Powered descent guidance is a critical part of any aerospace mission with a heavy payload in sparse atmosphere or vacuum. It is also finding use in terrestrial landing for vehicle recovery. Powered descent presents the challenge of landing a vehicle softly while using as little fuel as possible. While much work has been done on powered descent guidance and fuel optimality, little consideration has been made of ignition timing and powered descent initiation. The powered descent phase is typically specified by a predetermined condition, altitude, or time. Adaptive Powered Descent Initiation (PDI) is proposed and tested as a strategy of initiating powered descent guidance automatically for robustness and improved propellant performance. It effectiveness is tested by Monte Carlo simulation and results presented for a manned Mars mission. PDI is found to be especially effective in atmosphere as it extends the aerobraking phase without sacrificing safety and gaining propellant efficiency. Its closed-loop formulation ensures reliability in dispersed conditions reflective of practical mission conditions for Mars landing.