Parallel runway operation
Parallel Runways: Two or more runways at the same airport whose centerlines are parallel. In addition to runway number, parallel runways are designated as L (left) and R (right) or, if three parallel runways exist, L (left), C (center), and R (right).
Source: FAA JO7110.65 Air Traffic Control
Near Parallel Runways: Non-intersecting runways whose extended centre lines have an angle of convergence/divergence of 15 degrees or less.
Source: ICAO Annex 14 Aerodromes
The main objective of implementing simultaneous operations on parallel or near-parallel runways is to increase runway capacity and aerodrome flexibility. The largest increase in overall capacity often includes the use of independent approaches to parallel or near-parallel runways.
The safety of parallel runway operations in controlled airspace is affected by several factors such as the accuracy and use of the associated radar monitoring system, the effectiveness of the process of controller intervention when an aircraft deviates from the correct ILS localiser or RNAV course and the precision with which aircraft can and do fly the approach.
Modes of Operation
In ATC terms, the various modes of operation available for the use of parallel or near-parallel instrument runways are distinguished as:
Simultaneous parallel approaches
- Mode 1, independent parallel approaches: simultaneous approaches to parallel instrument runways where radar separation minima are not prescribed between aircraft using adjacent ILS; and
- Mode 2, dependent parallel approaches: simultaneous approaches to parallel instrument runways where radar separation minima between aircraft using adjacent ILS are prescribed.
Simultaneous parallel departures
- Mode 3, independent parallel departures: simultaneous departures for aircraft departing in the same direction from parallel runways.
It should be noted that when the spacing between two parallel runways is lower than the specified value determined by wake turbulence considerations, the runways are considered as a single runway with regard to vortex wake separation.
Segregated parallel approaches/departures
- Mode 4, segregated parallel operations: simultaneous operations on parallel runways where one runway is used for approaches and landings, and one runway is used for departures.
In the case of segregated parallel approaches and departures there may be semi-mixed modes of operations.
Semi-mixed parallel operations
- One runway is used exclusively for approaches while approaches are being made to the other runway, or departures are in progress on the other runway.
- One runway is used exclusively for departures while other is used for both departures and arrivals.
Mixed mode parallel operations
At least one runway is used for both take offs and landings.
Factors Affecting Simultaneous Operations on Parallel Instrument Runways
Factors which may have an impact on the maximum capacity or the desirability of operating parallel runways simultaneously are not limited to runway considerations. Taxiway layout and the position of passenger terminals with reference to the runways may make it necessary for traffic to cross active runways, a situation which may not only lead to delays but also to a decrease of the safety level due to the possibility of runway incursions by either arriving or departing aircraft.
Factors to Consider When Determining the Mode of Operations
Theoretical studies and practical examples indicate that maximum aerodrome capacities can be achieved by using parallel runways in a mixed mode of operation. In many cases, however, other factors such as the land-side/air-side infrastructure, the mix of aircraft types, and environmental considerations result in a lower achievable capacity.
Other factors such as non-availability of landing aids on one of the parallel runways or restricted runway lengths may preclude the conducting of mixed operations at a particular aerodrome.
Because of these constraints, maximum runway capacity may, in some cases, only be achieved by adopting a fully segregated mode of operation, i.e. one runway is used exclusively for landings while the other is used exclusively for departures.
The advantages to be gained from segregated parallel operations as compared to mixed parallel operations are as follows:
a) separate monitoring controllers are not required;
b) no interaction between arriving and departing aircraft on the same runway and a possible reduction in the number of missed approaches;
c) a less complex ATC environment overall for both radar approach controllers and aerodrome controllers; and
d) a reduced possibility of pilot error following undetected selection of the wrong ILS.
Parallel Runway Operation need to be carefully managed in such a manner as to minimise the risk of runway incursion or wrong runway use. Closely-spaced parallel runways may affect the pilots' situational awareness or lead to their distraction or confusion.
A potential problem with close parallel runway spacing is the possibility that an aircraft may make an approach to the wrong runway. Two scenarios can be considered:
- The wrong ILS frequency is selected. Pilot SOPs for approach clearance acceptance and subsequent setting of the required navigation equipment should be robust and attract 100% compliance. The role of the PM (and if present the augmenting crew occupying supernumerary seats) in a multi crew flight deck in cross checking that correct actions are taken is crucial.
- The wrong runway is visually acquired. If a pilot cleared for an instrument approach acquires visual reference with the aerodrome when some distance from landing, it is possible in the absence of the right level of crew discipline and interaction for alignment with the wrong runway to follow.
Safety-Related Issues Affecting Dependent Approaches to Closely-Spaced Parallel Instrument Runways
The minimum spacing between two aircraft in the event of a deviation is calculated using techniques similar to those used for independent parallel approaches.
Two factors apply:
- since the radar separation is applied diagonally, less distance between runways means a greater in-trail distance between the aircraft; and
- less distance between runways also means that the deviating aircraft crosses the adjacent approach track more quickly.
No special procedures have been developed as yet for simultaneous operations to near-parallel runways. Each situation is considered on a case-by-case basis and is dependent on a number of variable conditions.
New Concepts and Procedures
In order to maximise the capacity there are some concepts such as High Approach Landing System (HALS) that were developed and deployed (for a given period of time only) to allow aircraft to land simultaneously on closely spaced parallel runways at Frankfurt Airport. The concept involved adopting a second, strongly displaced landing threshold for the southern runway to mitigate against wake turbulence by flying above the vortices of the leading aircraft.