Shortly after completing Semper Invicta, you'll see an airship passing overhead. Use the Pip-Boy to tune into the military frequency named AF95. You'll receive instructions to meet Paladin Danse on the roof of the Cambridge Police Station.
If you thought the police station was the Brotherhood's main stronghold, think again. By boarding the Vertibird on the roof, you'll be transported to the Brotherhood's flagship, the Prydwen. While you're flying, feel free to fire away at any Raider encampments on the ground.
When you've arrived at the Prydwen, meet with Captain Kells shortly after disembarking. After that, attend the Elder Maxson's speech. After that, engage in a one-on-one conversation with him. Not only will he promote you, but he'll also give you a power armor. The power armor will be found at Station 03 on the main deck.
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