The rules under which aircraft are certified evolve over time. The simple Cessna singles are all certified under the old CAR3 rules dating back to the 1950's, which did not require establishing life limits and grandfathered onto todays Part 23 regulations. The Traumahawk was certified as a new design under the rules that has been updated in the mid 1970's. One of the updates was requiring airframe life limits. I think Piper simply did not want to spend the money to do the required engineering work and testing to go past 11,000 hours.
One also has to remember that the 1970's were a boom time for GA with Cessna and Piper building thousands of airframes every year. Schools would routinely use a trainer for a couple of years and then replace it with a new one, so 11,000 hours probably seemed like plenty. I don't think that even in their wildest dreams, did the light plane designers of that era think that you would routinely see trainers still on the line that were 35 + years old with flight times in over 10 K hours. The highest time C 152 I know of has 24 K + total time, all as an ab inito trainer, and is still working every day
Last edited by Big Pistons Forever; 11th Dec 2011 at 22:48.