I recently setup a new hexacopter and loaded ArduCopter 2.5.5. I'm using an APM 1. After plugging in the battery and using wireless telemetry to connect to the Mission Planner the altitude readings are all over the place:
The only workaround I've found so far is to turn on my hexa and make an initial link via USB. I then disconnect and reconnect via wireless telemetry...which is a pain since I have to remove and reattach the dome every time.