Historic tracks with black parts in them?

[flygpagen]

Hi! Sometimes tracks are coloured black, or have black parts in them. What does the black parts mean - missing altitude information or such?

[Runway 31]

It means an area of flight that was not tracked by a receiver

Alan

[flygpagen]

Thanks, but it can´t be all. Due to the forum I can not paste URL:s here but there are flights by small aircraft that are tracked in position 100% accurate, but the track is all black and the speed/altitude graph only displays speed. We´re trying to solve a problem related to the hardware in the aircraft, that´s why we need to know.
See also SEVPF flight 22 April 11:40 CEST.