Catherine's mother, Elizabeth Banks, said other cadets watched in horror as the glider plummeted.
"The glider stalled and then looked as though it was coming out of the stall. But, it didn't. It plunged to earth, disintegrating in the process," Mrs Banks said.
A wooden plaque was erected to honour the pair. But it had since fallen into disrepair, Mrs Banks said.
"I am replacing the wooden plaque with a stainless steel one on December 9, 2017 to mark the 20th anniversary of the accident."
Mrs Banks wanted to locate Dr Zulver's widow Geraldine, also a doctor, to inform her of the rededication ceremony.
"I hope that someone may remember Dr Geraldine Zulver and may know of her whereabouts."
She did not hold anyone to blame for her daughter's death and instead felt bad that she had not kept in contact with Dr Zulver's widow.
"Her loss would have been just as great."
Anyone with information on Geraldine Zulver can contact Mrs Banks at banks2110@gmail.com or call (04) 5268204.