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!