DFWHeavy
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How Accurate Of Runway Distance Markers?

Thu Aug 18, 2011 4:40 pm

I'm wanting to know how accurate the "1,000 ft" distance runway markers are.

For example: If you have an 8,500 ft runway and you line up to take off, the first number you would typically see is "7" for 7,000 ft remaining, which means there is 1,500 ft between you and the first number.
However, if you were to turn around, the last number you would see would still be "1" or 1,000 ft, but yet it would actually have to be 1,500 ft.

So how accurate are runway distance markers? How do you compensate for the example I listed above. Every runway that isn't exactly 6,000, 7,000, 10,000, 12,000, etc would have this problem.

Thanks!
Christopher W Slovacek
 
tdscanuck
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RE: How Accurate Of Runway Distance Markers?

Fri Aug 19, 2011 4:40 am

Quoting DFWHeavy (Thread starter):
For example: If you have an 8,500 ft runway and you line up to take off, the first number you would typically see is "7" for 7,000 ft remaining, which means there is 1,500 ft between you and the first number.
However, if you were to turn around, the last number you would see would still be "1" or 1,000 ft, but yet it would actually have to be 1,500 ft.

If you turned around, wouldn't the back of that sign just be black and there be another sign on the other side at the correct location for that number?

Tom.
 
ukoverlander
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RE: How Accurate Of Runway Distance Markers?

Fri Aug 19, 2011 4:46 am

Quoting DFWHeavy (Thread starter):
I'm wanting to know how accurate the "1,000 ft" distance runway markers are.


Is this a serious question?

They are 100% accurate. 1,000 foot markers are 1,000 feet apart.
 
kcrwflyer
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RE: How Accurate Of Runway Distance Markers?

Fri Aug 19, 2011 5:21 am

Quoting tdscanuck (Reply 1):
If you turned around, wouldn't the back of that sign just be black and there be another sign on the other side at the correct location for that number?
Quoting DFWHeavy (Thread starter):
So how accurate are runway distance markers? How do you compensate for the example I listed above. Every runway that isn't exactly 6,000, 7,000, 10,000, 12,000, etc would have this problem.

Well they can't be perfect... Our runway is 6300ft. so I'd say you have 150 more ft available than what each marker says. I'd assume they do them the same way at all airports.. just splitting the difference in the length above the even thousand.

Do any airports have them on both sides of the runway for each direction?
 
spacecadet
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RE: How Accurate Of Runway Distance Markers?

Fri Aug 19, 2011 6:31 am

Quoting kcrwflyer (Reply 3):
Well they can't be perfect...

Why do you say that? Distance measurement is both a simple matter and an exact science, and these signs are potentially pretty important.
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DFWHeavy
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RE: How Accurate Of Runway Distance Markers?

Fri Aug 19, 2011 12:55 pm

No, it isn't that simple. If you have a 9,500 ft runway, there will be 8 numbers; however, one of them has to be more than 1,000 ft apart, presumably the distance between of of the last ones and the threshold of the runway.

If it was "simple", then you would have 9 1/2 number.... and you don't! Think about it before responding.
Christopher W Slovacek
 
qf002
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RE: How Accurate Of Runway Distance Markers?

Fri Aug 19, 2011 1:01 pm

Quoting DFWHeavy (Reply 5):
If it was "simple", then you would have 9 1/2 number.... and you don't! Think about it before responding.

But the '9' isn't right at the end of a 9,500ft runway, is it? I always thought that there was just a bit of slack at the end (ie it's within the 9-10 zone, but there's no '10' because it's off in the distance) and the markers were always at 1,000ft intervals...
 
Cubsrule
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RE: How Accurate Of Runway Distance Markers?

Fri Aug 19, 2011 1:05 pm

With respect to 9,500 foot runways, it's a silly question, as there are (correct) markers for both directions.

But what about a runway that is 9,001 feet, 9,005 feet or 9,050 feet? I believe those runways WILL have only one set. There must be a cut-off somewhere.
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pilotfox
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RE: How Accurate Of Runway Distance Markers?

Fri Aug 19, 2011 1:34 pm

"The method chosen should be based on cost considerations and adaptability to the specific airport configuration. When using the preferred method or alternate method #2 for runway lengths that are not an exact multiple of 1,000 feet, one-half of the excess distance is added to the distance of each sign from each runway end. For example, for a runway length of 6,500 feet, the excess distance is 500 feet and the location of the last sign on each runway end is 1,000 feet plus 1/2(500) or 1,250 feet. If a sign cannot be installed at its standard location, a tolerance of +50/-0 feet is allowed for that sign. The sign should be omitted if it cannot be installed within this tolerance"

More info is in FAA AC 150/5340-18F STANDARDS FOR AIRPORT SIGN SYSTEMS

http://www.faa.gov/documentLibrary/m...Advisory_Circular/150_5340_18f.pdf
 
DFWHeavy
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RE: How Accurate Of Runway Distance Markers?

Fri Aug 19, 2011 1:55 pm

Thank you pilotfox.

For the people above. What I meant when I said "turn around" is, if you go to the first marker and turn around there would be a "1", but it is more than 1,000 ft.

So either one of the marker's at either end is off or they would all be off slightly to compensate. Pilotfox gave us more accurate information and I thank him for that.
Christopher W Slovacek
 
Cubsrule
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RE: How Accurate Of Runway Distance Markers?

Fri Aug 19, 2011 2:05 pm

Quoting DFWHeavy (Reply 9):
What I meant when I said "turn around" is, if you go to the first marker and turn around there would be a "1", but it is more than 1,000 ft.

