The COMAC ARJ21 is a Chinese-made regional jet airliner developed by the Commercial Aircraft Corporation of China (COMAC). It was designed to provide a cost-effective, fuel-efficient, and passenger-friendly aircraft to the regional jet market. It has a capacity of up to 90 passengers and is capable of cruising at up to Mach 0.78. The aircraft has been in service since 2016, and has become one of the most popular regional jet aircraft in China.

When it comes to designing the COMAC ARJ21, there are many aspects which are worthy of praise. Firstly, the aircraft has been designed with an emphasis on efficiency. The aircraft has a modern aerodynamic design, with a high lift-to-drag ratio and a low-weight structure. This results in a fuel efficiency of up to 19% better than some of its competitors, allowing airlines to reduce their operating costs.

The aircraft also features a modern cockpit, equipped with the latest avionics and flight management systems. The systems are designed to reduce the workload of the pilot, and to make the aircraft easier to fly. Additionally, the aircraft features modern engines, which are both fuel efficient and reliable.

The aircraft is also designed with passenger comfort in mind. The interior features a spacious cabin, with comfortable seating and ample legroom. It also features large overhead bins to store luggage, as well as a large galley for catering. The interior is also designed with noise reduction in mind, making the cabin quieter for passengers.

Overall, COMAC did a great job when designing the COMAC ARJ21. The aircraft is fuel efficient, reliable, and comfortable for passengers.

However, the aircraft does have some areas which could be improved. Firstly, the aircraft has a relatively limited range. This may be a limitation for some airlines, who may require a larger range for their operations. Additionally, the aircraft features a limited payload capacity. This may be a limitation for airlines who need to carry more passengers or cargo. Finally, the aircraft's current engines are somewhat fuel inefficient. While the engines are reliable, they are not as fuel efficient as some of its competitors, resulting in higher fuel costs for airlines.

Overall, the COMAC ARJ21 is a great aircraft, with many features which make it popular with airlines. However, there are some areas which could be improved, such as its range and payload capacity, as well as its engine efficiency. If these issues are addressed, then the aircraft can become an even more attractive option for airlines.