British Airways and Air Berlin have each completed test flights to examine the effects of operating through the volcanic ash cloud over Europe.
Each flight - operated by a Boeing 747 for BA and an Airbus A330 for Air Berlin - carried the airline's chief executive on board as an observer.
BA's 747 flew a 2hr 46min flight between London Heathrow and Cardiff.
It initially climbed to 10,000ft before conducting a staged climb - at intervals of 5,000ft, spending 5min at each altitude - until it reached 40,000ft, where it remained for an hour.
The flight path took the aircraft west of Wales before it returned, descending from 19,000ft to 15,000ft over Ireland and landing at Cardiff at 20:41.
"Conditions were perfect and the aircraft encountered no difficulties," says BA, adding that the jet will be subjected to a thorough technical examination at its Cardiff engineering station.
Air Berlin's test involved an A330 operating from Dusseldorf to Berlin Tegel.
The carrier says the aircraft climbed to an altitude of more than 12,000m (39,400ft) but says it landed in the German capital "without any problems", and showed no evidence of damage.
Air Berlin, which has expressed irritation over the continuing precautionary closure of European airspace, had already conducted a series of positioning flights with other aircraft, also without any apparent problems.