EXPRESS: Improving Operational Performance through Boarding Time Slack: Evidence from the U.S. Airline Industry

Flight delays have a significant impact on airlines, passengers, and the U.S. economy. Airlines also use flight on-time performance (OTP) – a metric based on the proportion of delayed flights – to attract travelers by demonstrating service reliability. It is well established that adding buffers to flight time and ground time improves on-time arrivals. However, such buffers can impose significant costs. The flight and ground times comprise multiple activities, and prior research has primarily focused on studying the impact of aggregate buffers. In this paper, we study the impact of increasing the buffer of a critical ground time activity, namely boarding time, potentially without increasing the aggregate ground time buffer, on flight delays. To do so, we use the context of Delta Air Lines' recent policy changes wherein the airline increased the boarding time for domestic narrow-body flights by 5 minutes and started compensating flight attendants for pre-flight boarding. Using a Difference-in-Difference-in-Differences framework, we analyze and quantify the impact of these policies on Delta’s flight delays. Our findings indicate a significant reduction in departure delays following the policy changes, with increased boarding time playing a critical role in improving on-time departures. Our analysis finds no evidence that boarding compensation contributed to the improvement in on-time departures. We show that the increase in boarding time can improve the likelihood of on-time departures, potentially without requiring additional total ground time or incurring additional scheduling costs. We observe that the benefit is more pronounced for longer flights, busier routes, and later departures in the day. Airlines are often criticized for relying on costly, ad hoc schedule buffering to improve OTP. However, our results demonstrate that carefully planning buffer time for critical activities can significantly improve operational performance.