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Chicago-Midway (MDW/KMDW) has five runways, oriented along the diagonals of the square airport. The longest two (13C/31C [1988 x 46 m] and 4R/22L [1964 x 46 m]) are ILS-equipped, and serve as the airport's primary air carrier runways; the other three (4L/22R, 13L/31R, and 13R/31L) are used primarily for small general-aviation operations. Of the three, 13L/31R and 13R/31L are available for use by larger aircraft in case of emergency:

Chicago Midway International Airport covers just over one square mile (650 acres or 260 hectares) [...] and has five runways: [...]

  • 13C/31C: 6,522 ft × 150 ft (1,988 m × 46 m), air carrier runway, ILS-equipped.
  • 4R/22L: 6,445 ft × 150 ft (1,964 m × 46 m), air carrier runway, ILS-equipped.
  • 4L/22R: 5,507 ft × 150 ft (1,679 m × 46 m), general aviation and air taxi.
  • 13L/31R: 5,141 ft × 150 ft (1,567 m × 46 m), general aviation and air taxi. Used as an Emergency Runway for commercial aircraft.
  • 13R/31L: 3,859 ft × 60 ft (1,176 m × 18 m), light aircraft only. Used as an Emergency Runway for commercial aircraft.

4L/22R, however, is apparently not available for use by air carrier aircraft even in an emergency, despite being longer than either of the other two general-aviation runways (and also twice and half again as wide as 13R/31L, the smallest runway at the airport), and the only one of the three to be useable when the winds are blowing along a northeast-southwest axis (the two designated emergency runways are both oriented northwest-southeast).

Why are Midway's two smallest runways available as emergency runways for large aircraft, but not the larger 4L/22R?

EDIT: As @RonBeyer suggests in his comment, one potential reason a runway of sufficient size might be unuseable for large aircraft could be if its bearing strength were insufficient (such that the pavement could collapse under the weight of a heavily-loaded jetliner). However, to hear it straight from the horse's mouth, 4L/22R is rated for aircraft imposing pavement loads of up to 30 short tons per single wheel, or 60 short tons per two-wheel bogie; although not up to the standards of the airport's two air carrier runways (each rated to 47.5 short tons per single wheel, 82.5 short tons per two-wheel bogie, or 125 short tons per four-wheel bogie) or emergency runway 13L/31R (rated to 40 short tons per single wheel, or 62.5 short tons per two-wheel bogie), it far surpasses the mediocre strength of runway 13R/31L, which is rated for a mere 6.25 short tons per single wheel... and is, nevertheless, a designated air-carrier emergency runway!

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  • $\begingroup$ 4L/22R is an asphalt runway unlike the others that are concrete. It has half the load carrying capability of the concrete runways. It also does not have an EMAS system installed unlike 4R and the other runways, so I'm guessing the safety overrun areas are not up to spec for that. $\endgroup$ – Ron Beyer Feb 15 at 2:27
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    $\begingroup$ If it were weight bearing capacity, why is Runway 13R/31L (PCN 42 /R/B/X/T Single wheel: 12.5) available but Runway 4L/22R (PCN 69 /F/D/X/T Single wheel: 60.0 Double wheel: 120.0) is not? $\endgroup$ – StephenS Feb 15 at 2:35
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    $\begingroup$ My current Jeppesen chart has the same "air carrier" note for 4L as for the 31 L + R. $\endgroup$ – Ralph J Feb 15 at 20:15
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    $\begingroup$ @RalphJ: A scan of said chart would be an excellent addition to your answer. $\endgroup$ – Sean Feb 16 at 16:49
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    $\begingroup$ @RalphJ: It's called fair use. Not a violation. $\endgroup$ – Sean Feb 16 at 17:36
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Prologue: the quote in the question is from Wikipedia, and the parts there that mention the designation to commercial aircraft emergencies have later been marked as "citation needed". Therefore, for as long as an authoritative source can not be verified, it might well be that in reality there is no such designation.

