Which attack method has NOT been attempted against an airport or airline since September 11, 2001?

Chemical or biological attacks have not been attempted against airports or airlines since September 11, 2001. This choice reflects the specific focus and methods employed by terrorists in the aviation sector during the years following that event, where the emphasis tended to be on more direct and immediate forms of violence, such as explosive devices, armed assaults, and hijackings.

Explosive devices have been used in various plots and incidents targeting airports and airlines, as have armed assaults. Likewise, hijacking had a significant historical precedent, and attempts have continued in some form post-2001, although they might not have always succeeded or been carried out with the same level of impact as in the past. The lack of chemical or biological attacks indicates a strategic choice by terrorists, likely influenced by the challenges of executing such complex attacks within the security environment of modern aviation.