Airframe Structure Failure and Survivability

There are many different variables that come into play during an aircraft accident. It is an investigators job to find out what caused the failure. Failure of an aircraft primary structure is ranked high on the list of risks aircrews would rather not face. Mechanical component failure which can lead to loss of control of the aircraft is not far behind. Another issue which must be addressed in any aircraft accident is the question and of crash survivability. Even if no one was injured the investigator must find out what worked and what didnt. If there were injuries, several questions should be asked. The chapters I will be covering will give the investigator a look into what causes structural failure, and how to determine crash survivability. An aircraft accident investigators job is to determine what caused the crash. Structural failure is a vital part of the investigation. The term structure failure means where the material fails to carry below it was intended to carry. A structure can fail in one of two general ways. One way is it can be fractured, which means broken into two or more pieces. Another way is when the structure shape is changed so that it can no longer carry its load. With this kind of failure, the structure is still in one piece. It could be bent, stretched, corroded, or so worn that it can no longer do its intended job. Over the next few pages, I will be going into detail about the different ways structures can fail. There are many reasons why an airplane structure can fail. As I have just gone over a few in the last paragraph, there are many different ways this can happen. In chapter 35, the book talks about overload. This is, when an in-flight load exceeds the weight the part was designed for. All structures to include bridges, buildings or airplanes are created to withstand only specific loads. It is unrealistic to assume that airplanes can be designed and built to withstand any conceivable load it can experience. If a structure is exposed to a load greater than which it was designed for, it will structurally fail. Be it deforming, or fracturing into two or more pieces. These are two general reasons why aircraft structures fail. Aircraft structures are designed to withstand loads generated by air at some maximum airspeed and the loads generated while maneuvering at some G load. Most aircraft can be flown at speeds and G loads which can place excessive loads on the aircraft structure. Aircraft that is directly exposed to onrushing air could be damaged as the dynamic pressure of the air stream is converted to static pressure pressing inward on the structure. Excessive speed can reduce the airplanes stability. The bottom line is that a lot of bad things can happen when an aircraft exceeds it’s redline airspeed. One of the clearest reasons for failure of a structural component is that the component lacked the proper strength to withstand the loads created while the aircraft is flown at its normal operating limits. There are numerous reasons why a structural component could be understrength. It is possible that the engineering of the structure was inadequate. The designer could have possibly made an error which was not caught during the testing phase. Another reason could be that wear and tear caused a weakening to the structure. Service life issues are normally divided into four sub areas; fatigue cracking, corrosion, wear and creep. The four of these progressive failures which cannot be undone as the aircraft accumulates flight hours ground-air-ground cycles. An aircraft structure can be weakened in a somewhat short period of time. Exposure to heat can greatly reduce a metals strength. For example, some aluminum alloys that are exposed to temperatures of 400Â ° for 5 minutes can reduce the alloys strength by 80%. Jet engine hot sections and compressor bleed airlines are made of materials such as stainless steel or titanium alloys which maintain most of their strength in relatively high temperatures. This problem happens when structure which is not been designed for high temperature is exposed to high temperatures. Some aircraft which can reach high supersonic speeds require the leading edge structures to be able to withstand the extremely high temperatures generated at these speeds. There are two general areas when it comes to aircraft structures. You have a primary structure, and a secondary structure. The primary structure is parts of the aircraft that are necessary to safely fly its mission. The following components are normally considered to be part of the primary structure: wing structure, fuselage structure carrying flight, ground and cabin pressurization loads, empennage, landing gear structure, engine mounts and supporting structure. The primary structures can be further sub categorized as either critical structure or principal structure elements. Critical elements are those whose failure would result in catastrophic failure of the aircraft. Principal elements are those that contribute significantly to carrying flight, ground and pressurization loads whose failure could or could not result in catastrophic failure. The following components are considered to be the secondary structure: aerodynamic fairings, tail cones, and landing gear doors. There are other mechanical components which carry flight critical loads. Even though these components have failure modes that are closely related to those exhibited by the primary and secondary