Fokker F-27 F-BYAH suffered aileron vibrations in flight. Modifications were made during December 1978 and January 1979. A test flight was planned on January 28. The airplane was flown by an Air Rouergue captain and co-pilot. Also present on board were a two engineers, a Fokker technician and an Air Alpes pilot. Planned flight time was fifteen minutes at an altitude of FL70 near the RZ beacon. F-BYAH took off from runway 31 at 17:15. Two orbits were flown near the RZ beacon after which the crew positioned the plane for an approach to runway 31. The aircraft struck a cloud-shrouded mountain while turning towards runway heading in preparation for an ILS approach. PROBABLE CAUSE: “Failure of the crew to level off at the prescribed altitude either due to inadequate monitoring when passing through 4400ft or an altimeter setting error by confusion between QNH and QFE. Other factors were a lack of flight preparation; non use of the radio altimeter and a possible CofG outside limits affecting the longitudinal maneuverability. Also, no approach card was evident in the cockpit for this aerodrome and the presence of an additional person on the flight deck may have distracted the crew.”