A sensor unit for a fingerboard latch assembly comprising a latch bracket and a latch rotatably mounted on the latch bracket comprises a sensor arranged to sense the orientation of the latch and a wireless, optical or other communication unit arranged to communicate the orientation of the latch sensed by the sensor. The sensor unit may be mountable on the latch and comprise an orientation sensor arranged to take measurements that are dependent on the orientation of the sensor unit, and a processor arranged to derive an orientation signal representing the orientation of the latch from the measurements, the communication unit being arranged to communicate the orientation signal. A monitoring system receives the sensed orientations from plural sensor units.