Keywords: Human-Machine Interface. According to accident statistics, human factors are a crucial factor affecting modern aviation safety. Globally, over two-thirds of flight accidents are directly caused by flight crew error. In my country's civil aviation accident statistics over 16 years, human factors accounted for 64%. If air traffic control and maintenance factors are also considered, this rate rises to approximately 80%–90%. Therefore, human factors are irreplaceable as a key factor in aviation safety. Human factors, also known as human factors in my country's civil aviation industry, refer to designing aircraft according to human anatomy, physiology, and psychology to meet human requirements for efficiency, health, safety, and comfort; optimizing the human-machine-environment system to achieve optimal coordination among the three, maximizing economic benefits with minimal labor costs. Modern aircraft are increasingly automated, and automatic control systems have reached a high level of sophistication. Simultaneously, modern aircraft are products that fully utilize human factors engineering. People have experienced the pleasant environment of modern aircraft cockpits, the efficiency and comfort brought by automation, and the high level of safety and reliability. Significant technological advancements have further highlighted the crucial role of human factors. I. SHEL Model Humans in specific human-machine interfaces can be described using the SHEL model. Human errors easily occur at the human-centric interfaces with hardware, software, the environment, and other people. These interfaces are also known as the four interfaces of the SHEL model: L-S interface, L-H interface, L-E interface, and L-L interface. Here, S (Software) represents software, H (Hardware) represents hardware, E (Environment) represents the environment, and L (Liveware) represents the human. The L-S interface refers to the relationship between humans and software, studying reasonable operating procedures, checklist procedures, and emergency procedures to simplify work processes and reduce human workload and intensity. The L-H interface refers to the relationship between humans and hardware, studying the mutual adaptation between humans and displays/controllers, and making human-machine interface design more suitable for human requirements. The L-E interface refers to the relationship between humans and their environment, studying the effects of noise, vibration, high and low temperatures, acceleration, biological rhythms, time differences, etc., on humans in specific environments, as well as adaptation processes and reaction patterns. L-L interface refers to the relationship between people, that is, the coordination relationship between people and their related people in the workplace. For example, in the cockpit environment, it is the management psychology and social psychology that studies the interpersonal relationships between crew members, between the crew and air traffic controllers and dispatchers, individual communication, crew cooperation and coordination, and the captain's leadership art. The core issue of the SHEL model is people. No matter how high the degree of automation of the machine is, people are always the most important dominant factor in the operating system. Due to the limitations of human beings, people are also a variable and unreliable factor. II. Functions of the human-machine system 1. Information reception Information comes from the aircraft, A/P (autopilot), ATC (air traffic control), and the external environment. Information reception is the pilot's monitoring of the above information. It is the basis for judgment and decision-making and even operation during flight. Judgment and decision-making is the central link of the human-machine system. People must receive accurate, comprehensive and timely information in order to make correct judgments and effective operations. In terms of information reception, it includes human-machine information chain, human-human information chain and air-ground information chain. (1) Human-machine information chain (PC): The accuracy, comprehensiveness and speed of human recognition of instrument devices, especially the timeliness of the response to warning devices. In autopilot mode, the aircraft engages in a complex flight process coordinated by the automaton. This process is often invisible, and the human's main task is to monitor and manage the aircraft's status, receive information, and compare it with the normal state. While this new human-machine division of labor minimizes the physical burden on humans, it brings new psychological problems, such as attention deficit, confusion, and lack of situational awareness. A tragic example is the B747 aircraft approaching below a safe altitude. When the ground proximity warning system (GPWS) was activated, the pilot not only ignored it but also tried to ignore the annoying sound to turn it off, resulting in a fatal crash. Therefore, it is required that once the GPWS is activated, the aircraft should be pulled up immediately until the warning ends. The issue of inappropriate human-machine interfaces should also be given sufficient attention. Current aircraft design must consider ergonomic principles. However, due to racial, cultural, habitual, and regional differences, mismatches in the human-machine interface can still occur. For example, the altitude of modern transport aircraft currently being introduced is measured in feet, while my country's flight rules require air traffic control to use the International System of Units (SI), i.e., altitude is measured in meters. This requires pilots to make conversions, increasing the possibility of flying the wrong altitude. In addition, the flight deck is marked in English, and the English prompts for important information create an unfamiliar working environment