Modern aircraft in WT now have nice, functional HUDs in many cases, and more and more of those aircraft are joining our hangars every patch, but all of those HUDs are lacking in ONE fundamental aspect: The correct displaying of flight path marker and attitude.
Reported cases:
Here I have listed just a few corresponding bug reports found in a just 5 minutes long search:
Community Bug Reporting System not disclosed
Community Bug Reporting System M2000, F-16, Harrier Gr.7
Community Bug Reporting System A-10
Community Bug Reporting System AJ37
Community Bug Reporting System F-16, A-7, Harriers, A-10
Community Bug Reporting System F-16
Community Bug Reporting System not disclosed
Community Bug Reporting System A-10
Community Bug Reporting System Harrier Gr.7
Community Bug Reporting System A-10
Community Bug Reporting System not disclosed
The reports are either aircraft specific or general, more or less complete and informative, but they all say the same: FPM and/or horizon / attitude lines are NOT working correctly in War Thunder, and should be fixed.
Please note that the issue is not restricted to only the aircraft listed in the bugs above, but really a common issue with all HUDs in WT which IRL feature the display of a flight path marker and/or horizon/attitude lines. As far as my experience goes, not a single HUD in WT has a correctly working FPM, and also the horizon/attitude lines of the HUDS don’t work correctly in many cases.
Precendent:
Ironically, the game DOES have a correctly working flight path indicator and horizon line / attitude ladder: The “HUD-only” view features exactly that as pale white lines, with the VV showing at low speeds and gear down, e.g. during finals…
References:
Two good examples of how it is supposed to look IRL can be seen in those two YouTube videos, showing real footage from the Cockpits of the A-7 and the Harrier Gr.7:

A7 Corsair HUD
Note in the A-7 vid, especially noteworthyand explaining the “how it should be” best is the sequence where a landing approach is shown (11:20 onwards): One sees very well how the pilot can use the FPM to determien the proceted flight patch, and thus in the case of landing, where the aircraft will touch down.

RAF Harrier GR7 HUD vid׃ Low level and practice bombing runs
Here it’s very well visible how the FPM is not “fixed” to the centerline of the aircraft, but shifts constantly, as also the flight path deviates constantly from the centerline.
Please note that those two videos were filmed by the aircraft’s HUD camera, which has a lower and more forward viewpoint than the pilot obviously, which results in the horizon bar actually shown with a slight upward offset relative to the landscape.
Pleas enote also that the way the Flight Path Markers is pretty much standardised in western aircraft. This obviously to facilitate transition of pilots from one type to another. The industry of cours strives to always create standards for gauges and displays, so they’re easily understood and interpreted correctly by everybody.
Sadly however - contrary to the rest of the gaming industry where correctly working FPS and horizon/attitude display on HUD’s are the norm - in WT we don’t seem to care or worry about implementing this correctly…