Air Frame Life Extension - F-15 Problem

[Astronuc]

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

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cheaper than new fighters, Corley explained that the F-15's age means other parts could soon fail, and also that it is outdated.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
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