Air Frame Life Extension - F-15 Problem

In summary: Structural integrity is a significant matter for aerospace engineers. Boeing is apparently investigating the possibility that it may be liable for the structural "defect" in F-15's. By then, one in theory collects sufficient data to allow for continued use. The responsible engineer needs to ask questions, collect evidence and ensure all data are correctly interpreted.
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Structural integrity is a significant matter for aerospace engineers. The Air Force is apparently investigating the possibility that Boeing is liable for the structural "defect" in F-15's. Life extension is a critical matter in aerospace and nuclear industries as highly engineered systems approach original design life. By then, one in theory collects sufficient data to allow for continued use. The responsible engineer needs to ask questions, collect evidence and ensure all data are correctly interpreted.

In aircraft construction, a Longeron is a thin strip of wood or metal, to which the skin of the aircraft is fastened. Longerons are attached to formers (also called frames), in the case of the fuselage, or ribs in the case of a wing, or empennage. In very early aircraft, a fabric covering was sewn to the longerons, and then stretched tight by painting it with dope, which would make the fabric shrink, and become stiff.

Sometimes the terms "longeron" and "stringer" are used interchangeably.
http://en.wikipedia.org/wiki/Longeron

AIAA Daily Launch E-newsletter said:
According to the AP (1/11, Lardner), senior officials say "the Air Force's aging jet fighter fleet" could become worse, "even after expensive repairs are made." Prior to the recent F-15C crash, "[t]he Air Force's dilemma [was] largely overshadowed by the equally urgent demands from the Army and Marine Corps for new equipment," and greater numbers of F-22s were deemed too expensive. Now, the Air Force has made "troubling...findings of a parallel examination that determined as many as 163 of the workhorse F-15s" have defective longerons. The problem, according to Gen. Corley, "would be best solved by" purchasing new fighters "rather than fixing jets that are 25 years old." While replacing parts is cheaper than new fighters, Corley explained that the F-15's age means other parts could soon fail, and also that it is outdated. Conversely, Corley said, "the F-22 is a modern plane that meets the Air Force's needs for an air combat jet."

Air Force investigating liability in F-15 defect. The Los Angeles Times (1/11, Barnes) reports, "The Air Force is reviewing decades-old contracts to determine whether" Boeing is liable for the longeron "defect that caused" an F-15 to break apart in mid-flight last November. Investigators say "ome of the longerons are too thin, or have ridges or rough surfaces that put too much stress on the structure." According to "Air Force officials,...Boeing's potential liability was difficult to determine because of a complex contracting history and the age of the aircraft." For example, the F-15 was "originally...designed to last 4,000 flight-hours," and later was "upgraded to last 8,000 flight-hours. The F-15 that crashed had 6,000 flight-hours." Lt. Gen. Donald Hoffman said, "Our question will be: What was the contractual arrangement at the time, and is there still residual liability? ... We have to prove that the flaw had significance."



http://www.aviationweek.com/aw/generic/story.jsp?id=news/F15S010908.xml&headline=Some%20F-15A-Ds%20Return%20To%20Flight%20Following%20Longeron%20Reviews&channel=defense [Broken]
Amy Butler (AviationWeek.com), Jan 9, 2008


This report (Airplane Stress Analysis) is historically significant. Our knowledge and understanding has changed dramatically since then, but the concerns are still the same.
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091143_1993091143.pdf
 
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The report discusses the various elements of stress analysis for aircraft components and construction, including longerons. It states that “Each component of the structure must be capable of withstanding the maximum stresses expected from flight loads and maneuvers.” It also explains that stress analysis requires engineers to accurately capture the loads placed on the component, as well as the material properties of the component itself. This is a critical first step in understanding the structural integrity of the component. The report goes on to discuss the types of structural defects that can occur in longerons, such as cracks, corrosion, and fatigue, and how they can lead to failure of the component. Finally, it examines the methods used to mitigate these defects and ensure the structural integrity of the component.
 
  • #3


As an aerospace engineer, I understand the critical importance of structural integrity in aircraft design and maintenance. The recent news of potential defects in the F-15's longerons is a concerning issue that must be thoroughly investigated to ensure the safety and longevity of these aircraft.

The responsibility of the engineer in this situation is crucial. We must ask the right questions, collect all available evidence, and ensure that all data is correctly interpreted. This is especially important as aircraft approach their original design life and undergo life extension programs.

The use of longerons in aircraft construction is a well-established and proven method, but as with any engineering system, it is important to constantly assess and improve upon design and manufacturing techniques. The potential defects in the F-15's longerons highlight the need for continuous monitoring and evaluation of aging aircraft, as well as the importance of using the most up-to-date materials and methods in their construction.

The Air Force's investigation into the liability of Boeing in this matter is also a critical step in ensuring accountability and addressing any potential flaws in the manufacturing process. It is important to determine the contractual arrangements at the time of production and whether there is any residual liability. The safety of our military personnel and the effectiveness of our aircraft must always be the top priority.

The report on airplane stress analysis from NASA is a valuable reminder of the historical significance of this issue. While our knowledge and understanding of aircraft structures has advanced significantly since then, the concerns and considerations remain the same. We must continue to strive for excellence in design and maintenance to ensure the safety and effectiveness of our aircraft.

In conclusion, the issue of structural integrity in the F-15's longerons is a significant matter that requires thorough investigation and evaluation. As responsible engineers, we must carefully examine all available evidence and work towards finding the best solution to ensure the safety and longevity of these aircraft.
 

1. What is "Air Frame Life Extension" for the F-15?

"Air Frame Life Extension" for the F-15 refers to a program that aims to extend the life of the F-15 aircraft by replacing aging components and making structural modifications. This is necessary to keep the F-15 operational and combat-ready for a longer period of time.

2. What is the problem with the F-15's air frame?

The F-15's air frame is facing structural fatigue due to long years of service and exposure to high stress and combat operations. This means that the aircraft's structure is weakening and may not be able to withstand the demands of future missions, jeopardizing the safety and performance of the aircraft.

3. Why is it important to address the F-15's air frame problem?

It is important to address the F-15's air frame problem because the F-15 is a critical asset for the military, used for air-to-air combat and ground attack missions. If the air frame is not properly maintained and strengthened, it could lead to costly repairs, grounding of the aircraft, and potentially compromising national security.

4. What are the steps involved in the Air Frame Life Extension program?

The Air Frame Life Extension program involves a thorough inspection and assessment of the F-15's air frame, identification of areas that need replacement or reinforcement, and implementation of necessary modifications and repairs. This may include replacing structural components, adding reinforcement brackets, and incorporating new technologies to improve the air frame's durability and longevity.

5. How long will the Air Frame Life Extension program extend the life of the F-15?

The Air Frame Life Extension program is expected to extend the life of the F-15 by at least 20 years. This will depend on the severity of the structural fatigue and the success of the modifications and repairs. However, with proper maintenance and continued upgrades, the F-15 may continue to serve for even longer than that.

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