What does the emergency dept do to prepare for something big like an airplane crash?

Mass-Casualty Plans. Emergency depts, along with the entire hospital and local responders (fire, police, ems) have pre-written response plans to any mass-casualty event. Plans typically identify those in charge, those responsible for patient care, and the various locations within the hospital that care will be provided. Additionally, large reserves of hospital staff (nurses, doctors, support) are typically called in.
ED Preparedness. Emergency departments prepare for biological and logistical disasters by planning, training and being part of a community based awareness system that if a disaster were to occur there would be a plan in place. For example, more physicians would be activated, police called in, triage system expanded, etc...
ATLS, practice. Emergency department personnel commonly practice for multiple casualty incidents or mci's. Many practitioners also take atls courses to prepare for such incidents.