What is the longest distance at which an aircraft can be visually identified?

In the IL-2 flight simulator, when you turn on "markers" for planes you can see a belligerent side, the aircraft belongs to, at a distance up to 6km (3.75 miles) as well as its type with a tail number.

But in some books I've read mentions that pilots were able to see an aircraft number, its specific painting/livery etc. of other aircraft they encountered.

So I wonder: in real life, at what distance can such things can be distinguished by a pilot with good sight (I understand that this depends on the size of an aircraft)?

The key piece of information here is the "Angular Resolution" of the naked eye.

Is called in this way because the key parameter is the perceived angle between two points when looking at them. Let's take 2 points that are distant 1 meter one from the other.

Let's then put them at a distance from your eyes, the angle you see between them can be computed as follows:

Image source

$angle\_in\_degrees = 2*atan(\frac{distance\_between\_them}{2*distance\_from\_you})*\frac{180}{\pi}$

• if they are 10 meters from you, they will appear about 5.7° apart
• at 100 meters from you, they will appear 0.57° apart
• 1000 meters, 0.057° apart

Note that two dots 10 meters apart and 100 meters from you will appear with the same 5.7° of the dots 1 meter apart and 10 meters from you: this is why, if you do not have other references, is nearly impossible to understand the distance from you of an object.

Now you are interested in reading something, in particular the aircraft number.

The naked eye cannot distinguish two points that appear less than 0.0128° apart (bright day, with no haze nor warm air distortion)

From what we see above, if the lettering is 1m tall, you could distinguish it at about 4 km. To actually read it, though, you will have to pick up the details, that will be much smaller: thik of distinguishing a "O" from a "D", you will have to notice that the left side is straight (or not), a detail much smaller than the total height of the letter and this will greatly reduce you maximum distance at which you can read.

Recognizing the livery instead is quite easier, as full portion of the aircraft will be painted, and in this case you only have to pick up the colours, not details of the shapes.

You could also distinguish the shape of on aircraft from a larger distance, given the much larger dimension of a full aircraft, but as for the letters, smaller details will not be recognizable until you get close enough, where "enough" depends on time of day, weather conditions and size of what you want to observe.

• You need 5 pixels to encode somewhat recognizable capitals and 8 pixels for decent ones, so the factor to divide the letters by should be something like that. Leads to about 500m-800m for 1m high letters. But I don't think they are as big. A small plane like C172 does not even have any continuous flat area where 6 such letters would fit. Nov 17 '14 at 22:15
• @JanHudec I totally agree that 1 meter is big, I used it for simplicity of calculation. Thanks for the pixel information, if you have any source, would you care to add it to the answer?
– Federico
Nov 18 '14 at 7:30
• I don't have reference; just judging from how fonts looked on old low-resolution screens. Nov 18 '14 at 7:31

As you can expect for a question of this nature, it depends. Atmospheric condition are a big factor, in hazy conditions you can't see as far. Background and altitude are other factors. It is easy to lose sight of a plane when there are many buildings behind it. If you know where to look, like a small plane that announced itself on final, in good conditions you might actually see them from 8 to 10 miles away. Just scanning for traffic you might see a plane from 5 miles away, but in many cases they are less than a mile away before you notice it. There is a difference between looking and seeing. You can look at something but not see it.

As far as reading the N numbers, I doubt that anyone could read one from more than half a mile away. The numbers are supposed to be 1 foot high, if I remember the regs correctly, so looking at one on the ground can give you an idea of the distance you can read it. Some older planes have very small numbers.

The shape of a plane helps to identify the type. Some planes like Mooneys have distinguishing features like their vertical stabilizer.

I'll also add The context. Or, everything one actually knows in a specific situation to identify a plane :

In real life, there are regions of the globe where you don't find planes a lot. When you know which airplanes are flying in the region, you may identify it even 10 miles away, based on factors like :

• the apparent size (compared to its motion) -> It's a white twin engine widebody ? -> 777F of Air France landing in Africa
• the visible colors -> It's the An28 of some airline (Completely Black)
• the time : Afternoon, a cyan 747 taking off from CDG -> Korean Air
• events : what looks like a shiny 747-200 at Orly -> Air Force One during some international summit..
• etc.

That also applies in the books you read : in the old days of world wars, you don't have much planes flying around unless it's foo or friend. So you can expect to encounter less different types of aircraft. Was-it a F4, a Spitfire or a Stutka, those could be identified from far away, while beeing just assumptions, fighter pilots were rarely mistaken.

Close combat like those describes by veterans : look at their palms : you rarely see a chased plane from behind, mostly from above for example : this gives several opportunities to read anything written big on the plane, both in bright and dark condition depending on the aircraft relative position to the sun. Changing luminosity helps reading aircraft numbering (you can experiment this during acrobatic airshows, looking at the plane from the ground)

And intelligence : During world war, you had spies, often civilians who gather information about the enemy's equipment. So, sometimes, fighter pilots knews how many enemy warbirds they may encounter, the models, the colors/markings, etc. and even the registrations, well before meeting them someday, if ever.

Finally : IL2 Sturmovik is a game. It's purpose is to entertain. If you were playing such game and expect to identify a plane 6 miles away just by looking at the screen, you may just barely notice a pixel dot and come closer.. closer... until it's too late. Arraagh ! X( (same thing applies to any game)

A game is not a game (and doesn't sell well) if you don't enjoy it, so, helping you identify friends and threats adds in the gameplay even if it's not really realistic.

• That's what it was actually like: by the time they're close enough to identify, it's too late. And the difficulty ()what they trained to do) was noticing them at all, especially behind you. Nov 14 '14 at 3:28
• @ChrisW: Indeed, spotting the enemy was the hard part. It helped a lot that each side had different types. Nov 17 '14 at 22:26