A method of countersinking a hole (4, 102) formed in a curved aircraft panel (2, 100) using a drilling apparatus, the drilling apparatus comprising a pressure foot and a cutting tool, the pressure foot being for contacting with a surface of the aircraft panel (2, 100) during a countersinking process, the pressure foot comprising a passage through which the cutting tool is to be passed during the countersinking process, the method characterised by comprising: providing a first digital model, the first digital model being a model of the aircraft panel (2, 100) including a model of the hole (4, 102); providing a second digital model, the second digital model being a model of at least part of the pressure foot including a model of the passage; arranging the first model and the second model such that a curved surface of the first model is in contact with a front surface of the second model, and such that an axis of the model of the hole (4, 102) is aligned with an axis of the model of the passage; using said arrangement of the first model and the second model, determining a first distance, the first distance being a distance between an opening of the model of the passage at the front surface of the second model and an opening of the model of the hole (4, 102) at the curved surface of the first model, the distance being in a direction substantially parallel to the aligned axes; determining a second distance, the second distance being a sum of: a distance between a tip of the cutting tool and the front surface of the pressure foot, the determined first distance, and a predetermined depth to which the hole (4, 102) is to be countersunk; and performing the countersinking process including moving the pressure foot such that a front surface of the pressure foot is in contact with a curved surface of the aircraft panel (2, 100), and such that an axis of the hole (4, 102) is aligned with an axis of the passage, and moving the cutting tool through the passage so as to countersink the hole (4, 102), wherein a distance through which the cutting tool is moved is equal to the determined second distance. providing a first digital model, the first digital model being a model of the aircraft panel (2, 100) including a model of the hole (4, 102); providing a second digital model, the second digital model being a model of at least part of the pressure foot including a model of the passage; arranging the first model and the second model such that a curved surface of the first model is in contact with a front surface of the second model, and such that an axis of the model of the hole (4, 102) is aligned with an axis of the model of the passage; using said arrangement of the first model and the second model, determining a first distance, the first distance being a distance between an opening of the model of the passage at the front surface of the second model and an opening of the model of the hole (4, 102) at the curved surface of the first model, the distance being in a direction substantially parallel to the aligned axes; determining a second distance, the second distance being a sum of: a distance between a tip of the cutting tool and the front surface of the pressure foot, the determined first distance, and a predetermined depth to which the hole (4, 102) is to be countersunk; and performing the countersinking process including moving the pressure foot such that a front surface of the pressure foot is in contact with a curved surface of the aircraft panel (2, 100), and such that an axis of the hole (4, 102) is aligned with an axis of the passage, and moving the cutting tool through the passage so as to countersink the hole (4, 102), wherein a distance through which the cutting tool is moved is equal to the determined second distance.