Alex: As, as in present day spelling of this town, is the same as Asch as in war reports.
Luc: your info likely from the 303rd BG Mission 364 account:
http://www.303rdbg.com/missionreports/364.pdf
Take a look at story of navigator Lt Howard Burr as well:
http://www.303rdbg.com/news/2010-12-28.html
At start of his account 'A Most Unusual War Story' it says:
After heavy flak damage from anti-aircraft fire and only one engine running, the Damon Crew was forced to land near the Belgian border.
I'm sure he would have written Brussels/Melsbroek if this was the case. But with As(ch) actually near the BE-NL border, it's likely that Y29 is the airfield where they landed.
Groet,
Leendert