## 3 Risks of Extended Ground Times
### Operational Constraints: Capacity Issues, System Disruption, Delay Formation Capacity issues:  
Since the airline deregulation act in 1978, air traffic started to grow continuously [16]. As of today, forecasts envisage an annual growth in air traffic demand by 5% [10]. To be able to handle the increasing traffic volumes, the provision of sufficient airport capacity is indispensable. The term capacity defines the capability of a facility to handle a maximum number of traffic units [17]. Nowadays, large hub airports such as Frankfurt or Amsterdam airport already operate at their capacity limits. Extended GTs and external events, such as adverse weather, can further reduce capacity [17]. Thus, new challenges arise from the severe congestion on the gate and taxiways. This increases the risk of delay formation induced by inefficient ground operations that lack resilience to the occurrence of unexpected events such as aircraft or passenger boarding malfunctions. Particularly, the execution of aircraft turnaround frequently causes significant delays [18]. Ground operations solely account for 32.6% of departure delays. Furthermore, delays from extended gate occupancy times can easily propagate throughout the air transport system [19]. It starts with the taxi operations that already face the difficulty of more aircraft movements due to the higher demand. If the taxiing starts behind the scheduled time, this further increases the coordination effort [20], and delay time accumulates.  
_System disruption:_ One characteristic of the air transport system is the definition of a system of systems. Figure 2 pictures the different complex systems that combine and shows the overall complexity of the aviation sector. Close ties between stakeholders result in interdependencies. Ground operations are an important and critical part of the transport system. Thus, an insufficient match of ground-based activities with upstream and downstream processes can unbalance the aviation network [4]. As the air transport system is widely connected, the delay of a single aircraft caused by extended GTs can induce network-wide disruptions. For instance, the workload of air traffic controllers increases with the complexity of traffic patterns. Irregularities caused by extended GTs, therefore, reduce tower controllers' ability to handle the expected number of traffic movements [14].  
Figure 2: The air transportation system as a system of systems. Derived and adapted from [PERSON] et al., 2019. p. 286 [4].  
Delay formation:For the absorption of delays, there are two options. Either using buffer times or waiting until the aircraft goes off rotation [7]. Consequently, it is important to alleviate delays and therefore know about critical processes that are more likely to cause delays than others. Critical processes refer to all processes that have the power to influence the TAT [21]. For certain GH operations, there is a strict chronological order resulting from the limited movement possibilities around the aircraft. Usually, those operations form the critical path. Figure 3 depicts the sequence of typical GH activities for a single-aisle aircraft and indicates the critical path. It shows that the riskiest processes are those inside the cabin, including cleaning and boarding. These processes foremost address airlines' responsibility. However, fueling operations can still become the critical path as they must be terminated before the embarkation process starts [3]. The literature provides various evidence regarding delay formation during turnaround. The IATA found that untimely GH services and missing passengers are major causes of turnaround delays [22].  
Operational constraints (capacity issues, system disruption, and delay formation) caused by long GTs have different origins. All in all, they increase the problem of capacity shortages and lead to delay formations that affect the system, especially during passenger operations.