The last airline I worked for had a practice of putting spares on the aircraft to 'protect the service' for any system that had multiple intermittent faults. We used to put the spares in the coat closet on the flight deck to just strapped them into the jump seat. After some queries were raised during EASA inspections at outstations the policy was changed to manifest the spares as freight and put them in the bulk cargo hold. Needless to say that after that we lost hundreds of thousands of dollars worth of spares that went missing down route when the baggage handlers decided to offload the boxes and then they were lost in the system..
