EMB ERJ 170/175 ER/LR
Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Communication Breakdown
Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Became Reoriented
Human Factors; Weather
We were rerouted. Taxiing out we noticed significant mileage difference. I stopped airplane parking brake on and called dispatch. I discussed with dispatch that we did not need an alternate. I read new clearance to him and asked if we were good on gas with alternate or did we need to go back to gate to refuel. He said we were good to go with reroute and would send numbers via Acars. I checked fuel on flight plan on MCDU 2 and also on progress page and was satisfied with fuel on board. We had 11.6 for takeoff. After Takeoff we received Acars message that said not to takeoff and that he would run numbers without alternate. I then realized I should have waited for numbers before we got in the air. The numbers we received via Acars were sufficient. We landed with 5.2 fuel load. Flight was completed safely there was never an issue with too low fuel.
Captain had called me for a reroute to destination. He waited on the phone to see if he had enough fuel to depart. When I submitted the reroute; I never changed the reroute number in the flight planning tool. The numbers that came back where the original route numbers; and I told Captain he had enough fuel to depart. I told him I would ACARS him the numbers. When I was preparing his ACARS with the new numbers; I realized I had the original route in. I immediately ACARS him and told him not to depart yet and that I ran the wrong numbers. At that point; I wasn't sure if he had departed yet or not. When I sent him the correct numbers; I realized he had departed already. He had enough fuel; but I had to remove the alternate from the flight plan; which wasn't required for the destination airport. The flight continued with no disruptions.
EMB-175 flight crew reported receiving a reroute at the gate but not informing Dispatch until after pushback. The route is significantly longer; but the Dispatcher made an error in calculations and told the Captain not to depart. Airborne the error is detected and the alternate is legally eliminated providing sufficient fuel for the flight.