## 4 Challenges for Ground Times and Respective Measures to Reduce Risks
### Improved Boarding as Example for One Important Key Driver  
The boarding and de-boarding procedures form part of the critical path for ground operations. They are often the last process to complete the turnaround and have direct impact on the success of the turnaround and profitability of airlines. Even if the boarding process can be outsourced to third-party service providers, it is often considered to be the responsibility of the airline operator. Reducing those times is beneficial. According to [PERSON] et al., the passenger egress and ingress can account for up to 40% of the TAT [27]. They also emphasize that the difficulty lies within the estimation of those times. There is always a residual uncertainty because no one knows how passengers will behave. Moreover, an estimation error directly influences the overall TAT. Reliable prediction of (de)boarding times is necessary for efficient scheduling and the determination of the value of new aircraft designs. Different approaches to optimize passenger embarkation have been proposed in the literature, while only a few studies address the disembarkation process. This is due to passengers having an intrinsic motivation to get off the plane as quickly as possible, which complicates structuring the egress [3]. The first studies on the boarding problem were conducted in the late 1970s [7]. Several methods help to model the boarding phenomenon and compare the performance of boarding strategies. Two examples are a stochastic model [6] and linear programming [29]. The general problem of the passenger embarkation is the row and aisle interference, which shall be avoided. Passenger movement is limited because of the dense cabin layout to fit in as many passengers as possible. Row or seat interferences happen when a passenger cannot immediately take his/her seat because another passenger is blocking access (e.g., blocked middle seat) [27]. Aisle interference usually results from passengers storing their hand luggage or entering their seat row, preventing other passengers to move forward [6; 49]. Both the selection of smooth and efficient boarding strategies or the implementation of cabin and aircraft layout advancements shorten the boarding process inside the cabin. The following table (Table 1) provides an overview of the research conducted in the field of aircraft boarding. It lists corresponding measures and links at least one relevant study to each of them for reference. The benefits assigned to the measures vary due to different underlying assumptions of the studies. The next paragraph briefly explains the most important methods summarized in the table. Adapting the proper boarding strategy is a key issue. While the effort of applying special boarding methods is moderate, the accompanying cost reduction is significant [50]. The success of boarding strategies depends on mutual factors. [PERSON] lists studies that tried to simulate the boarding process by concentrating on passenger arrival punctuality, their interaction, their physique and properties, or on the negative effect of group constellations, such as families [6]. He also found that the number of passengers has a significant influence. One additional passenger extends the boarding time by on average 4.5 s for single-aisle aircraft. There are five well-acknowledged methods to control passenger movement. First, the _random_ boarding method, which is used as the baseline scenario. Passengers access the aircraft without a special order on a first-come-first-serve basis. On contrary, the other methods predefine a sequence. The _outside-in_ method lets the passengers with window seats enter the aircraft first, followed by the middle and aise seats. For the _back-to-front_ boarding, the aircraft is pared into blocks. The resulting passenger groups enter the aircraft following the sequence from back to front. The _block_ boarding divides passengers into groups as well. However, it starts with the back group first, then the front rows, and lastly the centre rows.