When does engine synchronization occur?

• A. N1 +/- 1.5% or N2 +/- 2.5% with the right engine as master
• B. N1 +/- 1.5% or N2 +/- 2.5% with the left engine as master
• C. N1 +/- 2% or N2 +/- 3% with any engine as master
• D. Only during engine start

Answer: (B)

Engine synchronization is about keeping both engines at the same RPM so thrust is balanced and vibration is minimized. In this setup, one engine serves as the reference (master) and the other follows (slave). The left engine is the master, which means the system continuously trims the right engine to stay within tight limits of the left.

The tolerances shown—N1 within roughly ±1.5% and N2 within roughly ±2.5%—define how close the follower must stay to the master. Matching the low-spool speed (N1) more tightly helps ensure similar thrust levels between the engines, while the high-spool speed (N2) can have a slightly wider allowable difference but still maintain smooth operation. Synchronization isn’t just at engine start; it remains active whenever both engines are running to adjust for changes in throttle, loading, or air conditions.