Airship Down

Airship Down is an Institute main quest in Fallout 4. In this quest, the Sole Survivor, along with an army of synths, storms Boston Airport in an attempt to cripple the Brotherhood of Steel. This is done by hacking Liberty Prime into targeting the Brotherhood of Steel as Chinese Communists. Liberty Prime then destroys the Prydwen via his dual eye lasers.

Detailed walkthrough
Doctor Li or Doctor Watson greets the Sole Survivor and then leaves Rosalind Orman to explain the details of this quest. She explains that the target is the base of the Brotherhood of Steel operations in the Commonwealth - the Boston Airport. It is shielded by an electromagnetic shield that makes it impossible for the Institute to teleport troops into the base. The plan is that the Sole Survivor will take down the field by infiltrating a poorly-guarded part of the airport, and from there work a way through to the generators and destroy them. For each generator that is destroyed, the Institute will teleport some synths into the base. The player character is provided with a couple of Institute beacons, which are used to relay synths to the location in which they are thrown.

After that, the Sole Survivor will go to the main target - Liberty Prime. Doctor Li explains that it is an old weapon, and also tells about her role in making it operational. Working with Liberty Prime has given her good knowledge on how it works, which will now be used to manipulate it. Rosalind enthusiastically explains that a synth has been programmed with a computer virus and will transfer it to Liberty Prime, making it believe that the Brotherhood airship the Prydwen is a communist vehicle. The Sole Survivor is to escort that synth safely to Liberty Prime.

Following the conversation the player character is relayed from the Institute to the airfield in order to execute the plan. Firstly, one fights their way to the generators, and then to Liberty Prime. They then climb up to the head of the robot so that the hacker synth can begin uploading the virus. The player character must then defend the hacker against a massive assault from both the air and ground. Before finishing the upload, Elder Maxson attacks via a vertibird. If the vertibird is destroyed, he will fight on the ground. This gives the opportunity to obtain his Elder power armor and his unique weapon: Final Judgment. When the upload is complete, Liberty Prime will announce that it will attack the "communist" vessel, and fires its eye laser at the ship. As the ship begins to fall, the Sole Survivor will be teleported a short distance away to observe the fiery wreckage.

The quest completes, and the next quest starts.

Bugs

 * If the player killed all of the BoS members at the airport before destroying the last generator, they may not be able to complete the objective "Reach Liberty Prime".
 * Elder Maxson may not appear during the battle, making his unique items unobtainable.
 * If fast traveling away from the airport after killing BoS members, their weapons and armor may respawn on return, including that of Quinlan, allowing for extra loot. This may even occur if the player uses the lift to get to the third generator.
 * When asked to "work with members of the Institute," the quest will take the player to Evan (if Madison Li is killed or persuaded to join the BoS). They will be able to ask him "want to fill me in on what's happening", but he will never give them the information, but will instead repeat the same lines over and over again.
 * Dialogue with Dr. Li and Rosalind Orman will sometimes be unable to finish, as T.S. Wallace often pushes the two away from each other.