structures, they do not fall into either of those categories. Components like hydraulic pressure lines, drive shafts, electric alternators, and gear teeth in transmissions all have modes of failure which can give clues concerning the nature of the loads which caused them to fail. In aircraft accident investigator will have a firm understanding of the loads that airplane structured caries and the physical evidence that these loads leaves behind when they fail. The many types of loads are vectors and have both magnitude and direction. You can change the load by either changing its magnitude, for example increasing or decreasing the pounds of force being applied, or by changing its direction by applying force upward instead of down. Loads can take a finite length of time to be imposed. Even though loads can be applied in a very short period of time they can be applied or changed instantaneously. The fact that loads take time to apply and or change is important for the accident investigator understand. The book separates loads into three general areas; static loads, dynamic loads, and repeated loads. Knowing the difference between the three types is important because the nature of the load has a lot to do with the failure of the structure and the evidence left behind. If a load were applied so slowly that the structure to which the load was being applied to stayed equal at all times the load would be considered a static load. A static load can be either for short or long period of time. A dynamic load happens when the loads are applied fast enough to prevent the structure from carrying the load while remaining in equilibrium as the load is being applied. Dynamic loads can be divided into two subcategories, sudden, and impact. A sudden load will impose stronger internal stresses in the structure. Components such as landing gear are tested with dynamic loads. Impact loads are applied at faster rates than sudden, causing the structure to fracture almost every time. Impact loads are usually limited to high-speed bird impacts and crash tests. Repeated loads are just like they sound, loads that are repeated over and over again. Due to the behavior of dynamic impacts and longtime static loads, it makes them unlikely candidates for repeated loads. Short time static and sudden dynamic loads can be repeated over and over again. If a component goes through lots of repeated load cycles before it fails due to fatigue cracking it is said to have experienced high cycle fatigue. By lots of cycles the book means hundreds of thousands or millions or tens of millions of cycles. One of the ways investigators look at structure failure is to consider the time it took for the failure to occur. If the failure happened at the instance of a single load, it is called an instantaneous failure. If the failure took a period of time to occur, that is called a progressive failure. If a structural component contained a load that caused significant distortion, but did not exceed the materials yield stress, and the structure springs back to its original shape after the load is removed is called an elastic deformation/distortion failure. Now if the same events occur and the structure does not spring back to its original shape after the load is removed, that is called plastic deformation/distortion. This is a permanent shape change, unlike the elastic which is a temporary shape change. Now if the load reaches the point where internal stresses not only cause significant plastic deformation, they exceed the materials ultimate stress, the structure will then fracture and separate into two or more pieces. This is called a fracture failure. An experienced investigator can tell the difference between the five different types of structural failure. Another form that causes structural failures is corrosion. Corrosion is the natural disintegration of material as it is attacked by one or more substances in its environment. During the refining process, energy is added to metal ores and other raw materials in order to produce the mechanical properties necessary in structural components. Mother Nature the great equalizer, doesnt like variances in energy levels and sets to work trying to bring the material back to the low energy levels existing in the products of corrosion. When it comes to aircraft structural components, mother natures attack will reduce the strength and ductility of components turning strong metals into meek metallic oxides, hydroxides or sulfates. If these compounds are not removed from the structure they can worsen the problem by providing an environment which is ever more favorable to continued corrosion. There are many different forms of corrosion. Some can be the result of a direct chemical attack by reactive substances in the environment. Pitting is a common form of corrosion. Small holes that are randomly located across the metal surface are called pits and sometimes may be accompanied with a powdery residue. Even though pits may appear to have damaged only a small percentage of the surface, they penetrate deeply in a branching matter causing loss of strength and ductility which is way out of proportion to metals surface appearance. Chemical corrosion involves the reaction between a metal structure and some chemical agent. If you introduce corrosive acid on a metal wing, the acid and the metal will react to form new and undesirable compounds. A selective attack is when corrosive actions seem to favor one part of the components or assembly