Directive 86-19-10. Without the possibility of metal fatigue, the wooden wing has an indefinite life expectancy and is considered by some pilots to provide a smoother ride in turbulence. 1960s In January 1960, the Mooney company convinced Ralph Harmon to leave McDonnell Aircraft in St. Louis, Missouri, and take over management of the engineering efforts. He insisted on replacing the wood in the M20 with aluminum, and the all-metal M20B was completed by the end of 1960, less than a year after his arrival. In 1961, the company sold 222 M20B airplanes. The following year, the M20C was introduced and 336 were sold that year. Also known as the Mark 21 and later the Ranger, the M20C had several improvements over the M20B, including greater deflection on control surfaces, reduced cowl flap openings for better engine cooling, improved exhaust scavenging with a Hanlon and Wilson exhaust system, new battery access door, more powerful landing light, lightweight floor, an increased gross weight of , lighter empty weight, new instrument panel layout, and a higher maximum flap angle of 33 degrees. In 1963, the M20D was introduced, essentially an M20C with fixed landing gear and a fixed-pitch propeller. This had a slightly lower price than the M20C and was intended as a basic or trainer model which would have lower insurance costs and which would compete with the Piper Cherokee 180. It could be upgraded with retractable landing gear, and in fact, most of them were upgraded over the years. The M20D was produced from 1963 to 1966 with a total production of 161 units. In 1964 the M20E Super 21 was introduced. It was also based on the M20C but with a more powerful Lycoming fuel-injected engine. The company sold 366 M20E units that year. In 1965, a new feature was introduced to the M20. It was called "positive control" and was a single-axis autopilot produced by Brittain Industries. It maintained the wings of the airplane in a level position unless a button on the control wheel was depressed for turns and banks. It was a controversial feature, liked by some pilots and disliked by others. Production and sales of the M20 continued to increase. In 1966, a total of 760 units were delivered, including 280 of the M20C Mark 21 planes and 473 of the M20E Super 21s. A new model, the M20F Executive 21, offered more legroom due to