First of all, this is a bug. It may be fixed in further updates, so dont come back in 5 months time and say its not good. The plane has modded engines and the "infinite" fuel tanks. I managed to get it to 708,608 ft high and it can reach speeds of up to 10,243 mph before the vibrations kill it. However, if you go as high as possible in this and then drop down, at about 370000 ft, you start spinning wildly. This will continue until about 60,000 ft high. At this point, your height will be stuck at OVER 2 MILLION FT. Your speed will be out of control, when I took screenshots in flight, my highest was 2.7 MILLION MPH. When I went to location and chose Murphy Airport, the hangars were floating at regular height but the runway was at sea level. I forgot to see what happens at other maps. Credit for modded parts: (comment). Be sure to drop a comment about the top speed you capture, and any ideas you might have for me. Other than that, it is a really good plane, hope you enjoy it.

Edit: @tieler is the mod part supplier I used