Operations
Hobbs time
Time recorded by the Hobbs meter, which runs whenever the engine runs (typically oil-pressure activated). The usual basis for rental and instruction billing.
In practice
Hobbs time captures everything from start to shutdown - taxi, runup, holds - so it runs faster than tach time. Renters and students are usually billed on Hobbs because it reflects total engine operation. The gap between Hobbs and tach (commonly 10-20%) matters when comparing rates or reserve calculations, so a group should be explicit about which clock every number rides on.
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