above another. The primary type of selective attack is intergranular corrosion. This type of attack centers on the grain boundaries with in a metal component before consuming the grains themselves. Similar to pitting the damage from this kind of attack causes a loss of strength and ductility which is out of portion the amount of metal that is corroded. Another form intergranular corrosion is exfoliation, whose progress and go undetected until all structural integrity is lost. Grain boundaries attacked by this type of corrosion are normally flattened and or elongated grains of extruded or rolled metals. This type of corrosion can move undetected along the grain boundaries. Slow removal of material from the surface of the component by a mechanical action is referred to as wear. In most cases wear is undesirable, wear during break in on new or overhauled equipment is often a necessary ingredient in establishing proper operation and long service life. The type of wear the book talks about is the kind that leads to premature failure and breakdown. Abrasive wear happens when small abrasive particles cut into and remove material from surfaces of two components which are held together while moving. When this type of wear happens one question an investigator must ask is, where did the particles come from? Adhesive wear occurs when microscopic projections of the surfaces of the two components which are sliding across each other may contact, weld together and break off. A question that an investigator can ask is was the surface lubricated? Erosive wear is similar to abrasive wear in that foreign particles are cutting tiny chunks out of the surface. Its a little different from abrasive wear in that the abrasive particles gain their penetrating energy by a fluid that is carrying them along. How did the particles enter into the fluids is a question an investigator could pose. To prevent structure failure, the components go through an inspection called non-destructive inspection (NDI). NDI are inspection techniques which will not do significant harm to the object being inspected. Other names for this type of inspection is called non-destructive evaluation (NDE) or non-destructive testing (NDT). There are six specific techniques for these inspections. First, visual inspection is the simplest form and most common of the NDI process and uses your God-given gift of sight. To assist this type of inspection, illumination, magnification, and remote viewing are used to help. Another type is dye penetrant. This inspection is used to detect small surface cracks and discontinuities which may not be visible during strictly visual inspections. This technique is simple, but time-consuming. The component being inspected is covered with a colored liquid which is absorbed into surface cracks. The liquid includes a phosphorescent material which when exposed to ultraviolet light glows in the dark so small surface cracks are visible to the naked eye. Magnetic particle inspection provides another way to assist the eye by increasing the conspicuity of a surface crack. This process requires more specialized equipment then the dye penetrant process, it makes the crack even more obvious if properly used. This inspection makes use of the fact that when a magnetic field is induced in a component made of Ferro-magnetic material, surface cracks will alter the components magnetic field. When magnetic particles are placed on a magnetized surface it will align themselves along the magnetic field showing any variations caused by the cracks. If the magnetic particles are phosphorescent and viewed in a dark room under an ultraviolet light pattern around the cracks will be more visible. Eddy current is the first technique that is discussed that does not require direct viewing of the crack. This process involves the use of a probe to generate both an electromagnetic field and sense and evaluate the Eddy current generated in the material being inspected. When either or surface or near surface cracks are in the material it will alter the shape of the Eddy current and magnetic field it generates. This can be as simple as a twitch on a meters needle. The equipment needed for this type of inspection must be calibrated for the specific design being inspected and the size of the crack being search. Ultrasonic inspections make use of high-frequency sound to find surface and subsurface defects. The high-frequency sound waves are generated by a transducer and then beamed through the part being inspected. The reflective waves or the remnants of waves which penetrate the part are being measured with a receiver and electronically evaluated. There are two different ways the sound waves can be applied to and retrieved from the part being inspected; immersion of the part into a fluid which carries the sound waves to and from the part and direct contact inspection where the transducer and receiver are in direct contact with the part. The direct contact technique is much more mobile allowing use in the field of the aircraft or major fabrications. Radiographic inspection in its simplest form is not much different than that of an x-ray. Very short wave electromagnetic radiation are generated and directed through the part being inspected and towards unexposed radiographic film. Rays passing through cracks, flaws, voids and corroded areas will not be attenuated as much as raise passing through sound material. To the untrained eye, cracks, flaws, voids and corrosion may appear to