As stated above, the accepted standard for mutual aid availability is, what I heard it referred to as the 1/3 rule. It’s not a mandate and hopefully everyone plays nice. If a department has 9 engines & 3 ladders, then 3 engines and 1 ladder could be used for mutual aid. What is good about their system is that, in theory, the only empty firehouses which need to be covered are the ones in the city which has the fire. If Braintree has a 4th alarm going, then only the Braintree firehouses would need station coverage. The cities surrounding Braintree would still be able to cover their own stations because they didn’t send 50% or more of their apparatus.
I’m not sure what would happen if during Braintree’s 4th Alarm, Milton got a 3rd Alarm. Both fires would be pulling apparatus from many of the same cities. Probably a MetroFire dispatcher’s nightmare!! Again, hopefully some out of the box thinking, common sense, and understanding chiefs who would not denying the request to respond to another fire because they reached their 1/3 commitment. Situation dependent, but hopefully they would send additional equipment to the 2nd fire and request coverage.
This is where task-forces come in handy. If the fire is in the south part of MetroFire and there is a rapid deployment of apparatus, then a north task-force from various cities (6 engines, 2 ladders, 2 chiefs) could be dispatched to the effected area. Once they arrive at the assembly point, the engines and ladders can be parsed out to provide coverage where needed. (Basically a bulk movement of apparatus with one dispatch.)