A319 flight crew reported that the ground crew prematurely disconnected the air supply while doing an air-start.

Narrative

ZZZ ground crew is great. I operate through ZZZ regularly and appreciate their hard work. On Day 0 at XA:00 we had a near miss at Gate XX. On the previous flight from ZZZ1; our APU failed while starting the #2 engine and thus we had to accomplish an air cart start in ZZZ. The ground crew knew about this and we had several discussions with the ground crew; gate agent & station ops during the turn-around. When it came time to accomplish the engine start; communications were clear and standard. I instructed them to 'start air' and we began spinning the #1 engine. As the engine accelerated through ~20% N2; and the fuel flow indicated that the start sequence had begun; we heard the ground crew shut off the air supply and begin disconnecting the ground air cart. This was an extremely dangerous situation because the fuel control unit had begun supplying fuel to the engine and the igniters were firing. When the air supply was removed from the starter; the N2 rotation slowed and the start sequence effectively died. The amount of unburnt fuel and smoke that came from the engine caused concern with the First Officer and I; and I immediately asked the ground crew what had happened. He told me over the interphone that they had placed the ground air cart on the wrong side of the airplane and didn't realize we were going to start the #1 engine first. I relayed to him that I understood his concern but to NEVER disconnect air during an engine start. Having a good understanding of the situation and having seen this scenario play out before; I knew we had not exceeded any limitations. I instructed the First Officer to perform a 30 second dry crank once the air cart was reconnected on the other side of the airplane. All indications were normal and we followed the 30 second dry crank by a normal start of the #1 engine. The remainder of the pushback and cross-bleed start went according to plan; but I want to make sure that the ground crew NEVER disconnects ground air during a start attempt again. The consequences of doing so or attempting to start again without clearing the unburnt fuel with the 30 second dry crank could have been disastrous