The Flash documentation explains what this command does as follows:

A Boolean value that is true if the player is running in an environment that supports communication between Flash Player and accessibility aids; false otherwise.

The problem comes down to interpretation of this description. I have been misunderstanding this for over a year. I was shocked to find out that I had done so. Thankfully I have not depended on this command by itself.

The environment is the space in which the Flash player is contained. This could be a browser or the standalone application. The only supported environment that is read by this property is the Bowser and in fact – it is Internet Explorer in Windows.

This command actually checks for an environment that supports Microsoft Active Accessibility (MSAA). MSAA is the standard method of communication between accessibility technologies and applications on a Windows computer and what the Flash player uses to communicate with accessibility aids. You can find out more information on the Microsoft Active Accessibility Start Page.

This is standard to Windows and does not actually mean that accessible technology is present – never-mind active. If you run the following test – http://niquimerret.com/systemchecks/ you should get a true if you are in IE in Windows otherwise false. I have not found any other situation where it is true – let me know if you find one.

So what does this mean for Accessibility in Flash? Well it means that we need to look a bit further than just this property. The Accessibility.isActive() method is the better solution. This will feed back if the Flash Player is communicating with MSAA. It can take a few seconds for this to result in true so it is useful to use this only after about 2 seconds.

Note: There is no accessibility implemented in the standalone player.