Eds discussion of the different models is correct a C that has been upmodeled to a D can never revert back. I don't think its important if FAA records list an update or not. My lawyer friends tell me aircraft logbooks are legal documents and an entry into a logbook making a C a D makes it a D irrespective of whats on file in Ok city. What he did not address is that between 1946 -when the C,s were manufactured- and 1947 when the D models came out the Feds changed the regulations applying to the certification of light aircraft. In part the new regulations had some language applicable to fire standards ie the stainless sheet metal above the header tank. Frankly I am surprised that the feds permitted header tanks at all and it would be interesting to know what lobbying pressure from the airframe manufactures occured to permit the continuation of the practice in existing types. Frankly I can't think of any new post war type certificates issued with header tanks.
I talked to several graybeards who were active in aviation in the early postwar years and several reflected that they thought that the fed action- new licensing standards- to a large degree was based on experience in aircraft technology acquired during WWII. The feds wanted such technology applied to new models such as the Bonanzaa and Navion hence the Ercoupe D- and later models- was licensed under the new regulatory language. Coupes just got dragged along piecemeal. The FAAs mindset seems to be is a D would be a safer aircraft and to retrogress to a C would be reverting to a less safe aircraft. In reality a C and D are essentially the same airframe the elevator travel issue being nothing more than changing the position of the stops. The D I believe also had a larger trim tab which is addressed in the 1320 lb STC. In the real world of aerodynamics it does not make much sense but the rules are the the rules. I am sure that a lot of guys are more tuned into the history than I but this reflects the essentials. Len
