There are several factors that can influence a large scale human factors assessment and which need to be carefully considered during the planning stages. One of the main considerations is whether the assessment is to be performed using the real product, a purpose built facility, or software simulation. The paper describes how such issues impacted emergency egress assessments carried out by Human Factors engineers at BAE SYSTEMS on the newly designed Nimrod Maritime Reconnaissance and Attack (MRA 4) aircraft. These assessments were required to prove that the crew could safely egress the aircraft within the time limits set down by design requirements. The paper discusses some of the lessons learned from these assessments and how they may be beneficial for future Human Factors assessments across industry.