<h3>Objectives</h3> We measured the impact of a rapid sequence intubation checklist (RSIC) on first pass success (FPS) during endotracheal intubation by US flight crews, taking into consideration human factors of checklist utilization. We also examined the effects of the RSIC on definitive airway sans hypoxia or hypotension on first pass among adult patients (abbreviated as DASH1a). <h3>Methods</h3> This is a before-and-after observational study within a helicopter ambulance company. The RSIC was used by the crews (Flight Paramedic and Flight Nurse) over a three-year period. Data were evaluated for 8 quarters before- and 8 quarters after- checklist implementation, spanning December 2014 to March 2019. Data included: reason for intubation, use of checklist during intubation, and correlation with the prehospital difficult airway prediction tool, HEAVEN (Hypoxemia, Extremes of size, Anatomic disruption, Vomit, Exsanguination, Neck mobility/Neurological injury) compared with airway management prior to implementation of the checklist. The primary outcome was FPS, and this was compared among those who received RSI before the checklist, versus those who received RSI with the checklist. Also, definitive airway sans hypoxia on first pass among adult patients (DASH1a) was measured before and after RSI checklist implementation. Outcome scenarios were recorded to analyze effectiveness of the checklist. <h3>Results</h3> A total of 10,405 intubations were attempted during the study. First pass success was achieved in 90.9% of patients prior to RSIC, and 93.3% achieved FPS post-implementation (p=<0.001). In the epoch prior to the RSI checklist, there were 4015 (91.9%) of intubations that met the DASH1a metric. After RSI checklist implementation, DASH1a success improved to 94% (p<0.001). Reasons for intubation were similar between the cohorts, although there were more HEAVEN criteria, indicating more difficult airway characteristics, among the post-checklist cohort. It is also noteworthy that overall intubation success without hypoxia improved between the study timeframes, with 89.5% prior versus 93.1% of patients intubated without hypoxia after RSI checklist training, p<0.001. <h3>Conclusion</h3> Implementation of a standardized Rapid Sequence Intubation checklist is a best practice to optimize DASH1a and thereby prevent adverse events related to hypotension or hypoxia in the peri-intubation period. Data suggest that these observations held true even with more objectively difficult EMS airways, based on the higher prevalence of one or more HEAVEN criteria after the checklist implementation.