Technically, all hits are on timers. But critical hits timers are crazy high, it’s like 10 minutes or something (I don’t even remember now), so you can say they almost guarantee you an assist. Regular hit timer is only 30 sec in Arcade and 1 minute in Realistic, so if the enemy is spinning for over 1 minute after you hit it with a regular hit in Realistic, you will miss the assist. This is not a bug, this is just how the game works. Before severe damage was added to the game such situations were very rare, as enemies were dying much easier. Now it’s much more common, as your hits can be “cancelled” while the enemy is spinning without any purpose.
This will give you severe damage (not an instant kill), which will lead to the kill when the battle finishes, and the person who caused severe damage is still in the battle (that’s another bug that will prevent you from getting a kill - if you leave the battle early, having the enemy severely damaged, you won’t get a kill!).
Probably black horizontal stabilizers, like RunaDacino assumed.
Link to my bug report about this problem: Community Bug Reporting System
This is why, in my opinion, it made more sense in the old system that severely damaged planes had different nameplate color (grey). At least you knew what is going on with the enemy. Currently the enemy can already be severely damaged, and you have no way to know this. So you waste your time and ammo and all you can get is an assist or finisher rewards.
I don’t play Air game mode much lately, because I already spaded all propellers in Air Arcade last year, and I rarely play jets in Air Realistic, mainly during some events.
I don’t think I ever saw this bug in Air Arcade, when I played propellers. But I definitely saw this in Air Realistic, when I used my guns on the enemy, got a few good hits (that are visible on the scoreboard, so it’s not a sync issue), the enemy died a few seconds later and I didn’t get an assist. I’m not sure what’s going on there, I’m too lazy to check the replay to try to figure this out. I’m not even sure if that’s related to the game mode or maybe it’s another bug with the severe damage mechanics. It’s also possible that when the devs changed how critical hits work, some older regular hits became less important and their timer was reduced (set to 0 or maybe 1 second or something). But this is just guessing.
I actually mentioned this in the severe damage test feedback topic. I got this situation during the severe damage mechanics test. It looks like no one reported this bug properly yet (guess why?). My forum post was of course never seen by any developer (after all, it was only a severe damage test feedback topic).
I have some suspicions why this is happening, but there is no sense to talk about this here, it’s only guessing. And it shouldn’t really be our job to figure out bugs anyway.