Airport Radio Navigation Facilities: Runway Selection and Identifiers

Air traffic control (ATC) chooses runway for landing and takeoff based on wind direction and strength. The exact rules for runway selection may vary from airport to airport. Associated with runways at various airports in DCS are radio navigation landing aids such as:

• Instrument Landing System (ILS)
  
• Non-Directional Beacon (NDB)
  
• PRMG
  

Currently DCS decides runway selection for ATC and runway selection for landing aids based on two entirely independent sets of rules. The result is the runway as assigned by ATC and how landing aids are operating may or may not be in agreement. In general terms this is unreal, unhelpful, and should be changed.

Case Study: Vaziani

Runways: 13 (135.5°T) and 31 (315.5°T), same bit of concrete

Landing aids: 108.75 ILS (identifier IVZ) for runway 13 and 31

For wind speeds less than ~3.6kt as measured at 20m elevation runway 13 is the ATC-selected runway, regardless of direction. Runway 13 is the "calm wind runway." Runway 31 is only selected by ATC for landing if wind speed is greater than about 3.6kt and wind heading (wind blowing toward) is within 78° of 135.5°T. This is roughly half the circle but not quite. This angle cutoff applies to any speed of wind no matter how fast as long as it isn't below the "calm" threshold.

The ILS systems for runway 13 and 31 are mutually exclusive. Normally, when there is no wind, ILS for 13 is active and for 31 is not. The condition for ILS 31 active (and ILS 13 not) depends on the headwind component greater than ~3.6 kts. Headwind component is the cosine of the alignment with the runway multiplied by the wind speed.

The diagram summarizes the boundary conditions between runway 13 and 31 selection for ATC and for ILS. As you can see there exists a region where the ATC will select runway 31 while the ILS remains for runway 13, not a good situation.

Recommendation for design of the system is follows:

• ATC runway selection becomes the master. Landing aids are the slave, following by programmatic logic dependency on the ATC decision. By this arrangement the landing aid is never different than ATC.
  
• Review which landing aids should depending on the ATC runway selection. Landing aids should always operate except in clear cases where they conflict with other landing aids. E.g. if frequencies of aids are independent for both directions, both should operate. If there is only one landing aid for the non-selected runway it should continue to operate.
  
• Provision in the logic should be made to allow landing or area aid systems to cease operation in cases of partial non operation or tactical necessity.
  
• Review the Morse identifier letters for conflicting landing aids. E.g. Vaziani ILS 13 and IL31 LLZ in DCS have the same letters (IVZ) would never happen at a real airport. The identifier is required to be different for frequency-shared mutually-exclusive landing aids such that which one is operating can be determined solely by reference to which identifier is heard without any other information. The safety benefit of this requirement is obvious.