Consultation expired with comments. The proposed Special Condition (SC) has been editorially updated based on the comments received, and the corresponding Comment-Response Document (CRD) has been completed by EASA.
Details
Identification of issue
CS 25.993(f)/ CS 25J993(f) is a result of an accident on UA Flight 227 Boeing Model 727 airplane on 11th Nov. 1965, where fuel lines had been installed in areas where fuel line flexibility had not been required. It was determined the accident was a survivable crash. However, rigid aluminum fuel lines that were routed in an aluminum shroud became severed, releasing fuel under pressure, which was ignited by either sparking generator leads, sparks from the fuselage on the runway, or both. The subsequent fire caused numerous fatalities that could have been prevented if the post-crash fire had not occurred. Prevention of fuel line rupture and fuel leakage is the objective of CS 25.993(f)/ CS 25J993(f) and it states that fuel lines be designed to allow a reasonable degree of deformation and stretching without leakage.
During past certification programs, the “reasonable degree of deformation and stretching” has been misinterpreted as the expansion and contraction that would occur in normal service or in a minor crash landing. However, CS 25.993(f) was promulgated to ensure that “reasonable” design precautions are taken to prevent fuel leakage during or after a survivable crash; intent of the regulation is fuel line robustness against elongations and deformations that would occur in a survivable crash.
In addition, when CS 25.993(f) was created, installation of fuel lines outside the fuselage was not envisioned. However, Airworthiness Authorities during CATA harmonization exercise confirmed that same precautions shall be applied for fuel lines outside the fuselage contour as the resulting consequences would be equivalent to a fuselage internal fuel line leak after fuselage break-up.
Considering all the above, the following Special Condition is proposed.