## 4 Challenges for Ground Times and Respective Measures to Reduce Risks
### Improved Boarding as Example for One Important Key Driver
Finally, the _reverse-pyramid_ boarding method is a hybrid approach combining traditional back-to-front and outside-in boarding. Within the boarding groups, seats are sorted from window to aisle. Another method that promises increased efficiency is the _Steffens method_. In this method, passengers with assigned seats two rows apart from each other board the aircraft together (e.g., 6A, 4A, 2A). Firstly, all window seats from back to front, continuing with the middle, and finishing with the aisle seats [21; 50; 51]. The currently preferred method to eliminate boarding interferences is the back-to-front method. However, [PERSON] highlights that block boarding might perform better [53]. It appears that there is general uncertainty about the actual efficiency of each method. Nevertheless, each \begin{table}
\begin{tabular}{c c c c} \hline
**Category** & **Measure** & **Benefit** & **Reference** \\ \hline Strategic Boarding & Boarding methods, passenger management & 25\% & [PERSON]/[PERSON] 2012 [51] \\  & Seat assignment based on luggage & 3\% & [PERSON]/[PERSON] 2014, p. 96 [52] \\  & Avoidance of hand luggage & 12–34\% & [PERSON] 2018, p. 10 [6] \\  & Boarding methods based on the use of apron busses & 5–36\% & [PERSON] et al., 2019 [47] \\ \hline Aircraft design & Aircraft type & - & [PERSON] et al., 2013 [21] \\ \hline Aisle & Increased aisle width & 5–7\% & [PERSON] 2014, p. 120 [23] \\  & Increased aisle length & - & [PERSON] 2017, p. 35 [3] \\  & Twin aisles & 40–50\% & [PERSON] 2014, p. 94 [28] \\ \hline Door & Second (rear) door & 25–33\% & [PERSON] 2018, p. 377 [6] \\  & Quarter door & 3–24\% & [PERSON] 2014, p. 119 [23] \\  & Door size & - & [PERSON] 2014, p. 100–120 [23] \\ \hline Seat & Seat pitch & - & [PERSON] et al., 2018 [32] \\  & Foldable seat pan & 28\% & [PERSON] et al., 2016, pp. 6–9 [7] \\  & Side-slip seat & 19–28\% & [PERSON] 2017 [50] \\  & & [PERSON] et al., 2016, pp. 6–9 [7] \\ \hline IT & Connected aircraft cabin & - & [PERSON] 2018, p. 12 [6] \\ \hline \end{tabular} Source: Benefits in part taken and included from [PERSON] et al., 2016, p. 2 [7].  
\end{table} 
Table 1: Measures for improved boarding procedures. Figure 4: Grid model to represent different boarding strategies. Source: derived, adapted, modified, assembled and commented from [PERSON] 2017, p. 134 [53]. of them definitely increases efficiency compared to random boarding. Figure 4 uses the grid model to demonstrate the basic boarding strategies. However, executing those methods is limited to those flights that board their passengers via a jet bridge, accordingly only via the front door. If buses are used, it will almost be impossible to control the passengers according to the strategies since passengers board the aircraft in any sequence they like after leaving the bus. Building upon this problem, [PERSON] et al. consider 13 different methods for improved boarding when using apron busses considering the seat assignment of passengers [50]. On the contrary, [PERSON] states that the elimination of seat assignment from scratch could increase the motivation of passengers to access the aircraft as fast as possible [23]. Further studies take the quantity of baggage into account. In general, the time needed to complete the boarding decreases with a decreasing number of carry-on luggage items. Whenever a passenger needs to store his luggage, he/she blocks the aisle. The investigation of [PERSON] reveals that the boarding process would be significantly faster without hand luggage [6] because there would not be blocked or saturated overhead compartments that lead to counter flow movements of passengers. Besides eliminating hand luggage, there is the option to develop boarding strategies based on luggage characteristics. For example, assigning seats based on the number of luggage pieces allows for evenly distributed hand luggage, so the required time to find sufficient space in the overhead bins can be reduced [51]. Furthermore, it is effective when passengers with more items sit closer to the entry [6].