captjns:
On the 727 we would start all three, taxi from the apron, and once on the taxi-way... shut down number 3. In some cases, when we were below 150,000 (68 tons) we would shut down two and three,..
For most engines, shutting down & re-starting counts as an extra Low Cycle Fatigue (LCF) cycle on rotating parts. These life-limited parts must be
scrapped after a FAA-prescribed number of cycles.
Unless your carrier has some special dispensation about starting to only idle N2, and counting this as a partial LCF cycle, this practice can be false economy in a BIG way. Check with your engineering department!