Don will never retire because he has a passion for saving life. He takes every CFIT accident personally, like it was his fault, not as a failure but a challenge to do even better.
I have had the honour to have known Don as a professional and friend, and in a very very small way within industry to have worked with him.
As a systems designer Don's great attributes revolve around openness and listening; to pilots, operators, and engineers, all based on an endless desire to do better. New ideas and equipment first sought operational assessment and comment. He sponsored local demonstrations, regional meetings, safety forums, and reinforced a vast list of individual contacts.
Every CFIT accident was a personal challenge to learn everything that could be discovered about it to prevent similar events. Even after the successes of EGPWS, every CFIT incident or near miss was used an opportunity to learn and improve the systems; a fantastic role model for aviation safety.
In engineering, Don did not take 'no', or 'unable' lightly, and revelled in the challenge of FAA certification (he didn't suffer 'these' fools gladly).
Although it's some years since we met, the memory of his drive, energy, and passion will remain for ever.