*2.4. Impacts on the Turnaround Process*

One of the key elements of a competitive aircraft is the turnaround process. Compared to the current generation of regional aircraft, there are three major changes:


Mangold et al. [59] investigated possible solutions to refuel hydrogen aircraft. They concluded that the safety level can be as high as it is today for kerosene. To calculate the total time for the turnaround process, fixed values were used for segments where no fuel is transferred into the aircraft, and a flow rate was used to calculate the time for the actual refueling. For this aircraft, a single fuel line case with purging was assumed with the variables taken from Mangold et al. [59] as listed in Table 8.

**Table 8.** Calculation basis for refueling with liquid hydrogen by [59].


The other areas that we considered for the turnaround process were the passengers, baggage and battery recharging. For all of those segments, the equipment positioning and removal time was taken from Mangold et al. [59]. For the passengers, the boarding and de-boarding rates for a Type I door were taken from Airbus [60]. The cleaning rate can be estimated as 3 seats/min according to Fuchte [61] with an estimation of two cleaners for this aircraft. The baggage is loaded at two separate locations in one compartment each as bulk cargo. The Boeing 737 utilizes the same cargo storage strategy allowing for their loading and unloading rates [62].

Regarding the battery, the before-mentioned maximum rate of 2C for charging and discharging is applied for the energy transfer required on the ground. However, the strategy for the aircraft is to not have the charging time exceed the longest segment in the turnaround process, i.e., the passengers. This is managed by recharging the battery in flight to a level where the battery can reach the desired state of charge during the turnaround in the specified time frame. The selected values used to calculate the turnaround process are listed in Table 9.

**Table 9.** Turnaround calculation basis for the passengers, baggage and battery.


#### *2.5. Cost Calculation*

The costs are subject to different variables, such as depreciation, insurance, maintenance, fuel consumption, flight crew, cabin crew, landing fees and passenger services. These variables are grouped into direct operating costs (DOC) and indirect operating costs (IOC) [63]. The IOC depend on the relation between the airline and the client and, thus, is difficult to estimate. For this reason, it was not calculated in the present study. This study focuses on the evaluation of aircraft design in economic terms through the cost per available seat kilometer (CASK) and the DOC. A number of methodologies exist to estimate the DOC with high reliability, e.g., ATA, NASA and AEA1989 [64].

ATA is based on industrial statistics from the United States [65], NASA uses an estimation methodology considering interest [66] and AEA is a comparison methodology used in Europe [67]. In this paper, studies were conducted based on the AEA1989 model. The comparison with the reference aircraft was performed considering a depreciation calculation based on the AEA Method 1989 over 14 years. For the year 2035, consequently, a reduction in hydrogen prices between 1.5 €/kg and 5.5 €/kg is projected [68] as shown in Table 10, while an increase of 10% for fossil fuels is estimated compared to the current price [69].

**Table 10.** Price of hydrogen according to S&P Global [68].


On the other hand, Rethink Energy's model establishes, however, that the cost of green *H*<sup>2</sup> will fall to slightly over \$1/kg in 2035 as led by nations such as Brazil or Chile, generating the possibility of reaching even lower prices [70,71]. Consequently, achieving this price for green hydrogen reduces the level of emissions to zero, something that does not occur with blue hydrogen with 10% *CO*<sup>2</sup> emissions or the total release of gray hydrogen obtained only by steam methane reforming (SMR) or gasification [68].

In contrast, electricity prices will remain constant given the continent's transition to renewables. The implementation of these new environmentally friendly technologies affects the fee costs in the DOC. For the AEA method, it was necessary to add charges related to pollution and noise [72]. Those charges are considered for this study, although they are currently not applied at every airport.
