I don't know when your visit was, but I suspect that you may have got the wrong end of the stick! Aircraft accident imminent is only used when the aircraft will land with damage or sustain damage on landing, that is all. Generally if a 'technical problem' is the reason given then we will call a Local Standby. This will involve only the local AFS who will turn out and watch the aircraft land and then have a quick chat to the pilot before standing down. The next level is Full Emergency. This is a more responsive callout and some of the local emergency services will also come in just in case. Then we get to Aircraft Ground Incident, Aircraft Accident Imminent and Aircraft Accident. Basically, all the same but mainly vary according to the stage of flight or movement on the airfield. All bells and whistles as there has been damage sustained or people injured. There are other emergencies too, but this is a simplified view. Generally if a hydraulic problem is reported we will call a full emergency, but if the nose wheel will not extend then aircraft accident imminent is the one.
Hope this very simplified answer helps.