SMART 0 TSO: Smart zero-time since overhaul.

0= Zero

T: Time
S: since
o: overhaul
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Zero time

This refers to section 91.421 Rebuilt engine maintenance records. This rule allows the owner or operator to use a new maintenance record, without previous operating history for an aircraft engine rebuilt by the manufacturer or an agency approved by the manufacturer. So the rebuilt engine from the manufacturer starts life all over again as a zero-time engine.

https://www.aviationpros.com/home/article/10385867/parts-pri...
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Breathe new life into your aircraft with a ‘zero-time since overhaul’ (0 TSO) engine exchange at a fixed price.

Fly Like New at a Fixed Price

Breathe new life into your aircraft with a ‘zero-time since overhaul’ (0 TSO) engine exchange at a fixed price. The PT6T Engine Exchange Program, a P&WCSMART offer, is a cost-effective approach to resuming flight with engines that have an increased new part mix at an attractive and competitive fixed price.

https://www.prattwhitney.com/en/services/pwc-engine-services...
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والله أعلم