In the [airplane_geometry] section of the Aircraft.cfg file change
spoiler_limit = 19860
to
spoiler_limit = 60
The Avengers did not have dive brakes, but the gear could be put down to slow a dive.
If you would rather have it do that without damaging the landing gear you would need to edit the contact point data to something like:
point.0 = 1, -13.68, 0, -4.5, 3300, 0, 0.549, 180, 0.253, 2.5, 0.498, 5, 5, 0, 500, 500
point.1 = 1, 2.02, -5.917, -7.7, 3300, 1, 0.549, 0, 0.633, 2.5, 0.8, 7.5, 7.5, 2, 500, 500
point.2 = 1, 2.02, 5.917, -7.7, 3300, 2, 0.549, 0, 0.633, 2.5, 0.8, 10, 10, 3, 500, 500
Unfortunately this will not be as effective at slowing the dive as the phantom dive brakes.
A combination of using both would provide the proper visual appearance while changing the aerodynamics for a controlled dive.
p.s. Be careful with implementing these changes across the board, because not all of the available Avengers have this issue.