But, again, on a 9,500 foot runway, that's not true in most cases.
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DFWHeavy
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RE: How Accurate Of Runway Distance Markers?

Fri Aug 19, 2011 2:07 pm

Which would answer my original question, in that they are not really all that accurate. I consider accurate withing a couple feet of 1,000.

Cheers.
Christopher W Slovacek
 
Maverick623
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RE: How Accurate Of Runway Distance Markers?

Fri Aug 19, 2011 10:21 pm

Quoting DFWHeavy (Reply 5):
Think about it before responding.

A friendly note: you are asking for help and advice. It would be nice to be a little more polite and patient, especially when your posts contains incorrect assumptions.

Quoting DFWHeavy (Reply 5):


No, it isn't that simple. If you have a 9,500 ft runway, there will be 8 numbers; however, one of them has to be more than 1,000 ft apart, presumably the distance between of of the last ones and the threshold of the runway.

Not every airport uses the same signs for both directions, and for the ones that do, pilotfox explained the limitations.

Also:

Quoting DFWHeavy (Reply 11):
in that they are not really all that accurate. I consider accurate withing a couple feet of 1,000.

Runway markers are visual indicators to back up already known performance calculations, themselves having built-in margins. Visual indicators have limitations to their usefulness, hence why minimums are generally higher for visual approaches than for instrument approaches.

Since performance parameters will be slightly inaccurate (you simply cannot calculate exactly how your brakes are going to perform, nor exactly how much thrust your engine(s) will produce), you do not need such a high amount of accuracy in a visual indicator of runway length remaining. It's just like in math and significant figures: you can only be as accurate as your most inaccurate number.
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gunsontheroof
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RE: How Accurate Of Runway Distance Markers?

Fri Aug 19, 2011 10:47 pm

This might be better off in tech/ops--lots of the brainy folks over there don't venture into civ-av and might be able to elaborate further!
 
BMIFlyer
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RE: How Accurate Of Runway Distance Markers?

Fri Aug 19, 2011 11:11 pm

Guys, remember the signs show distance remaining. So....

On a 9500 ft runway, the first sign will be 500 ft from the end and will be a 9 - so 9000ft remaining.

The 500ft area you just started from is not really relevant.....

To make it easier, count from the opposite end of the runway to where you start your takeoff roll from  
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Maverick623
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RE: How Accurate Of Runway Distance Markers?

Fri Aug 19, 2011 11:16 pm

Quoting BMIFlyer (Reply 14):

On a 9500 ft runway, the first sign will be 500 ft from the end and will be a 9 - so 9000ft remaining.

The 500ft area you just started from is not really relevant.....

To make it easier, count from the opposite end of the runway to where you start your takeoff roll from

Yes, but some airports use both sides distance marker signs, even when the runway is not exactly x-thousand feet long.
"PHX is Phoenix, PDX is the other city" -777Way
 
vikkyvik
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RE: How Accurate Of Runway Distance Markers?

Sat Aug 20, 2011 1:44 am

Quoting tdscanuck (Reply 1):
If you turned around, wouldn't the back of that sign just be black and there be another sign on the other side at the correct location for that number?

Depends on the airport. Some airports (actually most that I've seen, I think) only have them on one side of the runway, so they are double-sided signs.

Then there are airports like BOS that have double-sided signs on BOTH sides of the runway.

From the AC that pilotfox posted:

Preferred Method. The most economical installation consists of double-faced signs located on only one side of the runway. Where this method is used, the signs should be placed on the left side of the runway as viewed from the most often used direction. However, the signs may all be placed on the right side of the runway where necessary because of runway/taxiway separation distances or conflicts between intersecting runways or taxiways.

Quoting BMIFlyer (Reply 14):
On a 9500 ft runway, the first sign will be 500 ft from the end and will be a 9 - so 9000ft remaining.

If you use the Preferred Method or Alternate Method 2 per the AC, then you wouldn't have a "9" sign on a 9500 foot runway.

Quoting DFWHeavy (Reply 11):
Which would answer my original question, in that they are not really all that accurate. I consider accurate withing a couple feet of 1,000.


I would say that "accuracy" in this case would mean whether the signs were installed according to the FAA's (or whatever regulatory body's) policies.

As it stands now, in general, you'll never have a "1" marker CLOSER to the threshold than 1000 feet. It could be up to 499 feet farther from the threshold, if I read correctly. And then all the signs between the "1" at either end would be 1000 feet apart (+50/-0 feet).

Now of course, I can immediately come up with a counterexample - BOS's runway 22R. The 1000 foot remaining marker is less than 1000 feet from the far threshold. But that was probably just installed before these regulations came into effect.

[Edited 2011-08-19 19:01:34]

[Edited 2011-08-19 19:02:12]
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ThirtyEcho
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RE: How Accurate Of Runway Distance Markers?

Sat Aug 20, 2011 4:50 am

If a "couple of feet" is crucial in your takeoff then you are flying a B-25 off the deck of the Hornet in 1942.
 
PGNCS
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RE: How Accurate Of Runway Distance Markers?

Sat Aug 20, 2011 3:15 pm

Quoting DFWHeavy (Reply 11):
Which would answer my original question, in that they are not really all that accurate. I consider accurate withing a couple feet of 1,000.

What exactly are you going to do with this super accurate (+/- 2') data when you are whizzing past these signs at 160 knots on the departure roll? Maverick623 made all the points that need to be made on the subject. Thank you also pilotfox for posting the relevant information.

Quoting ThirtyEcho (Reply 17):
If a "couple of feet" is crucial in your takeoff then you are flying a B-25 off the deck of the Hornet in 1942.

Well said!  

Computerized performance numbers are good at getting theoretical distances awfully close to BFL these days!

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