IRST not locking?

I’ve had several instances when I try to lock someone that is well within range, but the IRST system just won’t lock. The target is generally headed towards me, but given the number of times I’ve locked head-on targets with IRST, these stand out as odd. It becomes even more strange given that in these occasions I typically keep the target in my reticle as they close. Even as they get within cannon range (using a MiG-23ML), they still don’t lock. The skies are clear, so it’s not clouds obscuring IR, or any other phenomenon that I can discern.

In addition to these instances, I also have examples where I have a target locked with IRST, but my R-24T missile refuses to lock. Can anyone with experience with IRST explain what’s going on in these instances?

Depending on the target plane, even in front aspect it might be too ‘cold’ for the IRST. This is definitely the case with subsonics (they don’t go fast enough to generate all the heat on the skin that supersonics do) and non-afterburners. The test flight F-86 Sabers for example I have a hard time getting an IRST lock in front aspect even with the Su-27’s IRST. As for the second case, I’m not 100% certain, but I’m pretty sure the IR sensor on the IRST is more sensitive than the missile’s IR seeker. So your IRST might be picking up a target that the missile cannot see yet until you get closer or until it shows more of its side/rear

As I understand it, though, the missiles should lock to something that is already locked by the IRST, same as a radar lock?

I can’t say that I’ve run into any subsonics with my MiG-23, but I have noticed that I’ve struggled to lock Mirages a couple of times tonight.

No, IRST doesn’t work like a CW illuminator (the piece that guides radar missiles). IRST will tell your IR missile where to “look” but the IRST sensor doesn’t replace the IR sensor of the missile. It’s more similar to radar-slaving. Basically, once you fire the IR missile, it acts completely independently using its own seeker.

With radar missiles, the illuminator of the radar continuously feeds guidance info to the SARH missile to guide it to target

Don’t know if the Mirage is colder than average, I’d have to test that. The only target I know off the top of my head that afterburns but not really “hot” is the F-5 family