INTRODUCTION: Adverse weather and poor visual cues are common elements in night-time Helicopter Emergency Medical Service (HEMS) operations contributing to spatial disorientation and fatal accidents. Pilots are required to make weather-related preflight risk assessments to accept or reject a flight. This study's aim was to develop predictive risk assessment tools based on historical accident data to assist the decision-making process.METHODS: We analyzed 32 single-pilot HEMS night-time visual flight rules fatal accidents to identify contributory risk factors. Logistic regression analysis was used to develop prediction nomograms for nonvisual meteorological conditions (non-VMC), cause and nonsurvivable accidents as dependent variables. Risk factors such as temperature dew point spread, elevation difference, and years of HEMS pilot experience, were entered as continuous variables. Flight crew composition, pilot DTE (domain task experience) and flight rule capability, primary missions, and temperature dew point spread were entered as categorical variables. A point scoring matrix transposed model probability to likelihood and consequence severity.RESULTS: The nomograms correctly predicted the likelihood of entering non-VMC, accident cause, and sustaining a nonsurvivable accident in 75%, 55%, and 94% of cases, respectively. Using data from a recent nonsurvivable HEMS accident, the nomogram estimated a 92% probability (Very Likely) of nonsurvivable accident if visual cues were lost.CONCLUSION: These nomograms can provide preflight information to predict the likelihood of adverse safety outcomes occurring during a planned HEMS mission. While further development work is needed, this approach has the potential to improve HEMS operational safety.Aherne BB, Zhang C, Chen WS, Newman DG. Preflight risk assessment for improved safety in Helicopter Emergency Medical Service operations. Aerosp Med Hum Perform. 2019; 90(9):792-799.