# Evaluating Classical Airplane Boarding Methods for Passenger Health during Normal Times
## 1 Introduction  
The occurrence of the novel coronavirus COVID-19 has produced changes in the indicators used for evaluating the airplane boarding methods [1, 2, 3, 4]. In addition to the classical metric represented by the boarding time--namely the amount of time since the first passenger enters the aircraft and the last passengers occupies his/her assigned seat--highly used in non-pandemic situations (to which we are going to refer in the following as \"normal times\"), a series of health metrics have been proposed in the research literature for the times characterized by COVID-19, for the purpose of better evaluating the appropriateness of each boarding method for the new situation the world-wide air transportation industry was facing [1, 5, 6]. These metrics refer to the calculus of a series of health risk indicators through which the risk of contracting the disease by the passengers involved in an airplane boarding process can be evaluated [1, 7, 8, 9, 10, 11, 12]. As mentioned above, the research literature associated with the airplane boarding process pre-COVID-19 has been mainly focusing on answering the question: _What is the fastest airplane boarding method that will ensure the minimization of the airplane turnaroundtime?_[9]. To answer this question, a series of assumptions have been made regarding passengers boarding, such as the passengers' movement inside the aircraft [13; 14], passengers' personal characteristics [15; 16; 17], the presence or absence of passenger groups [18], the type and the quantity of the hand luggage [13; 17; 18; 19; 20], the occurrence of boarding interferences--namely the situations in which either the passengers are blocking the aisle while loading luggage in the overhead compartment (also called aisle interference) or the passengers are interacting with other passengers already seated on the same row as their allocated seat, when they need to make space to allow the other passengers to occupy their allocated seats (known as type-1, type-2, type-3, or type-4 seat interferences--as presented later on in this paper) [16; 18; 21; 22; 23]. In addition to these assumptions, some research papers have extracted data from field trials [24; 25], which have been used in calibrating the models for comparing different boarding methods in terms of boarding time. The boarding time has been the performance metric employed in the pre-COVID-19 situations in which improvements of the classical boarding methods have been proposed in the scientific literature [13; 18; 19; 20; 22; 23; 26; 27; 28; 29]. Throughout the COVID-19 pandemic, the research question accompanying the scientific literature dedicated to airplane boarding process has been: _What is the safest airplane boarding method that ensures a short boarding time?_[4; 9; 11]. For answering this question, the airplane boarding methods used by the airline companies have been tested while accounting for the imposed social distancing measures represented by leaving the middle seat empty [4; 11] or by keeping only 50% of the seats occupied in the airplane [1], and imposing a minimum 1-2 m social distance among the passengers while walking down the aisle to their assigned seats [1; 6; 12]. The performance evaluation of the boarding methods used by airplanes or proposed in the research literature for the COVID-19 times has been made through the use of health risk indicators (e.g., aisle seats risk, window seat risk, number of seat interferences, transmission risk, etc.) [2; 5; 6; 30]. As the signals regarding the recovery of the global aviation industry have been pointed out in the recent research literature [31; 32], there is an expected return to business-as-usual after COVID-19 in the near future [33; 34]. In this context, the lessons learned from the COVID-19 pandemic should not be left behind as the occurrence of another epidemic, pandemic, or endemic situations might arise unexpectedly any time.