Conversation so far:
What position did Ghulam Ishaq Khan hold in 1975?
Defence Secretary
How long was Ghulam Ishaq Khan the ministry of defence?
In 1975, Prime Minister Bhutto subsequently removed Khan from the State Bank, instead posting him at the Ministry of Defence. Khan was appointed Defence Secretary from 195 to 1977.
Did Ghulam Ishaq Khan hold any other positions besides defence secretary?
Yes, Minister of Finance (1977–85)
What did Ghulam Ishaq Khan do as ministry of defense?
As Defence Secretary, he helped manage Pakistan's atomic bomb project and had been directly associated with the program.
And was Ghulam Ishaq Khan successful managing Pakistan's atomic bomb project?
His involvement and support earned him the nickname as Mr Nuke by the U.S. diplomats, while the new media dubbed him as Baba Atom Bomb “
What about other Ghulam Ishaq Khan accomplishments besides his position as Defense Secretary?
Elected Chairman of the Senate in 1985, Ghulam Ishaq was elevated to the presidency after Zia's death in an air crash.

Question: When did this happen?
Self-contained rewrite: When was Khan elevated to the presidency?