I have resolved the issue with both GAI AN-124 and AN-225 disappearing through the ground in FSX...it was all to do with contact points!
Just for everyone reference if you have the issue add the following contact points to the Aircraft.cfg file:
GAI AN-124:
[contact_points]
static_pitch = -0.5
static_cg_height = 19.4
tailwheel_lock = 0
gear_system_type = 1
max_number_of_points = 11
point.0 = 1, 51.5, 0.0, -20.2, 1000.0, 0, 3.0, 70.0, 1.4, 1.0, 0.900, 12.0, 12.0, 0, 200, 230
point.1 = 1, -35.4, -13.5, -20.5, 3000.0, 1, 3.5, 13.0, 0.8, 2.0, 0.900, 15.0, 15.0, 2, 200, 230
point.2 = 1, -35.4, 13.5, -20.5, 3000.0, 2, 3.5, 13.0, 0.8, 2.0, 0.900, 15.0, 15.0, 3, 200, 230
point.3 = 2, 84.55, 0.0, -6.56, 100.0, 0, 0.0, 0.0, 0.0, 0.0, 0.000, 0.0, 0.0, 4, 0, 0
point.4 = 2, -39.43, 0.0, -17.13, 100.0, 0, 0.0, 0.0, 0.0, 0.0, 0.000, 0.0, 0.0, 4, 0, 0
point.5 = 2, -146.21, 0.0, 8.36, 100.0, 0, 0.0, 0.0, 0.0, 0.0, 0.000, 0.0, 0.0, 4, 0, 0
point.6 = 2, -52.78, -121.34, -5.12, 100.0, 0, 0.0, 0.0, 0.0, 0.0, 0.000, 0.0, 0.0, 4, 0, 0
point.7 = 2, -52.78, 121.34, -5.12, 100.0, 0, 0.0, 0.0, 0.0, 0.0, 0.000, 0.0, 0.0, 4, 0, 0
point.8 = 2, -142.61, -46.01, -5.12, 100.0, 0, 0.0, 0.0, 0.0, 0.0, 0.000, 0.0, 0.0, 4, 0, 0
point.9 = 2, -142.61, 46.01, -5.12, 100.0, 0, 0.0, 0.0, 0.0, 0.0, 0.000, 0.0, 0.0, 4, 0, 0
point.10 = 2, -138.88, 0.0, 50.06, 100.0, 0, 0.0, 0.0, 0.0, 0.0, 0.000, 0.0, 0.0, 4, 0, 0
GAI AN-225:
[contact_points]
static_pitch = -0.1
static_cg_height = 18.6
tailwheel_lock = 0
gear_system_type = 1
max_number_of_points = 3 //12
point.0 = 1, 96.2, 0.0, -19.25, 4000, 0, 3.0, 70, 0.96, 1.00, 0.8, 8, 10, 0, 220, 250
point.1 = 1, -11.7, -14.0, -19.35, 4000, 1, 4.3, 0, 1.03, 1.07, 0.8, 13, 13, 2, 220, 250
point.2 = 1, -11.7, 14.0, -19.35, 4000, 2, 4.3, 0, 1.03, 1.07, 0.8, 13, 13, 3, 220, 250
Thank you.