Capítulo 4: Trabajo de campo de investigación
IV.II Análisis descriptivo de los datos obtenidos en la encuesta
In some airports, the bleed air needed for the engine start is supplied from the ground. In this case, the APU is not needed and the main engines are started before the ground supply is disconnected and the aircraft starts moving.
After starting the main engines and disconnecting the tug, the aircraft taxis along the taxiway path determined by the pilot or assigned by SMC until the assigned takeoff runway. Speeds traveled depend on airframers’ guidelines for the aircraft type, airport regulations, weather and ground conditions. Maximum speeds are usually between 15 and 25 KTS (between 28 and 46 km/h) on straight sections and much lower in corners, at about 10 KTS (19 km/h).
2.1.4 Taxi-in
During the landing roll, the aircraft is braked to taxi speed and taxis to its assigned parking position. Speeds traveled are similar to those for taxi-out. All engines are normally running at idle in this phase and may be spooled up shortly to re-accelerate from a stop. Some airlines may have procedures for taxiing in with only one subset of the engines running to save fuel, although this is not common. If the crew is aware that a ground power supply will be missing or not available immediately at the parking position, the APU may be switched on during taxi-in to allow for the prescribed warm- up time before connecting the electrical loads. Upon reaching the parking position, the main engines are switched off.
2.2 Scenarios with a Novel Taxi Propulsion System
In this section, the possibility of using ground propulsion systems other than the main engines will be discussed.2.2.1 Key issues and limitations
A number of critical aspects need to be considered when evaluating novel taxi pro- cedures. A key one involves the main engine startup and cut-off. Jet engines must be warmed up for some minutes after start before applying take-off power [14]. In conventional taxi, this requirement is already fulfilled while taxiing at idle since taxi- out generally lasts much longer than the warm-up time. However, if different taxi procedures and technologies should be used to minimize the use of the main engines, the engine warm-up time would become a major constraint.
The size, layout and volume of traffic of an airport plays a role in assessing the benefit or even the need for ground propulsion systems. In a small airport with little traffic and short distance between terminal and runway, the taxi-out time might equal
the engine warm-up time, hence conventional taxiing would be unavoidable. However, taxi times are longer in larger and busier airports. Here, the optimal situation would theoretically be reached if the engines were started during taxiing exactly as much time before takeoff as the needed engine warm-up time; the final part of taxiing would be carried out conventionally with the main engine idle thrust. However, predicting the takeoff time with such accuracy is very difficult in practice, as this is ultimately influenced by factors such as the ATC clearance (depending in turn on the local air traffic), the pilots’ reactivity, conflicting ground traffic, and possible queues at the runway threshold. Also, safety concerns might prevent arbitrary engine start-up anywhere along the taxiways and might instead require this to be performed in a dedicated area with ground staff and fire protection [14]. The location of this area should be such that the aircraft can easily taxi back to the parking position or at least leave the main taxiways without hindering other ground traffic if the flight needs to be aborted. While such a requirement would limit the benefits of a ground propulsion system in the general case, time-consuming situations like queues at the runway threshold in larger airports or de-icing procedures could be exploited to start up the main engines.
After landing, especially after using thrust reversal, the engines also need to run at idle for a cool-down time of some minutes. Again, idling engines can be exploited to taxi conventionally in the first portion of taxi-in, whereas the ground propulsion system would be active in the second part. This scenario is not as critical from the point of view of safety as taxi-out, since engine cut-off does not pose particular safety risks.
2.2.2 Technical requirements
The ground propulsion system should be used for standard aircraft ground operations. This results in the first requirement that the system shall be capable of moving the aircraft freely in the usual two-degree-of-freedom domain of ground vehicles, i.e. longitudinal motion and coupled lateral and yaw motion. It must especially be able to drive the aircraft both forwards and backwards. This requirement should not pose a particular concern since the eligible technologies (e.g. electric motor drives) can normally be operated in both directions with little or no added system complexity.
The required performances represent a central aspect in the development of a ground propulsion system. A trade-off must be found between the concurring factors of sufficiently high performance on ground, system weight (if onboard), infrastructural constraints, and size of the power source for the system. The ground performances have an influence on the taxi time of the aircraft itself as well as on the interaction with other traffic. Therefore, operational requirements should be used to define minimal performances. While comprehensive fast-time simulation models at airport level and analyses of various regulations and procedures are needed to assess these effects precisely, some limited qualitative considerations will be made here to obtain a rough