With the FAA saying that Charlotte Douglas International Airport effectively can’t use its crosswind runway (Runway 5/23) when the airport’s three parallel north/south runways are in use, the attention has turned to building yet more parallel runways. This had been Jerry Orr’s vision for awhile now. Indeed, Orr’s long-term plan was for the airport to have have five parallel runways.
What’s getting lost in all this is that not all parallel runways are created equal. How much additional capacity an additional parallel runway would provide depends upon the distance between runways. Per FAA rules, runways that are at least 4,300 feet apart can operate fully independently of each other in all weather conditions. And that is the exact distance between the runway that was opened in 2010 (Runway 18R/36L) and the existing now-center parallel runway (Runway 18C/36C).
The proposed fourth parallel runway would be an infill project, and would be located just to the west of Runway 18C/36C. In other words, it’s less than 4,300 feet from both Runway 18R/36L and Runway 18C/36C. Thus, per regulation, use of the fourth runway would be dependent on the usage on both the runways beside it in poor (instrument) weather conditions — and that’s exactly when runway capacity usually become an issue. The rules are a bit complex, but the main impact looks to be the proposed new runway and Runway 18C/36C would operate in poor weather much like a single runway but with a bit of a capacity advantage — an aircraft could begin its takeoff roll on the proposed new parallel runway as soon as an aircraft touched down on Runway 18C/36C, instead of waiting until the arriving aircraft cleared the runway to begin its roll as would be the case if just alternating takeoffs and landing on Runway 18C/36C.
Note that the cost of the proposed new runway is much like the cost of adding Runway 18R/36L — diminishing marginal returns in other words. All the improvements that give a big bang for the buck at the airport have already happened. We’re to the point now where the airport will be spending ever more money on items that give ever smaller incremental increases in capacity.