I'm stuck at home with Covid, have a little bit of time to spend with this.
Remember the initial hypothesis on this forum was that FLIR1 shows a distant plane flying away, like the Chilean UFO. By distant, it was suggested >50Nm.
https://www.metabunk.org/threads/2004-uss-nimitz-tic-tac-ufo-flir-footage-flir1.9190/
We know this is impossible because with an elevation angle of 5-6º, that would put the plane way too high (especially if you account for Earth curvature in your calculation).
It's been hypothezised FLIR1 could show another fighter jet. Let's try to put a Hornet-size plane, 56ft, in Mick's simulator. I use a FOV of 0.35º to compare the sim and vid. In a previous post I've been using half of this for TV mode, following a remark by Fravor that TV mode allows for twice the zoom. But reading at the old FLIR1 thread (a lot of stuff in there), it seems that the FOV may be the same between NAR TV mode and NAR2 IR, i.e. 0.35º. I guess Fravor meant the FOV is just doubled between TV and IR, so MFOV TV is the equivalent of NAR IR, NAR TV the equivalent of NAR2 IR. Comparison of apparent size in NAR TV and NAR2 IR modes check out :
https://www.metabunk.org/threads/2004-uss-nimitz-tic-tac-ufo-flir-footage-flir1.9190/post-217405
The object spans about 10% of the 0.35º FOV in TV mode, a significant angular size right off the bat. If we try to put a F-18 in there, with some low or moderate tail angle because it cannot be a plane simply flying from left to right perpendicular to Underwood's F-18, we need to put the F-18 at ~5-6Nm to get a match in size.
You'll agree this is awfully close to not see it's a fighter jet. Also the tail angle stays small all along, as the object at that distance would need to fly in a fairly straight line (if it was turning Underwood would close on it and we would see a bigger change in Az angle, and reduction in apparent size).
I think we can confidently rule out a fighter jet here.
Another hypothesis has been a small airliner, or business jet. Especially because of this segment of the video, when we see protrusions that may suggest wings.
In this configuration the plane is seen from mid-side, with a significant tail angle. I didn't get anywhere with a 100-150ft target size, and it would be at <10Nm range. Not easy to find a configuration that fits that tail angle and angular size. It works for a big airliner, 250ft, at about 18Nm.
This configuration shows a 2500ft loss in altitude though. But in the sim the elevation angle is fixed to 5º, while it increases to 6º in the video. I guess that would compensate for it, at least partly, but I haven't made the calculation. Increasing speed of the object (here 300 Knots) decreases the loss of altitude, but tail angle decreases.
I don't think a big airliner at 18Nm is convincing either. Seems quite close not to recognize and/or ID a 250ft airliner. Underwood catches it in the middle of a turn, well that can happen. But no airliner exhaust features in IR, especially when seen from the rear at the beginning, is very suspicious to say the least.
Revisiting the main FLIR1 thread I realized all this had been pointed out back in 2018 by user igoddard. With less tools available at the time. He did some really great analyses that got mostly ignored. Not surprised.
https://www.metabunk.org/threads/2004-uss-nimitz-tic-tac-ufo-flir-footage-flir1.9190/post-217905
So let me throw a challenge here : can one of you find a plausible configuration for a plane in Mick's sim?
https://www.metabunk.org/sitrec/?sit=flir1
Because apart from a plane, I don't see which mundane explanation would fit the object we see in FLIR1.
I tend to agree with this post from 2018:
EDIT: added link to Mick's FLIR1 simulator