Definition
An indication on a twin-engine aircraft's tachometer or propeller synchroscope where the two needles representing each engine's RPM are not aligned, showing that the engines are turning at different speeds and are out of synchronization.
Plain English
The two engines are running at slightly different speeds, so the needles showing their RPM don't line up.
Context Anchor
Seen on helicopter RPM gauges, especially during engine-off practice, power changes, or checks of the drive system that connects the engine to the rotor.
Derivation
“Split” here means separated or apart. “Needles” are the pointer hands on an instrument. Together, “split needles” means the instrument pointers are separated, not that anything is physically broken.
Why Pilots Care
Prompts immediate correction to regain the desired track and avoid navigational error or missed approach.
Intuition Check
Do not read “split needles” as damaged or cracked instrument pointers. In this phrase, “split” means the two RPM pointers are separated on the gauge.
Example Sentence 1
After leveling off in cruise, the pilot noticed split needles on the tachometer and adjusted the right throttle until the engines synchronized.
Example Sentence 2
Maintaining centered needles prevented the split that would have occurred during the crosswind leg.