Garantías Mobiliarias Preconstituidas

Ground movement concerns the movement of vehicles, particularly aircraft, on the ground around the airport, Figure 2.3. This generally includes all holding areas, taxi- ways, inactive runways, and some intersections and transitional aprons where aircraft arrive, having vacated the runway or stands. Any bottleneck in the aircraft flow on the taxiways could therefore increase the ground delays and decrease the airport capacity. The Ground Movement Control (GMC), also called Surface Movement Con- trol (SMC), is responsible for the strategic assignment of aircraft to a runway, with the main aim being a reduction in delays, whilst operating within regulations and constraints based on traffic volume and weather conditions. Some of the regulations and constraints refer to the aircraft ground movements in taxiways, such as aircraft separation, cross points occupancy and procedural constraints, as used in Capozzi (2003). Different areas of potential economic and environmental interest have been identified in ground movement by Gelinas and Fan (1979) and Miller and Clarke (2004). An overview, categorisation and critical examination of previous research in ground movement is presented in Atkin et al (2010).

An overview of some of the ground movement operations and approaches used are presented in the following subsections, starting with Taxiing, followed by the scheduling and routing of trucks, de-icing/anti-icing machines and finishing with a short view of other ground movement operations.

2.3.1 Taxiing

Taxiing relates to the routing of flights from their entrance point via the airport ground infrastructure to their assigned stand (either local stands at a gate or remote stands on an apron) and back to the departure area. As such it links together the main airport operations.

Some research has taken place on understanding and solving the ground prob- lem, examples of which are throughput, congestion and terminal volume (amount of traffic), an example of this being the CSD (1999). Objectives evaluated were how to reach the destination as swiftly as possible, meet safety requirements and maximise

2.3. GROUND MOVEMENT 13

Figure 2.3: London Heathrow airport grounds and taxi ways.

utilization of the taxi ways while avoiding conflicts. Similarly more recently, Roussos and Kyriakopoulos (2009) presented an approach using a 3D aircraft collision avoid- ance system implementation, which uses repulsion fields, which could be adjusted for airport taxiing. I nevertheless anticipate some concerns regarding the time taken in solving the taxiing problem, since the potential fields have to be calculated per air- craft, and where the potential movements of an aircraft take account of other aircraft. An opportunity to use concurrency arises when obtaining the fields, and some speed is expected to be gained when passing from the original 3D problem to a 2D problem. Clare and Richards (2009) showed that in average taxi times can be reduced when using RHC approach compared to a First Come First Serve (FCFS) approach. Got- teland and Durand (2003) used GA for the minimisation of taxiing time, and Marn (2006) used B&B and Fix and Relax (F&R) methodologies for the aircraft routing and scheduling on the airport ground.

The importance of robustness when assigning flights to gates is emphasised by the fact that ground movement links together arrivals/departures runway sequencing with gate assignment, such that uncertainty and disruptions at these stages are likely to propagate to the gate assignments with potential undesirable consequences such as reassignment of flights to other gates or even cancelation of flights.

2.3.2 Scheduling and Routing of towing trucks

A stand is an area in the airport grounds where aircraft are parked, with a remote stand being one which is not located immediately beside the airport buildings, whereas gates correspond to those stands which are located at the airport terminal buildings. Aircraft need to travel from their assigned stand to the departure holding areas ready for departure. This operation may be executed by using the aircraft’s engines or by towing trucks. Whereas many aircraft are now able to move backwards on the ground using reverse thrust, the jet blast from the engines may cause damage to the terminal building and equipment, with the added hassle provided by engines close to the ground which may blow sand and debris forward and then suck it back, causing damage to the engine, Figure 2.4b. This does not happen when using normal thrust given that the air flow enters the front of the engine and leaves from the rear, whereas in reverse thrust the air also enters from the front but leaves from the lateral parts of the engine, Figure 2.4. This makes the towing trucks a preferable alternative to pushback when moving from gates to the departure holding areas. Additionally, the high price of fuel and an increase in environmental concerns has revitalised interest in using different means of reducing these factors, rendering previous research especially relevant such as Gelinas and Fan (1979); Fan (1990); Miller and Clarke (2004). This trend has been confirmed by the UK Airport Operators.

