Tried replaying it today, and cant get to the plane.
Ride the plain on the fake ending, fight the boss and beat it. Then I walk up the stairs and back at PAP room instead of plane.
One thing I remember I did that might trigger this error is that, before going to the plane, you need to shoot the red lamp and use the switch. Instead, I use the switch before shooting the red lamp.
Is this a known bug?
It's not a bug after climbing the ladder you should look around the buildable shield area, then you'll find a part...