Complex sentence: Another key policy of Ayling's time as CEO was the pursuit of a merger with American Airlines, however this was ultimately unsuccessful due to the conditions placed on the deal by regulatory authorities, the most painful of which was the sacrificing of landing slots at Heathrow.
Split into simpler sentences: Another key policy of Ayling's time as CEO was the pursuit of a merger with American Airlines. This was ultimately unsuccessful due to the conditions placed on the deal by regulatory authorities, the most painful of which was the sacrificing of landing slots at Heathrow.