This paper proposes a framework for the qualitative and quantitative characterization of maintenance work to support Maintenance, Repair, and Overhaul (MRO) organizations in performing capacity planning and scheduling. A quantitative assessment based on 372 maintenance projects collected at a Portuguese aircraft MRO confirms that a significant part of the maintenance work is stochastic in nature, given the amount of unscheduled maintenance. The proposed framework, entitled FRamework for Aircraft Maintenance Estimation (FRAME), is intended to allow MROs in managing this uncertainty throughout the maintenance planning process and comprises for that end a set of requirements for data treatment and a method for data analysis. The established requirements address important shortcomings found in the collected data that prevented the use of maintenance data for capacity planning and scheduling as is. The developed method for data analysis, entitled 3-Dimensional Maintenance Data Analysis (3D-MDA), is based on a space-time-skill coordinate system in which indicators are calculated from historical data to comprehensively characterize the expected maintenance work. Space refers to the aircraft work zone where maintenance is performed, time refers to the project work phase when maintenance is performed, and skill refers to the type of technicians required for maintenance to be performed. The established coordinates address the limitations of reviewed techniques by allowing accurate estimations of required resources for capacity planning and an extended range of constraints for maintenance scheduling. Being generic in nature, FRAME is applicable to maintenance in other industries, or even to other activities with due adaptations.