Frank,
Have you given any thought to shooting aircraft at a better time of day to capture a more colourful sky, or at night?
eg
http://www.pbase.com/cjsouthern/imag...0133/large.jpg
http://www.pbase.com/cjsouthern/imag...1403/large.jpg
http://www.pbase.com/cjsouthern/imag...6538/large.jpg
http://www.pbase.com/cjsouthern/imag...2905/large.jpg