There used to be an old adage, the more pavement, the better. Planning at airports used to favor as many direct routes from one point of an airfield to another, giving pilots more direct options. Over time airports expanded, runways, taxiways and taxilanes were added, often without removing existing ones that they were replacing. Often this would result in a very complex runway and taxiway system that essentially layered new airfield layouts on top of old airfield layouts that became increasingly complex and difficult for pilots to understand.
The FAA started focusing more on airfield geometry as a way to mitigate potential runway incursions (the act of entering a runway without proper authorization) and other airfield incidents. The potential for catastrophic loss of life can be quite high depending on the varying factors, in 2019 there were 1,764 runway incursions in the United States.
Through studies conducted by the FAA, certain recommendations were made in Engineering Brief 75 which became a guiding document for safe airfield geometry planning until the bulk of its recommendations were incorporated into Advisory Circular 150/5300-13A Change 1. Additionally the latest update to Advisory Circular 150/5340-1M provides more detailed guidance concerning pavement marking. These documents should be referenced when planning a new or rehabilitating an existing runway, taxiway or apron. The FAA may require that pavement geometry be brought up to standard if significant improvement work is planned. Many potential airfield geometry issues can be identified by FAA hot-spots (FAA Airport Diagrams contain hot-spot locations). Generally the FAA will prioritize funding for hot-spot and airfield safety mitigation projects.
If the airport has a history of incidents and geometry that does not meet the latest FAA standards, it might be worthwhile to conduct an Airfield Geometry Study. As a part of the study consider doing the following:
- Conduct a Safety Risk Management study for each potential issue area.
Each area being studied can be planed somewhere in this matrix depending on various factors. The existing condition and any potential mitigation should be factored in.
Image Source: Jean-christophe Dick - An analysis into past incidents using the Aviation Safety Reporting System ASRS, FAA Accident & Incident data among others.
- Interviews and workshops with key airport stakeholders including but not limited to:
- FAA
- Air Traffic Control
- Commercial Pilots
- Fixed Base Operators (FBOs)
- Ground Service Providers
- Airport Operations
Information from these efforts will allow the airport operator to determine the potential severity of an incident occurring versus the likelihood of it occurring, this will help determine if a geometry should be mitigated and in which priority.
Improved Airfield Geometry in itself is just one aspect of ensuring safety on the airfield, other aspects that should also be considered are pilot training, air-traffic control procedures, other operational measures among others. Keep in mind that when implementing a potential mitigation to an issue, it could have unforeseen adverse consequences. Ensure rigorous testing and stakeholder communication.
Common Issues
Here is a list of some of the measures recommended by AC 150/5300-13A Change 1, AC 150/5340-1M and/or Engineering Brief 75. The goals of these measures are to:
- Reduce the potential for loss of life or property
- Reduce runway incursions
- Reduce pavement over-runs
- Improve pilot situational awareness
- Reduce airfield geometry complexity
Taxi Method, taxiway fillets (pavement width)
Taxiway centerlines are designed so that a pilot can taxi with the “cockpit over the centerline”. Meaning that the pilot simply follows the line. Due to different aircraft characteristics, larger aircraft with longer and/or wider landing gear configurations may not be able to taxi using “cockpit over the centerline” without exiting the pavement in a turn. Instead pilots use judgmental oversteer which requires pilots to make judgement calls at each turn, increasing pilot workload and potential risk.
The FAA has provided guidance on how aircraft are classified based on their landing gear configuration, Taxiway Design Group (TDG) classification differs from Aircraft Design Group (ADG) in that it only takes into account the landing configuration (distance from front gear to main gear, and width of the main gear) and not the wingspan or tail height. While most Taxiway and Runway safety areas are defined by the ADG of the critical aircraft (aircraft type with at least 500 yearly operations), taxiway turns and intersections have design criteria that are defined by the critical TDG aircraft. It should be noted that the critical TDG aircraft may differ from the critical ADG aircraft for the airport. The FAA provides the criteria to be applied in AC 150/5300-13A Change 1 as well as a table of aircraft and their TDG classification. Taxiway fillets (radii of taxiway centerlines and additional pavement) CAD templates are also available online for free from the FAA.
Source: 150/5300-13A
Three node concept
When designing intersections, the best design possible is where there are a set number of possibilities for a pilot to choose from left, forward and right: three nodes. Do this to avoid complex intersections where a pilot could get confused by a multiplicity of options. Avoid having more than two taxiways intersect in one spot.
Source: EB-75, AC 150/5300-13A Ch.1
Improve pilot recognition of a runway entry
Improve visual aids such as enhanced runway entry markings, wigwags, centerline lighting, Runway Status Lights,etc… Taxiway runway intersections should be 90 degrees whenever possible, this allows a pilot to have good visibility in both directions before crossing (except high-speed exits).
Source: EB-75, AC 150/5300-13A Ch.1
Limit the number of runway crossings
Optimize taxiway flows to limit the amount of runway crossings. Potential solutions could include the planning of an end-around taxiway.
Source: EB-75, AC 150/5300-13A Ch.1
Avoid large expanses of pavement
Large expanses of pavement can be problematic for pilots as they do not allow for signage to be placed at the edge of the taxiway to be easily viewable by a pilot. It is also harder for pilots to distinguish between their designated location and another with fewer visual cues available. This becomes even more critical if the large expanse of pavement is located between a taxiway and a runway. This includes an overwide entrance to a runway.
