I've seen this a couple of times when taking on swarms of Vals. I stayed in the gunners' FOF too long...
F4F-4 Wildcat dp setup:
[SYSTEMS]
system.0=%system_name.0%,410,0 <---- nose
system.1=%system_name.1%,205,2
system.2=%system_name.2%,307,3 <---- engine
system.3=%system_name.3%,102,7
system.4=%system_name.4%,76,8
system.5=%system_name.5%,25,9
system.6=%system_name.6%,102,10
system.7=%system_name.7%,410,18
system.8=%system_name.8%,25,19
system.9=%system_name.9%,76,20
system.10=%system_name.10%,76,22
system.11=%system_name.11%,410,23
system.12=%system_name.12%,120,15
system.13=%system_name.13%,205,24
system.14=%system_name.14%,128,25
system.15=%system_name.15%,25,26
system.16=%system_name.16%,153,27
system.17=%system_name.17%,153,28
system.18=%system_name.18%,128,29
system.19=%system_name.19%,410,30
system.20=%system_name.20%,120,12
system.21=%system_name.21%,205,31
system.22=%system_name.22%,128,32
system.23=%system_name.23%,25,33
system.24=%system_name.24%,153,34
system.25=%system_name.25%,153,35
system.26=%system_name.26%,128,36
system.27=%system_name.27%,410,37
system.28=%system_name.28%,410,38
system.29=%system_name.29%,205,41
system.30=%system_name.30%,153,39
system.31=%system_name.31%,153,40
[EFFECTS.0]
; System = Nose Structure
effect.0=10,LIBRARY,fx_smkpuff_s,
effect.1=30,LIBRARY,fx_engfire_s,
effect.2=60,LIBRARY,fx_engfire_m,
effect.3=95,LIBRARY,fx_airexpl_l,
effect.4=100,BREAK,6,
[EFFECTS.2]
; System = Engine
Location = 0.003,0.039,0.953
effect.0=10,LIBRARY,fx_fuelleak_s,
effect.1=30,LIBRARY,fx_engfire_m,
effect.2=60,LIBRARY,fx_engfire_l,
effect.3=95,LIBRARY,fx_airexpl_m,
effect.4=100,BREAK,7,
It's a combination of the DP and the format of the damage texture in the aircraft's texture folder. The damage texture bmps of the USN stock fighters have the nose sections completely erased with black alpha (rgb 0,0,0). This means that once the damage to the nose reaches the max threshold, the sim calls the damage texture by way of the mdl file and the DP's 100% effect, which is to "break off", in the Wildcat example above (effect.4=100,BREAK,6,) and the nose is just gone, even if the engine hasn't reached its 100% damage threshold yet. In this case the bird continues to fly -- with engine sound! -- but the nose is gone. The sim code apparently states that the integrity of the nose structure is secondary to engine function for airworthiness -- LOL.
Relative to this, the damage dice of the enemy's weapons determines how many rounds the nose can take to reach break point. The Zero has a mixed armament config -- 20mm cannons and 7.7mm guns -- but by coded default the CFS2 AI only use the guns when flying AC with mixed arms (i learned this from studying AI performance in A2A work). So if the stock Zero's 7.7mm guns have a dice of 1d1*11 per round, then it only needs to hit your Wildcat's nose with approx. 30 rounds to call up a 100% break point since the Wildcat's nose structure has a lifepoint of 410 (system.0=%system_name.0%,410,0).
Because the larger nose section absorbs more hits first than the smaller engine that it houses, the answer is to make the nose structure more durable, so that the engine fails before the entire nose structure does. Raising the life point to 1000 means the nose can take 90 7.7mm hits before "breaking off". Before that point, the engine has already reached its max life (307) and you're either in the silk or dead.