for Chinese pilots, forming a barrier in human-machine information exchange. The Urumqi MD-82 aircraft crash is an example of this. Therefore, English has become the key to modern aircraft piloting. Whether the aircraft takes into account the human body size and ergonomic standards and specific requirements of Chinese pilots, whether it is convenient and comfortable to use, and whether it is prone to misoperation are also issues that should be noted. (2) Human-Automatic Information Chain (AC): refers to the crew's monitoring of the A/P. It should be distinguished from PA. The status of A/P has a great impact on the aircraft. Ignoring the AC chain will result in improper handling by the crew or human-machine confrontation. (3) Human-Human Information Chain (CC): refers to the information exchange within the crew to achieve information sharing and coordination. Information exchange within the crew is not optional, but a necessary process during flight. Because the design of modern aircraft has exceeded the capabilities of a single person, only through full communication, reasonable division of labor, and coordination among crew members can the overall efficiency of the crew be brought into play, and the safe flight of the aircraft be guaranteed. (4) Air-to-ground information chain (AG): Air-to-ground information transmission mainly relies on language and hearing, and mostly occurs during takeoff and approach. This is the busiest stage for the crew and also the period when the roles of human and A/P are exchanged. The polysemy of language itself and different understandings of the same sentence can cause semantic changes due to subtle hints. For example, "a certain aircraft turns right 30°", this sentence itself has a double meaning: one is to turn right to a heading of 30°; the other meaning is to turn right 30° from the existing heading. "take-off power", take-off as a noun can be understood as (engine) takeoff power; take off as a verb can also be understood as shutting down the engine (take off power), and these two understandings are exactly opposite. The most tragic accident in aviation history occurred in the Netherlands in the 1970s, when two B747 aircraft collided, resulting in 583 deaths. The cause was a misunderstanding of semantics! Achieving air-to-ground English communication places higher demands on Chinese pilots and air traffic controllers, and is also a severe test. Only when the vast majority of pilots and air traffic controllers have been trained and their actual English listening and speaking skills are very fluent and their semantic understanding is accurate can they naturally transition to English communication. Otherwise, it will increase the psychological pressure and mental burden on pilots and threaten flight safety. The main dangers in terms of information reception are: visual illusion; unclear or incomplete instrument reading leading to incorrect judgments; non-standard terminology causing different or unclear semantic understanding, or improper equipment application causing obstacles to the transmission of language information; insufficient crew communication, unclear or non-standard briefings, resulting in poor coordination; loss of situational awareness, sometimes manifested as "seeing but not seeing", lack of sensitivity to information, etc. (5) Human-external environment chain (EC): refers to human monitoring of the external environment. The crew not only receives information from instruments, but also from the external environment. Studies of Controlled Flight Into-Earth (CFIT) accidents show that over two-thirds of these accidents are due to a lack of awareness of terrain altitude, and many are caused by confusion between visual and instrument operations. 2. Information Processing: The core of information processing is judgment and decision-making, the quality of which is reflected in the richness of knowledge and experience, and the clarity, sufficiency, and timeliness of information. Decision-making ability is one of the fundamental components of a captain's professional skills. According to McDonnell Douglas's "1995 Commercial Transport Aircraft Accident Statistics," pilot error ranked first among landing/approach accidents involving commercial jet transport aircraft worldwide from 1958 to 1995, and inaccurate judgment was the leading cause of pilot errors. It is worth noting five harmful attitudes that affect a pilot's judgment and decision-making ability: ignoring authority, overconfidence, recklessness, impatience, and giving up. These pose a significant threat to aviation safety. For example, the dangers of "overconfidence" to safety are widely acknowledged in the aviation industry. International experience shows that pilots with 300-500 hours of flight time after obtaining their licenses are most prone to "overconfidence" and experience the most accidents. 3. Operation Operation is the result of judgment and decision-making, and also the requirement of operating procedures. Correct operation is the guarantee of normal aircraft operation, while inaccurate or incorrect operation will bring extremely dangerous consequences. Operation errors mainly include two aspects: firstly, the crew's division of labor and cooperation; secondly, the operating skills and proficiency. (1) The operation process refers to the overall effect of the crew on the aircraft. It requires the crew members to have a clear division of labor and cooperate in a coordinated manner. The main problems in crew cooperation in my country are: the division of labor is not standardized and not strict, especially in some emergency situations where there is no clear plan. The situation of "the captain being left to his own devices" often occurs. Communication between people is very important. In the past, even when using the autopilot to complete a flight maneuver, it often required