## 4 Challenges for Ground Times and Respective Measures to Reduce Risks
### System-Wide Challenges and Measures  
The air transport system connects all stakeholders to a certain extent. Hence, the performance of each stakeholder influences the overall punctuality of air traffic operations [6]. According to IATA, standardized procedures are priority. If the staff is well trained, they enable time efficiency and a better process flow through repetition and assure compliance with safety guidelines. Furthermore, those staff-related processes come along with simplified prediction effort [22; 27]. This is of special importance, because one major challenge for all stakeholders is the variability of GTs and the vulnerability to unexpected exogenous events. The completion of the turnaround process itself is of stochastic nature [3]. Therefore, it is difficult to estimate delay propagation. However, the ability to manage risks depends on the predictability of risk factors. Predictability means a reduction in uncertainties or at least the provision of time to react [14]. One possibility to do so is data analysis. Advanced simulation and modelling techniques rely on data collection of current, relevant, and refined data on the respective operational process. Researchers gladly use computer simulation tools. However, the amount of data currently available is scarce. This is due to a lack of data exchange and the commercial sensitivity of data of organizations such as aircraft operators or manufacturers. [PERSON] et al. call for data to be published and broadly available to better understand and quantify forthcoming challenges [32]. Not only data release to the public, but also the exchange of data in the air traffic system, is relevant for good operational performance. Different studies highlight the importance of collaborative decision-making. Precise information about the current status of operations should be available to all parties as each stakeholder holds back different information. For stakeholders, it is vital to have a holistic view of the air transport system as a local focus can neglect transnational effects. Therefore, 32 European airports implemented Airport Collaborative Decision Making (ACDM) as a big technological change. ACDM is part of the Single European Sky initiative and promises smooth collaboration of stakeholders to best utilize the local and network resources and the available capacity. It focusses on the transparent sharing of reliable timely information in aircraft handling across airport partners (airlines, airport operators, GH companies, and ATC) and with the European network management. This facilitates the planning for each department and improves the air traffic flow and capacity management [7; 26; 33]. ACDM requires appropriate processes and technical architectures that come at high costs. However, once the necessary facilities are in place, ACDM contributes to a general reduction in turnaround times and a 7% reduction in taxi times [20]. [PERSON] et al. propose to enhance ACDM through data-driven approaches [20]. This implies the implementation of Automatic Dependent Surveillance-Broadcast (ADS-B) receivers at the airside of the airport, which receive data from arriving and departing aircraft. Those receivers facilitate the communication between the ACDM partners and contribute to an advanced monitoring system. The goal is to use this broadcast data for integrated airport management, creating improved situational awareness. Data analysis should help to better predict future system states. Users can directly respond to congestion. ADS-B comes with two big advantages. First, the installation, data acquisition, processing, and storage require only a small monetary investment. Second, a worldwide implementationwould eliminate the problem of restrictive operational data access. It might be the starting point for data-driven and therefore improved operations. Another example for data-driven improvement of operation is the A-SMGCS (Advanced Surface Movement Guidance & Control System) [34].