De Havilland did a great job designing the Dash-8 Q400. It is a reliable, cost-effective regional turboprop aircraft that can carry up to 80 passengers and cruise at up to 300 knots. The aircraft is powered by two Pratt & Whitney PW150A turboprop engines, which provide excellent fuel efficiency and low noise levels. The Q400 is also equipped with a full suite of advanced avionics and a digital flight control system, making it easy for pilots to operate.

In terms of design, De Havilland did a great job creating an aircraft that is efficient, reliable, and cost-effective. The PW150A engines provide excellent fuel efficiency and the digital flight control system ensures that the aircraft is easy to operate. The Q400 also has a spacious interior, providing ample room for passengers and cargo.

Where De Havilland can do better is in terms of maintenance and repair costs. The Q400 is an older aircraft and parts can be expensive to replace. Additionally, the PW150A engines are known to be finicky and require regular maintenance. De Havilland could improve the aircraft’s reliability and reduce maintenance costs by updating the engine design.

Overall, the Dash-8 Q400 is a reliable, cost-effective regional turboprop aircraft that De Havilland did a great job designing. They can, however, do better by reducing maintenance and repair costs, which could be achieved by updating the engine design.