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In-flight acoustic measurements have been made on the exterior and interior of a twin-engined turboprop airplane under controlled conditions to study data repeatability. It is found that the variability of the harmonic sound pressure levels in the cabin is greater than that for the exterior sound pressure levels, typical values for the standard deviation being +2.0 and -4.2 dB for the interior vs +1.4 and -2.3 dB for the exterior. When insertion losses are determined for acoustic treatments in the cabin, the standard deviations of the data are typically ±6.5 dB. It is concluded that factors such as accurate and repeatable selection of relative phase between propellers, controlled cabin air temperatures, installation of baseline acoustic absorption even for "untreated" configurations, and measurement of aircraft attitude should be considered in order to reduce uncertainty in the measured data.