# An Unsupervised Learning Approach for Analyzing Unsafe Pilot Operations Based on Flight Data
## 4 Experiment
### Experiment Results and Interpretation  
Through the K-means algorithm, the adverse events caused by pilots' forward stick inputs are divided into four types, and the results are shown in Figure 5, where the \(x\)-axis indicates the feature index while the \(y\)-axis represents the corresponding feature values. From the clustering results, the reasons for different types of adverse events can be interpreted as follows:  
* The first type (headwind influence): This type is represented by yellow lines. The characteristic of this type is that its headwind parameter values are significantly higher than other types. When the aircraft encounters heavy headwinds during the flare, the wind will have an effect of increasing the aircraft pitch angle in a short period of time. If the pilot is not prepared for this situation, then he may apply the forward stick operation to counteract the wind effect. For this type, the pilot should pay significant attention to the wind conditions during the landing stage.
* The second type (high pitch influence): This type is represented by red lines. The characteristic of this type is that the cumulative value of the pitch angle change (PITCH_C) and maximum pitch angle of the aircraft (PITCH_M) is very large before the pilot applies the forward stick inputs. Meanwhile, the height is relatively high, and the influence from the wind is insignificant. So, it can be concluded that the aircraft has endured a continuous increasing in the pitch angle, and its pitch angle finally becomes too large. This is usually caused by the pilot's unawareness of the aircraft status and attitude, i.e., keeping a stable pitch angle with a relatively small vertical speed. As a result, the pilot applies the stick forward operation to directly reduce the pitch angle of the aircraft in order to avoid the tail strike risk. For this type, the pilot should be aware of the pitch attitude of the aircraft, especially when it is close to the ground. If the pitch angle becomes too high and it was not likely to stabilize in time, then the pilot should initiate a go-around.
* The third type (long flare influence): This type is represented by blue lines, from which we observe that the height parameter almost keeps unchanged at a very low attitude during the one second. In this type, although there are some tail winds (WIND \(<0\)), its impact is insignificant. The result indicates that the aircraft keeps flaring at a relatively low height. Most frequently, this is caused by the pilots excessive applying  
Figure 4: The silhouette coefficient of K-means clustering results with respect to \(k\).  
of back stick inputs, which will quickly reduce the aircraft vertical speed. When the aircraft keeps flaring at a relatively low height due to the pilot's excessive reduction in the vertical speed, then they may further perform the forward stick operation to make the aircraft touch the ground as soon as possible so as to avoid the runway overrun [21,32] risk. For this type, the pilot should be aware of the vertical speed of the aircraft and avoid reducing the vertical speed too much before entering the flare.
* The fourth type: This type is represented by green lines. This type of flight does not show a significant low height or large pitch angle, which means the aircraft was not enduring an abnormal situation. For this type, the pilot should get more flight training to improve their landing skills.