Also, what I said is that the plane was either ascending or descending. Maybe it was at 6km and it went up to 8. Or it went down to 3km to engage or start the landing process. I don't know. This results are done with the data that you gave us, and I simply calculated with that. I did not check it with other sources on the internet, I have no idea if this is true or not.
But from what you gave us, it makes sense. The Plane was flying at rather low altitudes and speed, ready to engage
Yes it's definitely a possibility. It makes more sense than having a plane flying at 300 or 400km/h. But that remains rather unlikely, considering the context (Turkish warnings), the risks of AA weapons, also considering that you are losing altitude and you should have an increased speed (since the word used is depth we can dismiss the possibility that the su 24 was getting up). All of this remains completely odd and very hard to believe. Plus talking about "depth" for this is still strange, but they are Turks, so...
Well you said yourself the alt of manpads is 4km so 6 should be fine. There's no real need to fly at supersonic speeds. 600 - 700km/h will be enough at a height of 6km to make leading extremely difficult for any small arms of which most only have a range of 2km. If DCS is worth 2 cents as an indicator then its not an unusual speed to go by (600km/h - 700) especially if checking for ground targets. Maintaining supersonic speeds in high air density is extremely resource inefficient given the air friction working against you and the amount of engine thrust you have to maintain to hold the speed. Hence the wings can fold back for the SU-24 to offer less air resistance.
The 300 - 400km/h would indeed be unusually slow but then I am using what the press said in regards to distance and time and I am pretty sure the press is getting it wrong. Olaf's interpretation makes much more sense and uses both measurements.
Olafs's interpretation is that the plane lost altitude (or increased in altitude, which is a possibility that I dismiss for the reasons explained above), so basically it would have been at 3600m at the exit point of the border. In this case, indeed, the plane would have been flying at 600km/h, which is still very slow, probably to bomb a target at the other side of the border. In this case, the plane would be flying at low altitude and very low speed. That seems completely unlikely for the reasons already given. Moreover, it relies on the interpretation that "depth", in this letter, means altitude, which is odd in itself.
However, if you consider that the plane remained at 6000 meters, then it couldn't be flying at 600 km/h, but either around 300km/h or 400km/h as we said above.
Data comes from the leaked Turkish letter to the United Nations.