Several accidents have shown that crew members’ failure to speak up can have devastating consequences. Despite decades of crew resource management (CRM) training, this problem persists and still poses a risk to flight safety. To resolve this issue, we need to better understand why crew members choose silence over speaking up. We explored past speaking up behavior and the reasons for silence in 1,751 crew members, who reported to have remained silent in half of all speaking up episodes they had experienced. Silence was highest for first officers and pursers, followed by flight attendants, and lowest for captains. Reasons for silence mainly concerned fears of damaging relationships, of punishment, or operational pressures. We discuss significant group differences in the frequencies and reasons for silence and suggest customized interventions to specifically and effectively foster speaking up.