Recently on my ADS-B plane-plotter, I notice commercial airliners?
Recently on my ADS-B plane-plotter, I notice commercial airliners flying in a convoy, sometimes there will be up to six aircraft all separated by about a mile or two with a 2,000ft approx. difference in altitude. Is this new and what's going on? Heres the very likely answer to what you are seeing. There are airways, like freeways in the sky, that crisscross the US and probably most other large countries. There are Low Altitude Airways for aircraft below 18,000 feet MSL and High Altitude Jet Airways for higher flying aircraft like airliners. Each of the lines with the black rectangles is an airway. Notice the number of airways and this is just a very small geographic area. ATC keeps aircraft separated either by distance between aircraft or by 2000 foot altitude differences. All of that is consistent with what you observed.