Koos started hang gliding in 1993 in France and immediately knew this was going to be his sport. He picked up XC flying first in the Alps, later in Holland. In 1998 he started entering competitions and soon ended up organising the Dutch nationals and XC courses alongside. With some friends he learned tandem flying and tried to set records. After nearly making a 120km triangle in Australia on a tandem he reset his goals and made a tandem world record in France of 25km which still holds. During the Pre-Worlds in Hay in 2004 he learned that even with low cloud base you can get far in flat lands. In 2005 he set a new Dutch record of 218km starting from the Belgian border, crossing the whole country and landing near the North Sea. Since 2004 he is Dutch Champion. He has a strong interest in the psychological aspects of flying and is involved in safety. As a Forensic doctor he joined the medical advisory commission in Holland and explored human factors and altitude physiology. In 2009 he became active member of the HG subcommittee of CIVL.