Abstract

Under Title 14 Code of Federal Regulations Part 139, Certification of Airports, certificated airports are required to conduct output-based proportioning tests when using fluorine-free foams (F3s) to ensure their aircraft rescue and firefighting (ARFF) vehicle is in an operationally capable state. Output-based testing with F3 can be restricted by environmental regulations and can incur significant clean-up costs. This research identified the currently approved MIL-PRF-32725 F3 concentrates and evaluated whether a surrogate liquid could be used in place of F3 concentrates during ARFF vehicle foam proportioner verification testing. Two MIL-PRF-32725 F3 concentrates and matched surrogate liquids were evaluated. Bench-scale testing with a custom-built, vacuum-based fixture and full-scale output-based testing with an ARFF vehicle were used to evaluate surrogate performance under controlled and operational conditions.

Results indicated that the surrogate replicated F3 flow behavior when its viscosity was closely matched to the target concentrate. Vacuum-based testing with surrogate liquid well matched to its F3 concentrate showed an average difference in flow rate across all discharges of 0.71%. When the surrogate liquid was less well matched, an average difference in flow rate across all discharges of 4.61% was observed. During output-based testing, the average difference between F3 and surrogate proportioning rates was 0.38% proportioning rate, with differences ranging from 0.01% to 0.93% proportioning rate. Differences in proportioning rates during output-based tests can be attributed to a lack of temperature control during testing. These data are expressed in proportioning rate percentage (i.e., a portion of a nominal 3% proportioning rate). Conductivity-based proportioning rate measurements produced results comparable to tank drop-based proportioning rate measurements. Discharges of F3s had an average difference of 0.32% (standard deviation (SD) = 0.41) proportioning rate and discharges of surrogate had an average difference of 0.33% (SD = 0.29) proportioning rate, between the two proportioning rate measurement methods. The results indicate that a properly mixed and viscosity-matched surrogate liquid can serve as an effective substitute for F3 concentrates during output-based ARFF vehicle foam proportioner verification testing.