—Safety, Efficiency, and Comfort This article, written from the perspective of a test pilot, discusses the principles that should be followed in human-machine interface (HMI) design from several aspects, proposing that safety, efficiency, and comfort are complementary requirements. The impact of the cockpit HMI on flight extends beyond its influence on the pilot's physical, psychological, and conscious activities; it directly affects pilot efficiency and flight safety. The complexity of HMI design lies in the fact that humans are the users, and the suitability of the interface must be proven through human use. Safety is the first principle of HMI design . Flight safety is the primary requirement for flight. The factors determining safety are extremely complex, including external environmental factors, aircraft and system factors, and human factors, among which human factors are the most complex. The safety of the HMI is assessed primarily from the following aspects. ● Does it cause direct physiological harm to the pilot? The entire flight process for a pilot includes entering and exiting the cockpit, takeoff, maneuvering, instrument flight, weapon launch, landing, emergency landing, and ejection. During these processes, the human-machine interface may cause physiological harm to the pilot, which designers must strive to avoid during cockpit design. For example: Are there any sharp surfaces on the exposed surfaces of the cockpit that could cause cuts to the pilot's body? Can the pilot restraint system eliminate or minimize the overload and impact injuries suffered by the pilot under high overload conditions (such as impacts, collisions, etc.)? During ejection, will the cockpit system cause direct or indirect harm to the pilot, such as strangulation, cuts, bruises, blows, etc.? Are the aircraft equipment easily damaged, causing harm to the pilot, such as canopy damage, system leakage, fire, decompression, frostbite, hypoxia, etc.? ● Is it prone to causing pilot errors? Pilots are human, not machines, and errors in judgment and use are inevitable. Therefore, cockpit design should have a certain degree of fault tolerance and error prevention. For example: Modular layout of equipment functions makes the human-machine interface more rational and logical, preventing pilot misoperation; special warnings for dangerous switches remind pilots to prevent accidental operation, and their locations should be specially positioned, as far away from frequently used switches as possible, with anti-misoperation steps to prevent pilots from accidentally colliding with them while operating other switches; efforts should be made in logical design to invalidate certain pilot misoperations, such as retracting landing gear on the ground or deploying parachutes in the air; rational design of instruments and displays avoids pilot misinterpretation; warnings of dangerous conditions and incorrect operations enable pilots to promptly detect faults and errors and correct them as quickly as possible. ● Ability to overcome the dangers of complex environments: Environments have a significant impact on flight safety. For human-machine interfaces, the most important aspect of enabling aircraft to fly in complex environments is the ability to overcome the effects of low visibility and clouds. This requires the aircraft's navigation interface to accurately guide the aircraft to the landing airport and land safely under low weather conditions; a good cockpit attitude display facilitates pilot judgment of the aircraft's attitude and reduces the likelihood of pilot misinterpretation. ● Safe Flight Under Malfunctions: Malfunctions in various aircraft systems are unavoidable during flight. From a design and manufacturing perspective, we must maximize system reliability and minimize equipment failure rates. Simultaneously, from a human-machine interface (HMI) design perspective, we should implement redundancy design. For critical systems, displays, and instruments, redundant designs should be adopted. For example, attitude displays include head-up displays (HUDs), electronic flight instruments, and backup instruments; landing gear systems have normal and emergency deployment options; and ejection systems have normal and emergency ejection options. Efficiency is a fundamental requirement for military aircraft in modern warfare. From a HMI design perspective, efficiency should be reflected in the following aspects: ● Display systems should display the information most relevant to the pilot. Traditional aircraft cockpit display systems provide very limited information, resulting in generally high pilot workloads. Modern avionics and integrated fire control systems have significantly increased the information capacity of cockpit display systems, significantly reducing pilot workloads. Examples include multi-functional display systems with "one HUD and three DVRs" and integrated warning systems combining sound, light, and text information. How to effectively utilize limited cockpit space to provide pilots with more useful information is a shared challenge for designers and pilots. The cockpit display system should provide pilots with the information they need most, tailored to different flight phases. For example, throughout the flight, attitude information is of utmost concern to pilots. During the cruise phase, pilots are most interested in the aircraft's position and aerodynamic parameters; therefore, the display should primarily provide attitude, aerodynamic, and position information. During the attack phase, pilots are most concerned with target and weapon information; therefore, in addition to necessary attitude and flight parameters, the display should mainly show attack-related information. When an aircraft malfunctions, pilots are most concerned with malfunction information. Thus, the display system must provide pilots with the information they need most, based on their specific requirements. ● Providing a User-Friendly Display Interface: The cockpit display system should provide pilots with