Interesting thread.
In daylight hours, perfect visibility conditions, an airliner sized aircraft will be visible as a speck in the sky at about 40 km. This is based on the human eye’s angular resolution (approx, .02 degrees). This also assumes absolutely no impediments to vision. ie: scratches or reflections on the window.
The reality is that it is virtually impossible to see another aircraft at that range. Aviation safety experts performed a field study in which pilots attempted to detect another aircraft (DC-3) approaching on a collision course. Over various conditions, the average distance at which detection by the pilot occurred (“detection distance”) was from 5.5 to 8.7 km. Of greater relevance to this study, the subject aircraft also carried an experimenter who knew exactly the approach angle of the target aircraft, and “kept constant vigil with his nak’d eye” until he detected the intruder aircraft. This “threshold distance”, over the same conditions, averaged from 17.3 to 23 km.
In short, if you know exactly where to look, you might spot another aircraft at 20 kms, but if you only know he’s out there, “somewhere”, good luck. At normal airline cruising speeds, two aircraft flying toward each other have, typically, less than 3 seconds within the “detection range”. This is why we have implemented automation in the cockpit to assist with collision avoidance.
If you know there is an aircraft to your 10 o’clock, there is no guarantee you will ever see him. The idea of implementing a slider to adjust the distance from 40 miles to 20 miles is simply ridiculous. As a long time commercial pilot, I would welcome the day that a little sign would pop up over each aircraft within 50 miles, so I would have plenty of time to decide which ones are a threat to my airspace.
I have done a lot of formation flying and it is always a bit dicey during reform exercises. It can be very difficult to see aircraft that are only a couple miles apart.
If you have other aircraft turned on but can’t see any… look harder.