Time to Major Overhaul MOH
I see ads for aircraft for sale that will show engine time but with a note that it is hobbs time. 
How does one use Hobbs to determine actual engine time for overhaul?
3 Replies
Excellent answer. Makes more sense out of the MOH 
Hobbs is merely a timer from when the power is applied.
Tach is usually set at some rpm number to read actual clock time, slower if below rpm, faster if above rpm level. 
1147 Posts
If that's all there is, you just use it.  There is no FAA- accepted conversion factor from Hobbs to tach.  But if it's for personal use, the manufacturer's recommended overhaul interval is merely a guideline, not mandatory, so you can overhaul on condition rather than hours.  So, it's a lot more important that you monitor your engine's condition with things like oil analysis, engine monitor trends of EGT/CHT, valve guide wear checks, exhaust valve visual borescope inspection, etc.  You may find your engine can go well beyond the book TBO or maybe it runs out of life before that number comes up.

The problem with buying a plane is that you may not have all that trend/condition data available from the seller, which means a more careful inspection of the engine and its records at pre-buy.