Mechanical Backups? Thinking about installing an MGL ASV-1 or a Kanardia combo. They both have Airspeed, Altitude, and Vertical speed all in one place with nice big(ish) numbers. Thinking about putting it right in the middle with a slip ball right below it. I will make sure to have plenty of battery power in the event of an engine out. Anybody have opinions about the necessity of mechanical backup instruments?