The push-pull CLT concept finally became popular with the retractable gear C-337 Skymaster that was developed in 1964 and introduced in 1965. Finding space for the retracted wheels and struts was a headache, and working out the details of gear door closure both on the ground and in flight was also difficult. As mentioned earlier, a low wing configuration would have eased these problems. However, we had worked the basic principle out on the C-210, and we followed that same concept. In addition, we gained much needed over-the-nose visibility with a 2.5 degree change in wing angle of incidence and a more downward-sloped cowl. Other important changes were the fixed overhead cooling air scoop for the rear engine, a 4-inch increase in the elevator chord, ventral fins shortened by 6-inches, and a new landing gear geometry to place the airplane four inches closer to the ground. The overhead fixed scoop for the rear engine was designed by Wichita University professor Kenneth Razak. To eliminate the cooling fan (from the C-336) a cowl flap was mounted on each side of the rear cowl to regulate the cooling air flow. Ken, an aerodynamics expert, had had a longtime consulting relationship with Cessna, mainly in boundary later control (BLC) research projects. He and I also co-owned a Cessna 195 for a dozen years. Dick Kemper was given the project test pilot assignment, and he performed the maiden flight on March 30, 1964.

In an effort to increase the downwash over the horizontal tail for minimizing nose-down pitch with flap deflection and increasing elevator power for landings, a unique differential flap travel arrangement was installed in the early production airplanes. Those flap sections between the fuselage and booms would be deflected 40° while the outboard flaps would be deflected to a maximum of 25°. This worked very effectively. However, an ice-laden C-337 making an instrument approach at night in Cleveland, Ohio experienced a sudden nose-down pitching motion when the flaps were extended. This resulted in a crash into a home near the airport which was survived miraculously by all occupants of the airplane. It was apparent that an ice build-up on the leading edge of the horizontal stabilizer could not tolerate the increased localized downwash angle due to 40° of flaps. Thus the underside of the tail stalled, causing the loss of elevator control when the airplane pitched downward.

Since it was inappropriate to install a deicing boot on all C-337s, a decision was made to simply remove the differential flap feature. Cessna Service Letter 65-43 dated April 19, 1965 stated, "to improve the stall characteristics of the Model Super Skyrnaster, the inboard flap travel has been reduced from 40° to 25°. This change provides for a much smoother stall recovery under a gear down, flap down and partial power configuration."  This was a mandatory change that involved the modification of the existing inboard flap bellcranks, replacement of the existing flap push-pull rod, and adjustment of the rods, bellcranks, and cables.