Ignition problems with MJU-J 7A/B flares

After firing MJU-7A/B flares from F16 fighters severe soot markings were found on the aircraft around the flare dispenser. Also parts of the safety and ignition (S&I) device were found in the flare magazine. Because of the potential risk the Royal Netherlands Air Force (RNLAF) asked TNO to investigate the risks, possible causes and solutions for these problems during flare ejection. After preliminary research the cause of the problems was found in the functioning of the BBU-36/B impulse cartridges. Due to incorrect functioning of the cartridge the acceleration of the flare pellet out of its casing is too slow. Hot gasses from the impulse cartridges are then able to bypass the S&I device and ignite the flare inside the casing. An experiment was performed to determine the “worst case” damage caused by in-casing ignition of multiple flares. The BBU-36/B impulse cartridges have been characterized by performance tests in a closed vessel and an acceleration test. Pressure curves and high-speed video images showed that the actual performance of the impulse cartridge was strongly dependant on the test applied. It was learned that, in contrast to the acceleration test, the conventional closed vessel test does not reflect the behaviour of an impulse cartridge in reality. Flares were characterized in relation to the force needed to push the flare pellet out of the casing. It was found that certain combinations of impulse cartridge and flare lots were responsible for the high number of in-casing flare ignitions during flight.