TBO in hours is measured by the actual run time of the engine.
But the TBO "in years" begins on the FIRST RUN of the engine.
The Line Maintenance Manual states the limit applies to "calendar life since initial start of operation".
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Note that TBO start time is not the same as the warranty start time!
The warranty conditions service letter (not TBO time) says the following...
This warranty will be in effect:
− in case of the warranty period being measured in hours of operations: from the date the Product is first put into use.
− in case of the warranty period being measured in months: from the date of delivery to the first retail consumer, but in no event later than SIX (6) MONTHS after BRP-Rotax invoice date regarding its sale of the Product to an authorized Rotax Distributor/Dealer