Actually, should the nature of the emergency require, any runway (or even taxiway) on any airport is available for any aircraft if the crew so wishes. Some more or less famous examples of such situations (Air Safety Network):

Air Canada flight 143 (The Gimli glider, fuel starvation)

United Airlines flight 232 (Sioux City, total loss of hydraulics)

Air Astana flight 1388 (Portugal, reversed aileron cables)

If you are out of fuel or experiencing severe control issues or something of such magnitude, regardless of what you are flying there is no way ATC is going to tell you "sorry you can't land on the 4L, turn to heading..."

This might seem like splitting hairs, but that's how it is. To state that some runway is (or is not) available for emergencies of a certain aircraft type, is somewhat of a moot point.

After you've spoken the three magic words, the ATC has to, and more importantly, they want to do everything they possibly can and more to help you out. You are finally free as a bird, if only you did't have an emergency to deal with...

Speaking of splitting hairs, Gimli glider did not land in an airport at all, but it's the most famous case of the sort...

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    $\begingroup$ While I completely agree with this answer, it doesn't seem to answer the question of why some of the runways at KDMW are designated as "emergency runways" $\endgroup$ – expeditedescent Feb 15 at 18:58
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    $\begingroup$ Again, I agree. But whoever wrote the quote in the question seems to think otherwise. I assume they got that idea from somewhere $\endgroup$ – expeditedescent Feb 15 at 19:09
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    $\begingroup$ Yep I just added the "Citation needed" tag myself a few minutes ago. Been digging around a bit, can't fint the source of those claims. Likely written by someone with limited understanding of the subject $\endgroup$ – expeditedescent Feb 15 at 19:20
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    $\begingroup$ The gimli glider did land at an airport, it just so happens that the airport was converted into a drag strip. It was an airport before that. $\endgroup$ – Ron Beyer Feb 16 at 3:54
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    $\begingroup$ Yes, and the hair is now split twice 😃 $\endgroup$ – Jpe61 Feb 16 at 8:06
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The other two 31's (not 31C) are "emergency use only" for large jets. They won't be used for anything but an emergency. While 4R is the usual runway used in that direction, 4L could be used in other-than-emergency cases. That's why it lacks the notation mentioned.

How does that matter? Let's say 31C is closed for repairs. Absent a compelling emergency, big jets won't use 31R or 31L. However, if 4R were similarly closed, 4L could be used as normal ops, as long as the aircraft performance allowed.

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  • $\begingroup$ Why is 4L/22R useable as a spare non-emergency runway, but not 13L/31R (which is almost as long as 4L/22R, has a significantly higher weight-bearing strength, and would allow almost-normal operations with 13C/31C and 4R/22L both closed, even if the winds are running along a northwest-southeast axis [instead of the northeast-southwest axis supported by 4L/22R])? $\endgroup$ – Sean Feb 15 at 17:30
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    $\begingroup$ @Jpe61 No. I'm explaining what the text quoted in the question means. As your answer points out, everything is available for emergency use; the quoted text means that those runways are only available in that case to large aircraft. $\endgroup$ – Ralph J Feb 15 at 20:08
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    $\begingroup$ @Jpe61 Just adding some context from the world that I work in to the text that somebody else posted. See my comment above from my Jeppesen manual if you need something "sourced". (Though I won't be posting a screenshot of a copyrighted product.) $\endgroup$ – Ralph J Feb 15 at 23:01
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    $\begingroup$ Does Jepp mention the "Used as an Emergency Runway for commercial aircraft"? Sorry, don't mean to harras you with this, I just hate it when Wiki has stuff like this with poor, or god forbid, no citation. People are lost enough without the internet being full of alternetive facts. $\endgroup$ – Jpe61 Feb 15 at 23:13
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    $\begingroup$ @EdwardFalk The original post is what I'm addressing here. To the extent that it is "citation needed", I'd suggest voting to close the question. $\endgroup$ – Ralph J Feb 16 at 18:47

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