On 30th_{June 2010 the Airport Operators Association (AOA) launched new guide-}

lines to reduce aircraft ground emissions, which amongst other initiatives, outlined aims to increase taxiing with no engine, which may increase the use of towing trucks during taxiing. Merlin (1983) and UNIQUE (2005) both considered the use of towing for taxiing aircraft as a means of reducing contamination. However the following disadvantages, which were mentioned by controllers, were not considered:

a Forward thrust. b Reverse thrust. Figure 2.4: Engine thrust.

2.3. GROUND MOVEMENT 15

own propulsion systems, thus increasing congestion on the ground.

• The propulsion system of an aircraft requires reaching a specified temperature before take-off, which means that when it reaches the departure sequencing point the aircraft has to start its engines and wait until the engine take-off temperature is reached. This may entail some extra work for the GMC in the instruction to start engines, and introduce uncertainty as to when the aircraft is ready for take-off. Any uncertainty may affect current flight assignments to BSSs and gates.

The assignment of towing trucks (also called tugs) to aircraft is pre-calculated, based upon the planned stand allocations and arrival/departure times. Perturbation in the flight arrival sequences may affect their stand allocations which may therefore have to be re-allocated. It is believed that towing truck assignment may not always be re-allocated in these circumstances, i.e. the stand allocation moves but the tow plan does not. One area of investigation is the effect of this lack of re–planning. Importantly, how often does stand re-planning mean that the departure times for these towed aircraft are no longer achievable?

Other points for consideration at London Heathrow airport are presented below. • Towing truck teams are qualified in specific aircraft types, so there are issues if

they are re-allocated.

• Towing trucks which are not towing aircraft control themselves (driver), with the responsibility of keeping out of the way of other aircraft.

• Towing trucks which are towing respond to instructions from the tower, GMC. • There are eleven handling agents and over ninety airlines involved at London

Heathrow airport.

• Aircraft from remote stands normally do not require towing.

No previous work directly related to scheduling and routing of towing trucks was found, although various scheduling papers could be relevant, such as Du et al (2008) which considered the assignment of flights to oil tank trucks that may have different fuel capacities and Kolischa and Hartmann (2006) investigated heuristics for the resource constrained project scheduling. The scheduling presents similarities to and has interdependencies with the AGAP and ABSSAP, while the routing has similarities to the routing of taxiing aircraft. The model for the AGAP presented in Chapter 7 considers the use of towing trucks. Also the minimisation of the number of

towing operations is one of the objectives in the AGAP model presented in Chapter 7.

2.3.3 Scheduling and Routing of de-icing/anti-icing machines

The predictions of cooler winters in the UK, Lockwood et al (2010); Seidenkrantz et al (2009), have prompted some airport operators to order more de-icing machines revealing an increasing influence of these resources on the ground operations of air- ports in the UK. Norin et al (2007) describe a decision tool for the de-icing process which uses a Greedy Randomised Adaptive Search Procedure (GRASP) whereas in Norin et al (2009) an optimisation algorithm to schedule de–icing trucks is developed which is integrated within a simulation model in which the results show a reduction in flight delays and waiting times.

Flights needing de–icing/anti–icing require this operation to be scheduled, which is normally conducted at the gate, but may also be executed in a remote location. In both cases, these operations need to be considered when assigning flights to gates as they may affect the assignment and length of time the flight needs to spend at the gate.

2.3.4 Other ground operations

Several operations need to be completed at the gate before a flight is ready to start its departure process. Aircraft may be fuelled by oil tankers or underground pipelines. Aircraft located at gates without pipelines have to be fuelled by oil tankers which may have different fuel capacities. Du et al (2008) presented an Ant Colony Optimisation (ACO) algorithm with Max-Min and Rank-based Ant System with an heuristic called Earliest Due Date First (EDD) to solve the multi-objective assignment problem of oil tankers to flights with a minimisation of the number of oil tankers required, the total start time for servicing flights, and the total flow time of oil tankers. Similarly, flight catering requires vehicles to transport and place the required supplies next to the aircraft in readiness for its next flight. These operations increase the airport ground traffic, and if they were to be considered in conjunction with the other operations presented in this thesis, will in turn further increase the complexity of the overall problem.