We investigate the circumstances required for aircrew and passengers to experience an increased radiation exposure rate from a solar energetic particle (SEP) event occurring during a flight. The effects of the 14 July 2000 National Oceanic and Atmospheric Administration S3 class SEP event are examined using ground-based and satellite measurements together with coincident measurements made using a tissue equivalent proportional counter (TEPC) on board a Virgin Atlantic Airways flight from London Heathrow to Hong Kong. In this paper we present the first measurements made during a SEP event using a TEPC at flight altitudes. Our results indicate that there were no increased radiation levels detected during the flight due to the SEPs, but the measurements agreed well with the CARI-6 model calculations made using a heliocentric potential value derived immediately prior to the SEP event. In addition, a prolonged increase in the >85 MeV particle flux is observed for up to 2 days after the SEP onset by the SAMPEX spacecraft at latitudes >55¿.