The reason why I need that "if ((uS / US_ROUNDTRIP_CM) != 0) &&" is that the returned value from the sensor is 0 (zero) when the UAV (quadcopter) is flying higher then the sensor can sense. The sensor is "seeing" from 0 upto 3 to 4 meters. The quad max range is 300 meters. So when I only us "if (uS / US_ROUNDTRIP_CM > 50)" the landing gear folds down because it gets a 0 (zero).
And zero is less then 50 ( < 50 ).