This paper reports on the successful first deployment of a new, autonomously operating measurement system on a Grob G 520 Egrett aircraft, which was used as a chase aircraft to perform in-flight aerosol and trace gas measurements of engine exhaust from other aircraft. A suite of custom-built and commercially available instruments was selected, modified, and adapted to operate in the unpressurized compartment of the Egrett over a wide range of ambient temperatures and pressures. We performed these first in-flight emission measurements at cruise altitudes between 7.6 and 10.4 km (FL250 and FL340) behind a Piper Cheyenne, a twin-turboprop aircraft powered by Garrett/Honeywell TPE 331-14 engines over Texas in April 2022. The instrumentation and inlets on the Egrett were designed to measure non-volatile particulate matter (nvPMDp>10), total particulate matter (tPMDp>10), nitrogen oxides (NO and NO2), water vapor (H2O), carbon dioxide (CO2), and contrail ice particles. All instruments were operated in relevant plume conditions at cruise altitudes and distances ranging from 100 to 1200 m between the two aircraft. The instruments proved to have high reliability, a large dynamic range, and sufficient accuracy for measuring the emissions of the turboprop engine. We derived the emission indices (EI) for tPM, nvPM, and NOx at cruise. The particulate emission indices range from 9.6 to 16.2 × 1014 kg-1 (particles per kg fuel burned) for EItPM and from 8.1 to 12.4 × 1014 kg-1 for EInvPM (medians). For NOx we find rather low EINOx between 7.3 and 7.7 g kg-1 for EINOx (medians). Furthermore, the tPM aerosol size distributions have been measured in the exhaust plume, taking into account the size-resolved sampling efficiency of the instrument. The analysis of the size-resolved emission index indicates a log-normal distribution with geometric mean and standard deviation at Dg=27.5±2.0 nm. This geometric diameter value is in the range of jet engine soot emissions previously measured in flight. The measurements help to constrain the climate impact of small-class turboprop engines and need to be compared to larger turboprop aircraft in the future. The current work provides a benchmark for future alternative H2 propulsion systems, such as fuel cells and direct combustion engines.