# An Unsupervised Learning Approach for Analyzing Unsafe Pilot Operations Based on Flight Data
###### Abstract
Excessive vertical overload not only gives passengers a bad flight experience but also largely increases the airlines' maintenance costs. Severe incidents may even threaten the lives of passengers. According to [6], the most important reason for the increase in the vertical overload is due to the pilot's inappropriate applying of forward stick inputs which makes the aircraft nose down before it touches the ground. This operation usually reduces the pitch angle of the aircraft, which will further reduce the lift force the aircraft can gain. If this happens in the flare phase [7], i.e., the few seconds before touchdown, then with a high probability the aircraft will touch ground with an excessive vertical overload, which causes the hard landing safety incident [8, 9]. Therefore, we define the event in which a pilot performs this forward stick operation as an adverse event. Actually, from the Airbus A320 Flight Crew Techniques Manual (FCTM), a pilot should avoid applying any nose down inputs during the flare phase to avoid the hard landing or bounced landing risk. However, our statistical results from about 86,504 flights show that this unsafe pilot operation occasionally happens. In view of the above phenomenon and given that all the pilots have undergone professional training, why would they perform such unsafe stick operations in the flare phase? Although the Airbus company conducted two case studies on this topic (A Focus on the Landing Flare, [[https://safetyfirst.airbus.com/a-focus-on-the-landing-flare/](https://safetyfirst.airbus.com/a-focus-on-the-landing-flare/)]([https://safetyfirst.airbus.com/a-focus-on-the-landing-flare/](https://safetyfirst.airbus.com/a-focus-on-the-landing-flare/)), accessed on 22 November 2022), systematic research, especially based on a large volume of flight data, so as to give more insights and a comprehensive overview of this phenomenon, is still missing. The Quick Access Recorder (QAR) [10, 11] is an airborne flight recorder designed to provide quick and easy access to raw flight data, which can record multivariate time-series flight parameters during the entire flight. Compared to other data recording systems like the DFDR (Digital Flight Data Recorder), black box, etc., the QAR has the advantages of a higher sampling rate, more parameters, and faster data transmission. Recently, the QAR devices have been widely adopted by airlines to improve flight safety [4, 12, 13, 14]. The flight parameters collected by the QAR devices include the state parameters of the aircraft, such as the speed, pitch angle, radio altitude, acceleration, etc. They also contain the parameters of the pilot operations, such as the pitch control, roll control, throttle lever position, etc. Some external environmental parameters, including the wind speed, wind direction, and temperature, are also collected by the QAR device. Our work is based on a QAR dataset with 86,504 flights from the domestic airlines in China. We mainly aim to fill the aforementioned research gap, give more insights, and establish a comprehensive overview about the unsafe pilot stick operations during the flare phase. Specifically, our work will answer the following key questions: (1) Are there any typical reasons among the pilots who applied forward stick inputs during the  
Figure 1: Statistics of serious aircraft incidents from 1959 to 2019.  
flare? (2) What are the key contributing factors related to this unsafe stick operation? and (3) How is this phenomenon related to different airlines, airports, and pilots? To this end, we first carefully preprocessed the raw flight data through data cleaning, parameter transformation, etc. After that, we selected key features that are helpful for explaining this operation based on the experience of flight experts, and then clustered these features through the K-means algorithm [15, 16]. From the clustering results, four main categories are summarized, corresponding to four influencing factors, i.e., the headwind influence, high pitch influence, long flare influence, and pilot personal influence.