The Covid-19 pandemic is the worst crisis aviation has ever faced. Which one was the biggest one before that pandemic?
It took nearly 3 years for aviation travel numbers to return after the attacks and arguably we are still seeing lasting impacts on changes to broad aviation travel procedures as a direct results of the attacks even 20 years later.
Hindenburg disaster may be a candidate. This fire firmly ended the era of the Zeppelins and other great airships filled up with hydrogen that were flying transatlantic routes at that time. Helium filled airships, even if much less flammable, somehow shared the decline as well.
The aviation needed to switching into very different fixed wing aircraft, using technologies developed during WWII.