On May 30, 1:51*pm, Marc wrote:
On May 30, 12:31*pm, Bill D wrote:

In the three year interval from 2009 to 2011, the number of active
tugs increased from ~175 to ~ 210. *That was a mystery but,
apparently, the additional tugs are being used to tow privately owned
single seat gliders.

How are the number of active tugs determined?

Marc

By getting on the phone and asking people. That number is cross
checked against the "Where-to-fly" database.