Mark

The explanation above is wrong dealing with the flapper.

When a blower is operating..... it pulls "its own" flapper flush to the mount. This isolates the plenum and the secondary blower system from the airflow. All air is drawn from the tubes on the cargo pit ceiling. Air is pulled through "both" sensors and then exits the running blower.

In other words..... it the flapper fails on the running blower, air is drawn backwards through the non-running blower and not through the tubes and sensors.