Since the important issue has yet to highlighted here, I will explain,
I refer to the B737NG here, but this may apply to other types as well,
With the airplane below 16,000 feet, including on the ground, a vacuum compressor provides the required motive forces to flush the toilets,
Unfortunately, this compressor is provided with a pressure switch that shuts it off above FL160, with the cabin pressure differential taking over.
Again unfortunately, the vacuum compressor is not informed of the lack of cabin pressurization, and will continue to remain at rest in these circumstances.
The result is obvious, the toilets won't flush above FL160 without cabin differential pressure.
Just a thought....
