Curious.

[BabyBear]

Can anyone tell me at what height an aircraft reg cannot be read (by someone with normal eyesight) from the surface?

Cheers,

BB

[what next]

It depends on the size of the lettering I would say.

[Ralis]

yep, lettering size and location, under wing is extreamly visable

[znww5]

In brief, CAP 523 in the UK states that wing lettering must be at least 50cm high, fuselage or tail lettering must be at least 30cm high.

So now we need an optician to tell us the typical reading distance for 50cm and 30cm high lettering, there must be one on Pprune somewhere!

[IanPZ]

I'm not an optician, but I do know that 20:20 vision means you can read letters 20mm high from 20 ft.

Given that its related to your ability to focus sufficiently to see detail at a specific distance, and that lenses work in a linear way, then I think it comes out as follows.

If a letter is 20mm high, it can be read from 20 ft away
So if the letters are 50cm high on the wing, that is 25 times bigger
By extension, you should be able to read them from 25x 20ft

So you should be able to read the letters on the underside of a wing at 500ft

Any guess as to whether that makes sense?

IPZ

[BabyBear]

Don't know whether that makes sense, or not?

My question was based on the belief that there is a specified size and therefore related to that size on the underside of the wing.

The reason I ask is that yesterday as 4 aircraft flew over in very loose formation I naturally looked up out of curiosity and was surprised at how easily I could read the reg. of the one which went directly overhead. It never crossed my mind that they were particularly low so I was surprised at being able to read it (not that it would have mattered too much as it was over the sea).

They seemed to be enjoying themselves.

BB

[znww5]

IanPZ's maths seem to make sense and I seem to recall an earlier discussion of the wing reg having to be readable at 500ft, so that would make sense. However, the regs don't stop people from using letters larger than the minimum if they wish, so the aircraft could have been higher . . . or BB has the eyesight of a hawk

[thing]

that 20:20 vision means you can read letters 20mm high from 20 ft.

A curious mix of measurements. Mind you, aviation is full of them so hardly surprising that opticians mix them up as well.

[Fuji Abound]

I agree, I thought the popular thinking was that if the aircraft is over 500 feet you cant read the letters.

I think its about right as well judging from personal experience !

[Mark 1]

The 6/6 standard visual acuity implies that you can differentiate lines 1 arc minute apart. The smallest discernible character is thus 5 arc minutes.

So someone with standard vision will be able to read 50cm registration letters at about 360m or 1200' in ideal conditions.

And if you score 6/5 or 6/4 you should be able to read them even higher.

So your formation may have been quite legal.

[foxmoth]

So someone with standard vision will be able to read 50cm registration letters at about 360m or 1200' in ideal conditions.

And if you score 6/5 or 6/4 you should be able to read them even higher.

So your formation may have been quite legal

Perfectly legal even below 500ft - a bit harder of course to keep clear of all the bits you need to,especially in formation so for that reason above 500ft much better, but no LEGAL reason you need to be higher - any considerate formation would be higher though as it tends to be noisy.

[NorthSouth]

foxmoth:

Perfectly legal even below 500ft

Not if someone underneath's reading your reg it ain't!
NS

[Obi_Wan]

20:20 vision

Apologies for the thread drift, but, the accepted definition of 20:20 vision is as previously stated, however, both an optician and an AME have defined it to me as being, "20:20 means you can see at 20 feet what someone with perfect eyesight has, if your eyesight is worse the first number gets smaller, as in 20:40 means you need to be at 20 feet, and someone with perfect normal eyesight can see it at 40 feet".

As for the original question, it depends on so many things. How clear are the letters, (I've see some italic letters), it depends if the letters are actually standard size (some aircraft so appear to have smaller under wing lettering), whether the aircraft was straight and level, was it a normal SEP, could the person on the ground have had binoculars (depending what prompted the original question, whether a ground based person old have had cause to grab their binoculars!)

[foxmoth]

Not if someone underneath's reading your reg it ain't!
NS

Yes it is - as long as the SLANT distance from them to you is more than 500ft!

[BabyBear]

I am not questioning the legality of the flight, I don't believe he was lower than 500ft and even if he was I had no issue with how the aircraft was flown. I was in a kayak in the sea and other than we paddlers the pilot, even if below 500ft, was acting properly. Interesting point though, flying within 500ft of something you don't know is there.

I was directly under this aircraft, there were 4 of them in, as I said, very loose formation, only to the extent it was obvious they were together. I was simply surprised to look up and without as much as thinking about the reg, read it. When at home I often look up and try and establish if the aircraft overhead is known to me by reading the reg, it's not often I can. That said there was one today that I would put lower than the one in question that I couldn't read.

I have to conclude the lettering was larger than the legal requirements.

BB

[John R81]

When pulling sentry duty at an active small arm's range, close to a military airfield, we were given a Very pistol loaded with a red flare. We were told that when the Buldog trainers came over, if we could read the reg, fire the flare as they were breaching the 500ft safety exclusion.

No idea if that's right, but thems was the orders.

I spent many hour WILLING one to be low enough to give me the excuse!

Sadly.................... nooooooooo!!!!