Protean War Engine

Crew is an activated ability of Vehicle cards. “Crew N” means “Tap any number of other untapped creatures you control with total power N or greater: This permanent becomes an artifact creature until end of turn.”

A creature “crews a Vehicle” when it’s tapped to pay the cost to activate a Vehicle’s crew ability.

If an effect states that a creature “can’t crew Vehicles,” that creature can’t be tapped to pay the crew cost of a Vehicle.

Some Vehicles have abilities that trigger when they become crewed. “Whenever [this Vehicle] becomes crewed” means “Whenever a crew ability of [this Vehicle] resolves.” If that ability has an intervening “if” clause that refers to information about the creatures that crewed it, it means only creatures that were tapped to pay the cost of the crew ability that caused it to trigger.