1894 28 July 477/3 He is pretty well known as the undownable fighter of the harrow trust.
1957 Apr. 50/2 The undownable question remained: Why were tau and theta [particles] exactly alike in every respect except this one?
2002 A. J. Bellamy vi. 168 The loss of what was considered to be a virtually ‘undownable’ aircraft was a huge blow.