A McDonnell-Douglas DC-8 NASA research aircraft inadvertently flew into an airborne volcanic ash plume from the 26 February 2000 eruption of Hekla Volcano. Filter samples from the aircraft were compared with normal use and pristine clean filters using SEM, energy-dispersive x-ray spectrometer, and Nicolet FTIR spectrophotometer analyses. These analyses confirm that the DC-8 encountered airborne volcanic ash from Hekla Volcano. This result is supported by independent onboard heated aerosol observations at the time of the encounter. The analyses further demonstrate the ambiguous nature of the dual band thermal IR (split window) method for detecting volcanic ash from the point of view of aviation safety. They also highlight the utility of in situ aircraft filter-based observations of volcanic aerosols for scientific purposes.