the most intuitive and visual information, enabling them to quickly and accurately obtain information. This includes standardizing head-up display (HUD) images, mapping navigation screens, simulating digital information, and translating text information into Chinese. From an optimization design perspective, the clarity and logic of the display screen, the rational arrangement of various screen positions, and the readability of the characters should also be considered. Furthermore, screen switching and adjustments should be convenient and quick. To truly achieve optimized human-machine interface design, pilots should participate in the cockpit design and evaluation. Furthermore, pilots must be objective and comprehensive in their evaluation of the human-machine interface (HMI), and should not be limited by traditional aircraft cockpit layouts or past flight habits. To obtain objective and reasonable assessments, multiple pilots should participate in trials to obtain statistical data. ● Pilots must be able to quickly and effectively control various systems. The cockpit contains numerous switches, levers, and other controls with diverse functions, and these are used very frequently. To enable pilots to use them quickly and correctly, the design should consider a rational arrangement based on different functions: control components should have good accessibility; a HOTAS (Hardware Over-The-Air) design should be adopted, allowing pilots to switch systems and change functions without taking their hands off the stick; frequently used switches and levers should be placed in easily accessible locations; control components should be convenient and flexible to use; and the direction of operation or movement of control components should conform to the pilot's physiological habits. Comfort is an indispensable factor . Modern technology is human-centered, and the design of modern military aircraft increasingly considers human factors. The comfort of the HMI is not merely a matter of feeling; it also affects the pilot's psychological and physiological state, directly impacting pilot performance and even flight safety. We must not underestimate its importance. Pilots' perception of comfort has both a rational and an emotional aspect; this issue is complex and unavoidable. Visual Factors – A Harmonious and Pleasant Cockpit Appearance We have this feeling: harmonious colors and images give a sense of peace and pleasure, while a dark and cluttered environment gives a sense of anxiety and unease. Pilots experience the same thing with the appearance of their cockpit. In cockpit design, attention should be paid to: smooth and flat surfaces; harmonious and orderly arrangement of displays, instruments, and equipment; harmonious and unified colors within the cockpit; a wide field of vision for the pilot; and soft lighting. Auditory Factors – A Quiet Cockpit Strong noise can cause irritability and fatigue. Cockpit noise is unavoidable; the question is how to reduce the noise experienced by the pilot. It is necessary to identify the sources of noise and address them accordingly: improve the performance and quality of the communication system to reduce radio noise; adjust the engine to reduce engine noise; improve the helmet's noise reduction function; and increase the structural strength of the aircraft to avoid the humming sound of aircraft vibration. Tactile Factors – Seat and Flight Equipment Comfort The comfort of seats and flight equipment is a crucial aspect of human-machine interface comfort. This includes ensuring the seat's shape is suitable for the pilot's body without any compression or obstruction; the parachute straps are of moderate tightness, allowing for comfortable wearing; the helmet's size and shape are appropriate, and the visor is soft and comfortable; and the anti-G suit and compensation suit are comfortable to wear, with the anti-G and compensation systems functioning well. Spatial Factors – Spacious Cockpit A cramped space can create a feeling of confinement, and pilots are highly sensitive to their surroundings. Providing pilots with a slightly more spacious environment, allowing them room to move, is essential. A slightly higher seating position provides a wider field of vision and also creates a sense of spaciousness. All of these factors contribute to improved pilot comfort. The Relationship Between Safety, Efficiency, and Comfort In human-machine interface design, designers often prioritize functionality and performance—the principle of efficiency—and tend to overlook or underemphasize safety and comfort. However, these three aspects are interconnected and mutually influential, and should be considered holistically during the design process. First, a safe cockpit environment reduces pilots' anxieties, allowing them to focus most of their energy on flying, thus improving work efficiency. Otherwise, pilots have to worry about avoiding mistakes, misreading data, and expending considerable energy on flying, increasing their workload and inevitably reducing efficiency. Flying in an uneasy state puts immense psychological pressure on pilots, making it impossible to reduce workload and leading to a lack of comfort. Conversely, a user-friendly and efficient cockpit interface makes piloting easier, with high-information displays allowing for comprehensive monitoring of all aircraft systems. This reduces the probability of errors and enables timely and correct handling of malfunctions. An efficient cockpit interface reduces pilot workload, fatigue, and psychological pressure, allowing for a more pleasant flying experience and a greater sense of comfort. A comfortable cockpit environment keeps pilots energetic, coordinated, quick to identify problems, calm, and accurate in handling them, thus reducing the chance of errors. Therefore, the design of the cockpit human-machine interface should consider all three aspects, with a focus on key areas. At the same time, we should increase investment in scientific research and design a cockpit interface that pilots prefer.