This step can only be done after flying the plane to the bridge three times.
After that has been done, head to the Citadel Tunnel's massive spiral staircase. Specific numbers must be entered in the three volt meters at the bottom. It's nearly impossible to enter all four numbers in one afterlife, so get a teammate to revive the downed player, and have another teammate finish the number sequence when the first teammate falls.
These numbers are: