Report and Recommendations of IAU (International Astronomical Union) Commission 50 (Identification and Protection of Existing and Potential Observing Sites), prepared by F.G. Smith, UNESCO Subvention -1977 - DG/2.1/414/44:
-Interference from Aircrafts (report excerpt): Condensation trails from aircrafts and the cloud build-up which often results from them, have very serious effects on atmospheric transparency. Aircraft lights may interfere directly with astronomical observations. It is essential that regular air routes, and where possible individual aircraft, should avoid the sky visible from observatory sites, since exhaust products and heat may affect conditions for astronomical observations.
-Recommendation: Aircrafts should not be allowed in the sky more than 10 degrees above the horizon, as viewed from the observatory site, nor within 5 km horizontal distance from the site .