be just another shadow on the film. Orientation of the x-rays so as to illuminate the discontinuities and proper interpretation of the film are therefore important aspects in ensuring the thoroughness of the inspection. One of the issues which must be addressed in any aircraft accident is a question of crash survivability. Even if no one was injured, the question of what worked and what didnt work should always be asked. Did the restraint systems operate as designed? Whether or not escape hatches were needed, could they have been opened if needed? Did the emergency lighting system work? How crash survival equipment is designed is based on experience, history, and sound engineering judgment. To know if these components work correctly design requires an accident for verification. The accident is a chance to validate our judgment and we cannot afford to pass it up. To make investigation of aircraft crashes a little easier requires a systematic approach by breaking down a complicated series of events into smaller, more digestible bites. The approach chapter 36 goes over is the CREEP method. CREEP stands for: Container, Restraint, Energy absorption, Environment, and Post-crash factors. The first four of the five CREEP elements relate to the dynamic portion of the crash itself. These four factors are concerned with the initial and any subsequent impacts with the terrain, the associated deceleration forces acting on the aircraft and its current occupants, and the deformation and dislocation of aircraft structure and its contents. The fifth factor relates to the occupants attempts to egress the aircraft before suffering additional injuries not directly resulting from the dynamic portion of the crash. In order to survive a crash it is first necessary to provide a living space for the occupants during the dynamic portion of the crash. If the space is crushed or punctured, the chances of survival fall drastically. This factor is container. Now if the occupants have been provided with adequate living space, the next series of questions should deal with the restraint of the crew and its passengers and equipment and components around them. Occupants of any moving vehicle must be protected from injuring collisions with in the vehicle, for example being thrown against the sides of the living space or having objects such as cargo or equipment thrown at them. The strength of all restraints should be sufficient to prevent injury at the force levels which can be expected during the most severe but survivable crash. The investigator should examine all restraints system failures to determine if there failure contributed to injuries experienced by the crew or passengers. The deceleration forces created during a crash may be high enough to cause fatal or serious injuries, even if a safe living space, adequate crew and passenger restraints, and a delethalized flailing envelope are provided. Since crew and passenger bodies are not strongly attached to the airframe, the design of the aircraft structure and seeds may cause the acceleration forces experienced by the crew and passengers to be either amplified or attenuated. A soft deep seat cushion can greatly amplified the vertical Gs experienced by someone sitting in the seat. The deep seat cushion deforms at high loads absorbing energy as it gives can greatly reduce the vertical crash loads to which a seat occupant is subjected. Hopefully, the designers will build a secure box around the crew and passengers and secure them to it. Although we may be able to restrain the torso, it is normally impractical to secure the head and limbs of the crew and passengers. The volume through which the unrestrained extremities can be expected to move shouldve been the legalized to the maximum degree possible. Obstructions which could cause injury should either remove from within the flailing envelope or padded to reduce the severity or probability of injury. This is the environment that the creep method covers. All too commonly, crew or passengers survived the dynamic portion of the crash, only to suffer additional injuries or death when they are unable to safely exit the aircraft in a timely manner. The two primary factors in the causation of fatalities during otherwise survivable crashes is, post-crash fire and inability to quickly exit the damaged aircraft. Fire is the most significant post-crash hazard by a long shot. Not only can the fire kill and injure directly through heat, the toxic fumes and smoke produced when material and the aircraft interior burn are more often the direct cause of death. This post-crash condition is a top priority in controlling to prevent death. Design of airplane exits is predicated on the normal parked attitude and configuration. Obviously, this is not always the case. Sometimes occupants will have to exit from an airplane that is an abnormal attitude and perhaps in a very unusual configuration. Part 125 airplanes have specific emergency exit acquirements levied on them, many general aviation airplanes have only one exit which can be easily jammed in the airplane ends up inverted. In conclusion, nobody is expert on all types of structural failures. With so many different variables, it takes a highly detailed investigator to pinpoint what kind of failure causes a crash. And without a systematic approach of investigation of a crash, the investigators are left with an accident that is difficult to determine whether or not the occupants should have survived the impact.