the captain, co-pilot, and sometimes flight mechanic to work together. In this standardized cooperation, communication was automatic. But today, either the captain or co-pilot can independently give the autopilot an instruction. The autopilot works silently, and the action is so simple and subtle that it is difficult for the other party to detect it. Without knowing, another person may perform another operation in a different way, which will result in a contradiction between the previous and subsequent operations. The crash of China Airlines' A300-600 aircraft in Nagoya is a painful lesson. Due to human error, the GO Lever device was activated and the aircraft began to go around. The crew was unaware of the AC status, leading to a "confrontation" between humans and the automatic system. (2) Human operating skills and proficiency. It is mainly reflected in the pilot's manual skills. Statistical results show that the phenomenon of misoperation caused by insufficient skills and experience is relatively prominent, and it often becomes a link in the event chain. III. Solutions 1. Strictly follow the rules and regulations and eliminate violations of rules and regulations. There are already quite a few accidents caused by violations of rules and regulations. Our analysis of the lessons learned from 16 years of accidents using the elementary event analysis method proves that compliance with rules and regulations is the most important factor (as shown in the attached figure). It should be emphasized that standard operating procedures, which comprehensively consider safety, efficiency and ease of operation, are the result of careful design and experience accumulation, and some are even obtained at the cost of blood. Deviating from standard operation is equivalent to deviating from the barrier of aviation safety. 2. Integrating Line Operation Flight Training (LOFT) with In-Depth Crew Resource Management (CRM) Training: The core of this training is to strengthen crew coordination and cooperation, fully utilize various information resources and control methods within the cockpit, and maximize the overall functionality of the crew to ensure safe, efficient, and economical flight. This training is tailored to the automation level of the aircraft's cockpit equipment and incorporates practical line flight scenarios, employing flexible and diverse methods. Currently, CRM is widely implemented in major airlines worldwide and has yielded positive results. 3. Emphasizing the Crucial Role of Captain Competence: Captain competence is the most important aspect of the human factor. First, the captain is the most important and critical frontline leader and organizer of the airline's safe operation, the commander and gatekeeper of the last line of defense for safety. The captain's technical level and work ability represent the overall level of the airline's safe operation. Captain competence reflects the overall competence of the flight crew and serves as a benchmark for the entire airline's safety technology. The captain is the direct organizer and key executor of crew division of labor and cooperation, playing a vital role in flight operations and CRM training. The captain's non-technical factors also play a crucial role in flight safety. Strengthening the training and selection of qualified captains and strictly reviewing their qualifications are effective ways to improve the overall quality of captains. 4. Fatigue is a major enemy. The harm of fatigue to flight safety is often unexpected and unpredictable. The severity of the dangers of fatigued piloting lies in its potential to make pilots careless, prone to sadness and lack of concentration, and prone to sluggish and irregular movements. Fatigue can also make crew members more easily agitated, dull-witted, and depressed, thereby impairing cooperation and coordination among crew members. Fatigue caused by prolonged sleep deprivation is even more dangerous, causing disorientation and hallucinations. Without exception, fatigued pilots exhibit a severe decline in work ability or complete loss of work capacity. All of these pose a significant threat to flight safety and increase the likelihood of human error. 5. Pilot training in handling complex situations should be strengthened. With increasing automation, pilots are increasingly becoming managers and decision-makers, thus requiring more knowledge. Experts estimate that traditional skills-based training can only cover 25% of the required capabilities; increasing knowledge training is an inevitable result of aviation development. The curriculum should not only include content on new equipment and technologies, but also, in accordance with the requirements of the International Civil Aviation Organization (ICAO), content on human factors. This includes topics such as the limits of human ability, how to make scientific judgments and decisions, how to control emotions, and how to maintain situational awareness under highly automated conditions. Handling special environments is crucial; for example, weather conditions are second only to pilot error in landing/approach accidents, with heavy rain, fog, darkness, and wind accounting for the majority, all of which significantly impair visual judgment. Handling special environments requires pilots to possess composure under pressure, extensive flying experience, excellent teamwork, and outstanding technical skills. 6. Properly Understanding the Relationship Between Efficiency and Safety: Efficiency and safety are not contradictory, but rather complementary. Safety is fundamental to efficiency and the basic principle of aviation operations; reputation is the soul of an airline, and safety is the cornerstone of that reputation. Safe operation does not mean no operation or reduced operating volume, but rather eliminating all unsafe factors and improving safety productivity.