Image Source: Jean-christophe Dick
Overwide entrance to a runway
Source: EB-75, AC 150/5300-13A Ch.1, AC 150/5340-1M
Full Parallel Taxiways
A full parallel taxiway is easily recognizable by pilots as the standard access route to a runway end and helps avoid runway incursions. Dual parallel taxiways also reduce the risk for incursions by allowing aircraft to queue for departure instead of using an overly wide expanse of pavement at the runway end.
Source: EB-75
Aligned taxiways / In-line taxiways
An in-line taxiway is a taxiway whose centerline is aligned with a centerline of a runway. The use of this type of taxiway can be confusing for pilots as the location of the holding position may be ambiguous and can lead to a runway incursion. In the words of the FAA: “Aligned Taxiways: Forbidden”.
Image Source: Jean-christophe Dick
Source: EB-75, AC 150/5300-13A Ch.1, AC 150/5340-1M
High-speed exit from one runway into another
It is highly inadvisable to provide a high-speed runway exit from one runway that leads directly into a parallel runway. A possible result of this kind of configuration is a pilot exiting a runway at high speed may be expecting the next intersection to be that of a taxiway and not a runway. Best practices are to have high-speed taxiways terminate into a center parallel taxiway, with aircraft crossing the other runway at a further intersection.
Image Source: Jean-christophe Dick
Source: EB-75, AC 150/5300-13A Ch.1
Co-located taxiways
It is not advisable to have high-speed exits on a runway in opposite directions that start from the same point, these are referred to as “co-located” taxiways. Best practices would be to displace them sufficiently so they appear to be independent of each other.
Source: EB-75
Acute angled taxiways
Avoid runway exits that require a greater than 90 degree turn to exit. This requires an aircraft to slow considerably and increases runway occupancy time. An increased occupancy time may require an aircraft on approach to the same runway to execute a go-around.
Source: EB-75, AC 150/5300-13A Ch.1
90 degree intersections
Whenever possible, use 90 degree intersections, especially at runway/taxiway intersections. A 90 degree intersection allows a pilot on a taxiway to have optimal visibility of any potential traffic on the runway in both directions.
Source: EB-75
Straight access from an apron area to a runway
Avoid having taxiways connect straight to a runway from an apron area, even if they cross taxiways. A pilot should be forced to “consciously” make a turn between the apron area and the runway to promote situational awareness.
Source: AC 150/5300-13A Ch.1, AC 150/5340-1M
Avoid runway crossings in the center third
Avoid taxiway crossings that cross the runway in its center third. This area is referred to as the “high-energy” portion. Should an aircraft cross a runway without authorization, an arriving aircraft will have bled too much lift to take-off go-around (TOGO) yet retain too much speed to stop in time; departing aircraft similarly will have too much energy to stop, but not enough lift to take off and avoid an aircraft. Design taxiways and procedures to use either end thirds of the runways or consider installing an end-around Taxiway.
Source: EB-75, AC 150/5300-13A Ch.1
Using runways as taxiways
Inadvisable to use a runway as a taxiway as errors in communications could lead to a runway being used for a takeoff or landing while an aircraft is taxiing. Standard operating procedures that promote safety around a runway are weakened if a runway is habitually used as a taxiway.
Source: EB-75
Long taxiway paths with a history of incursions
Long parallel taxiways that cross another runway can lead to incursions in certain cases. During an airfield geometry study, multiple cases of runway incursion incidents occurred when pilots were taxiing on a long taxiway crossing a runway, in several of the instances, pilots did not realize they had crossed the holdine onto a runway. A potential mitigation is to relocate the runway crossing intersection centerline so as to not be aligned with the long taxiway centerline, forcing a “jog” in the path.
Image Source: Jean-christophe Dick
Source: EB-75
Avoid V-shaped runway geometries
Runways that have thresholds located close to each other angled in V can present issues with pilots as one runway can easily be confused for another, using the wrong runway could have serious consequences especially if the wrong runway is inadequate for the type of aircraft or load.
Image Source: Jean-christophe Dick
Source: AC 150/5300-13A Ch.1, AC 150/5340-1M
Minute Case Study:
Comair Flight 5191, August 26, 2007
On August 26, 2007, a CRJ-200 with 50 occupants was preparing for take-off at Lexington Kentucky Blue Grass Airport. The flight was cleared for take-off on runway 22. However the pilot in charge taxied on to runway 26 instead. Both runways thresholds were located in close proximity and accessible from the same taxiway. The runway the flight was cleared to use was 7,003 feet long, however the runway the pilot taxied onto was only 3,500 feet long, well below the minimum required distance for takeoff for that aircraft and load.
The flight overran the runway end and resulted in 49 fatalities.
Consequently, runway 8-26, the shorter general aviation runway was completely demolished and replaced with a new with runway with increased spacing from the primary commercial runway. The new configuration renders the possibility of taxiing on to the wrong runway highly unlikely.
Airfield geometry was not the only cause for this accident, issues with crew procedures, crew management and air traffic control procedures equally were causes. However, if the geometry was more closely aligned with the FAA recommendations, the possibility of this incident occurring would have been significantly reduced.
Sources:
Other Issues
Other topics are included in Engineering Brief 75, AC 150/5300-13A Ch.1 and AC 150/5340-1M. The above list refers to the most common issues encountered. Review and refer to these documents for more potential issues to look for.
Jean-christophe Dick worked on the Long Beach Airfield Geometry Study, the Tucson International Airport Airfield Safety Enhancement Study and other airfield reviews at airports in the US and China.
