An old CAR "Licensing of Aerodromes" document indicated that taxiways may not be designated by O, I, or X.

Is this still in effect today? Why were (or are) those not used as taxiway identifiers?

My guess is that the shapes of those letters could cause confusion such as an "X" meaning something like "closed" or I looking like a vertical line "|".

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    $\begingroup$ The 1990 Wayne County Airport runway collision happened because they missed the turn onto taxiway "Oscar 6", and again when instructed to turn onto XRay but instead turned onto Runway 03C. I believe DTW renamed the runways/taxiways after this (but not sure if one correlates the other) because there is no longer a taxiway "O6", "X" or a runway 03C. $\endgroup$
    – Ron Beyer
    Commented Jul 22, 2019 at 5:12
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    $\begingroup$ @RonBeyer that airport is known to most people as "Detroit Metro", DTW, or just "Detroit", where it would be perfectly normal for two jetliners to be. There is another major airport in Wayne County that is airline-capable, but it is GA only. When you don't use Metro/DTW by one of its proper names, people might think you mean the other... $\endgroup$ Commented Jul 22, 2019 at 18:26
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    $\begingroup$ @Harper That's the title of the Wikipedia article I linked to, which is why I used it. I personally refer to it as DTW (and did so in my post), so there shouldn't be any confusion about what airport I'm referring to. $\endgroup$
    – Ron Beyer
    Commented Jul 22, 2019 at 20:22
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    $\begingroup$ That page is amazingly sparse. I assume the 727 was able to successfully take off before returning, given the relatively small amount of damage for such an awful situation. $\endgroup$
    – zymhan
    Commented Jul 23, 2019 at 14:06
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    $\begingroup$ @zymhan: According to the accident report (PDF), the captain "rejected the takeoff and stopped the airplane using maximum braking." $\endgroup$ Commented Jul 23, 2019 at 18:51

2 Answers 2


Because those symbols are easily confused with the number 0, the number 1 and the symbol for a closed runway/taxiway (✕) respectively.

(1) Numbers by themselves, and the letters "I" and "O" must not be used because they could be mistaken for a runway number.

(2) The letter "X" must not be used because a sign with an "X" could be misconstrued as indicating a closed taxiway or runway.

Source: FAA Engineering Brief No. 89 on Taxiway Nomenclature Convention (via SKYbrary.aero).

Most aerodromes do refrain from using these identifiers, but there are also examples of them being used. For example, Copenhagen (EKCH) has a taxiway I(ndia).

  • $\begingroup$ Interesting. Nice catch $\endgroup$ Commented Jul 22, 2019 at 19:05
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    $\begingroup$ So does ICAO, from Annex 14: Recommendation.— When designating taxiways, the use of the letters I, O or X and the use of words such as inner and outer should be avoided wherever possible to avoid confusion with the numerals 1, 0 and closed marking. $\endgroup$
    – user14897
    Commented Jul 22, 2019 at 19:07
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    $\begingroup$ @ymb1 Thanks! Was looking for an ICAO reference, but stumbled upon the FAA document first :) $\endgroup$ Commented Jul 23, 2019 at 6:14
  • $\begingroup$ Something very similar happens in rehearsal letters in music: music.stackexchange.com/questions/86569/… $\endgroup$
    – user44653
    Commented Oct 22, 2019 at 1:55

"X" marks a closed runway or an airstrip where aircraft may not land. See this temporary marker as an example of how it is used.

Temporary "X" indicating closed runway

I have tow-launched hang gliders from an old WWII airstrip in Suffolk (England), which was shared with microlight pilots. We had one of these markers which we laid out before flying, to prevent other aircraft from inadvertently landing there whilst we were towing.


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