How far can a pilot see?

it is a matter of eyesight quality first, but then, you need some training as to avoid focusing to something too close as NEEMA said. A trained pilot with good eyesight can see quite far but it is hard to get a measurement (you’d need to know the exact position of the aircraft you see).

Once, at the Ferté Alais airshow I spotted Alphajets of the Patrouille de France on their last turn inbound over Orly (25km away, which is some 13.5 nautical miles (15.5 statute miles).. without smoke of course. At altitude, the air is much clearer (less haze, pollution and so on), so, if you have good eyesight and are used to look “far” (again), and the glass you look through is of good quality as well, I’d say that for an aircraft of the size of an airliner, you can spot it at distances of more than 20 miles

Unfortunately, without external stimulation (typically such as at altitude), the eye relaxes to settle focussed at but a ( very) few feet in front of the viewer.

If they were leaving a contrail, definitely.

Without? Five miles would be a lucky spot unaided, even if it was an A380. But if it was showing up on TCAS then knowing where to look it might well be possible.

Moggy

I was asking because I was looking at https://www.flightradar24.com/54.15,-3.69/6 the other day and saw the line of aircraft crossing the Atlantic Ocean. I realise they are not following close enough to wave at each other but was wondering if the aircraft following would be able to make out the lead 5km ahead?

Umm .. I actually took the original question to mean “how far away is the horizon” ie the limit the visibility of a spot on the ground. In which case I don’t have the maths capability to work it out. The first answer offered above has limitations though, just as the question is vague. From the ground an aircraft at 39,000ft could be visible, providing the air is clear and stable. The fact that the aircraft is likely to be leaving a contrail leading the eye to it helps considerably. From 39,000ft looking down at the ground, it is not so easy due to both ground clutter and pollution haze in the lower denser atmosphere layer. I suspect on re-reading the question, which is in the general aviation section it could be asking, “how far away could a pilot at 39,000ft physically see another aircraft?”, say in an air-to-air combat scenario or theoretical VFR avoidance. Don’t know. If the observer at 39,000ft is looking away from earth out into space then, with less atmosphere and at night, will far distant stars/planets will be clearer due to less atmosphere to see through, or you need to be even further out of the atmosphere to make a difference? Vague question, vague answers, but has made me ponder.

The same as at 0ft as long as there are no obstructions.