Found one:
faction_weight = bu_im_grp, 2
encounter = area_assault, 4, 0.500000<--this means encountr at D4
faction = bu_im_grp, 0.500000
but..
in faction_prop you only have D15-19
gd_im_ge_fighter6_d15-19 a couple of times (not sure why you need several, one would do the same job.
If you want the custom IMG faction to always use eagles you could just change the:
npc_class = lawful, class_fighter, d15, d16, d17, d18, d19
to
npc_class = lawful, class_fighter, d1, d2, d3, d4, d5, d6, d7, d8, d9, d10, d11, d12, d13, d14, d15, d16, d17, d18, d19
instead.
Even if you have no intention of using lower difficulty settings you still need to add 1-19 for each faction. If you had a random isiion you could end up with the game crashing when it tries to find some of the lower difficulty entries.

i've yet to change that eagle to a newer custom ship

update with new ships on all:

System:

[EncounterParameters
nickname = area_defend
filename = missions\encounters\area_defend.ini

[EncounterParameters
nickname = area_assault
filename = missions\encounters\area_assault.ini

[Zone
nickname = Zone_ST12_pop_ambient_01
pos = -32346, 0, -31952
rotate = 0, 0, 0
shape = SPHERE
size = 16567
sort = 1
toughness = 4
density = 3
repop_time = 25
max_battle_size = 7
pop_type = br_p_grp, base_cluster_law
relief_time = 35
faction_weight = bu_rb_grp, 20
encounter = area_defend, 4, 1.000000
faction = bu_rb_grp, 1.000000

[Zone
nickname = Zone_ST12_pop_ambient_02
pos = -33135, 0, -31557
rotate = 0, 0, 0
shape = SPHERE
size = 5128
sort = 51
toughness = 4
density = 3
repop_time = 25
max_battle_size = 2
pop_type = br_p_grp, base_cluster_law
relief_time = 35
population_additive = false
faction_weight = bu_im_grp, 2
encounter = area_assault, 4, 0.500000
faction = bu_im_grp, 0.500000

Faction Props:

[FactionProps
affiliation = bu_rb_grp
Legality = lawful
nickname_plurality = singular
msg_id_prefix = gcs_refer_faction_bu_rb
jump_preference = jumpgate
NPC_Ship = bu_rb_jedi
NPC_Ship = bu_rb_snow
voice = pilot_f_ill_m01
voice = pilot_f_ill_m02
mc_costume = mc_fc
space_costume = pi_pirate1_head, pi_pirate8_body, comm_br_darcy
firstname_male = 524722
lastname = 228908, 229207
rank_desig = 197029, 197030, 197031, 5, 9
formation_desig = 197808, 197820
formation = fighters, fighter_basic

[FactionProps
affiliation = bu_im_grp
Legality = unlawful
nickname_plurality = singular
msg_id_prefix = gcs_refer_faction_bu_im
jump_preference = jumpgate
NPC_Ship = bu_im_tie1
NPC_Ship = bu_im_tie2
voice = pilot_f_ill_m01
voice = pilot_f_ill_m02
mc_costume = mc_fc
space_costume = pi_pirate1_head, pi_pirate8_body, comm_br_darcy
firstname_male = 524723
lastname = 0
rank_desig = 0
formation_desig = 197808, 197820
scan_chance = 0.0
scan_announce = False
formation = fighters, fighter_basic

NPC Ships:

[NPCShipArch
nickname = bu_rb_jedi
level = d10
loadout = jedi_load
ship_archetype = jedi
state_graph = FIGHTER
pilot = pilot_company_ace
npc_class = lawful, class_fighter, d1, d2, d3, d4, d5, d6, d7, d8, d9, d10, d11, d12, d13, d14, d15, d16, d17, d18, d19

[NPCShipArch
nickname = bu_rb_snow
level = d10
loadout = snow_load
ship_archetype = snowspeed
state_graph = FIGHTER
pilot = pilot_company_ace
npc_class = lawful, class_fighter, d1, d2, d3, d4, d5, d6, d7, d8, d9, d10, d11, d12, d13, d14, d15, d16, d17, d18, d19

[NPCShipArch
nickname = bu_im_tie1
level = d10
loadout = tie1_load
ship_archetype = tiefighter
state_graph = FIGHTER
pilot = pilot_company_ace
npc_class = lawful, class_fighter, d1, d2, d3, d4, d5, d6, d7, d8, d9, d10, d11, d12, d13, d14, d15, d16, d17, d18, d19

[NPCShipArch
nickname = bu_im_tie2
level = d10
loadout = tie2_load
ship_archetype = tieinterceptor
state_graph = FIGHTER
pilot = pilot_company_ace
npc_class = lawful, class_fighter, d1, d2, d3, d4, d5, d6, d7, d8, d9, d10, d11, d12, d13, d14, d15, d16, d17, d18, d19

Still CTDs with this update

Edited by - BlasterUK on 7/2/2006 7:25:31 AM

A few bits i'm not keen on:
toughness = 4
density = 3<--quite low
repop_time = 25
max_battle_size = 2<--quite low too, the encounters give more
pop_type = br_p_grp, base_cluster_law<--Br_p_grp? they aren't in the zone
But, for my money its the custom names that have been given in faction_prop. I tried making some myself with already working encounters and it was CTD every time they spawned.
Give it a go with existing names just to see if it will work. It might even be best to use aleady existing npcship entries just to see if the factions work first. Do these factions work in any other zones or is this the first one you have worked on?