology are what we should be grateful for, to begin with. Corrosion preventative program initiates at the very start of the manufacturing process.

Differences in heating time and temperature variation during the manufacturing process put the alloys in a vulnerable position for corrosion because it could alter their grain structure.

Modern airliners today are constructed using advanced alloys composition made from aluminum, titanium, magnesium, and others. These alloys are difficult to corrode.

Unfortunately, titanium and magnesium alloys are much more difficult to manufactured and expensive. Therefore, limited areas on the aircraft are using these alloys.

Aluminum alloys get the honor to make up more than half of the aircraft structure material. What the engineers do to protect it from corroded is by covering the surface up with a thin layer of pure aluminum cladding, also known as alclad. This ultra-thin film act as a shield for the surface underneath it because pure aluminum is anti-corrosion.

What if the film damaged due to scratch? Fret not, the use of corrosion inhibiting compound and chromate conversion coating are there to the rescue.

The corrosion-inhibiting compound act as a water-displacing compound that eliminates the water/alloy relationship. It will prevent water or moisture from reacting with the alloy. It is ideal for preventing corrosion due to its long-lasting properties and ease of application.

The corrosion-inhibiting compound’s easy application and proven effectiveness make it the choice for aircraft. Image by Author.

The person in charge needs to obey all the required procedures to store an aircraft or when putting it in long-term parking to lower the risk of corrosion, especially during this pandemic. Desiccants are necessary for certain areas as well as in the engines.

If you happen to see a lot of aircraft park strategically to each other in a desert, it is because the condition is suitable in the long run. Low humidity makes the desert a perfect place to store aircraft, be it in Alice Spring, Teruel, or Victorville.

Other regular actions to decrease the chances of corrosion are;

  • Lubrication
  • Applying primer and top it off with paint
  • Regular inspection

For the last 50 years, the aviation industry is still learning to deliver the most trusted means of transportation. An enormous amount of hours are consumed to analyze the suitable material and design to create an evenly distributed load path in the structure.

The learning curve is getting higher and higher for understanding the interval needed before each crack or corrosion-related detailed inspection.

The aviation industry is regularly reviewing all related incidents regarding these two enemies to propel safety even further.

Rest assured because we have learned and implemented all bits and pieces of yesterday’s findings to achieve this kind of today’s achievements.

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