Airframe Structure Failure and Survivability

Topics: Structural load, Structural failure, Wear Pages: 10 (3606 words) Published: May 16, 2013
Aircraft Accident Investigation SFTY 330


There are many different variables that come into play during an aircraft accident. It is an investigator's 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 didn't. 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 400° 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...
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