Airborne microorganisms in the upper troposphere and lower stratosphere remain elusive due to a lack of reliable sample collection systems. To address this problem, we designed, installed, and flight-validated a novel Aircraft Bioaerosol Collector (ABC) for NASA's C-20A that can make collections for microbiological research investigations up to altitudes of 13.7 km. Herein we report results from the first set of science flights—four consecutive missions flown over the United States (US) from 30 October to 2 November, 2017. To ascertain how the concentration of airborne bacteria changed across the tropopause, we collected air during aircraft Ascent/Descent (0.3 to 11 km), as well as sustained Cruise altitudes in the lower stratosphere (~12 km). Bioaerosols were captured on DNA-treated gelatinous filters inside a cascade air sampler, then analyzed with molecular and culture-based characterization. Several viable bacterial isolates were recovered from flight altitudes, including Bacillus sp., Micrococcus sp., Arthrobacter sp., and Staphylococcus sp. from Cruise samples and Brachybacterium sp. from Ascent/Descent samples. Using 16S V4 sequencing methods for a culture-independent analysis of bacteria, the average number of total OTUs was 305 for Cruise samples and 276 for Ascent/Descent samples. Some taxa were more abundant in the flight samples than the ground samples, including OTUs from families Lachnospiraceae, Ruminococcaceae and Erysipelotrichaceae as well as the following genera: Clostridium, Mogibacterium, Corynebacterium, Bacteroides, Prevotella, Pseudomonas, and Parabacteroides. Surprisingly, our results revealed a homogeneous distribution of bacteria in the atmosphere up to 12 km. The observation could be due to atmospheric conditions producing similar background aerosols across the western US, as suggested by modeled back trajectories and satellite measurements. However, the influence of aircraft-associated bacterial contaminants could not be fully eliminated and that background signal was reported throughout our dataset. Considering the tremendous engineering challenge of collecting biomass at extreme altitudes where contamination from flight hardware remains an ever-present issue, we note the utility of using the stratosphere as a proving ground for planned life detection missions across the solar system.

Flight testing for aeroservoelastic clearance is an expensive and time consuming process. Large degree-of-freedom high-fidelity nonlinear aircraft models using computational fluid dynamics coupled with finite element models can be used for accurately predicting aeroelastic phenomena in all flight regimes including subsonic, supersonic, and transonic. With the incorporation of an active feedback control system, these high-fidelity models can be used to reduce the flight-test time needed for aeroservoelastic clearance. Accurate computational fluid dynamics/finite element models are computationally complex, rendering their runtime ill suited for adequate flight control system design. In this work, a complex, large-degree-of-freedom, transonic, inviscid computational fluid dynamics/finite element model of a fighter aircraft is fitted with a flight control system for aeroelastic oscillation reduction. A linear reduced-order model of the complete aeroelastic aircraft dynamic system is produced directly from the high-order nonlinear computational fluid dynamics/finite element model. This rapid runtime reduced-order model is used for the design of the flight control system, which includes models of the actuators and common nonlinearities in the form of rate limiting and saturation. The oscillation reduction controller is successfully demonstrated via a simulated flight test using the high-fidelity nonlinear computational fluid dynamics/finite element/flight control system model.

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