A method of monitoring an auxiliary power unit, APU, onboard an aircraft, the method comprising: obtaining measurement data indicative of performance of the APU; mapping the APU to a first degradation group from among a plurality of degradation groups based on a relationship between the measurement data and historical measurement data associated with the first degradation group; obtaining contextual data associated with operation of the aircraft; and determining a maintenance recommendation for the APU based on the contextual data using a predictive maintenance model associated with the first degradation group, wherein: obtaining the measurement data comprises obtaining exhaust gas temperature measurement data during operation of the APU; mapping the APU to the first degradation group comprises identifying the APU as belonging to a first degradation group of APU lifecycles from among a plurality of degradation groups based on a relationship between the EGT measurement data and historical EGT measurement data associated with the APU lifecycles of the first degradation group; the predictive maintenance model is determined for the first degradation group based on relationships between the historical EGT measurement data associated with the APU lifecycles of the first degradation group, historical contextual data associated with the APU lifecycles of the first degradation group, and historical maintenance data associated with the APU lifecycles of the first degradation group; and a maintenance schedule for the APU is influenced by the maintenance recommendation. obtaining measurement data indicative of performance of the APU; mapping the APU to a first degradation group from among a plurality of degradation groups based on a relationship between the measurement data and historical measurement data associated with the first degradation group; obtaining contextual data associated with operation of the aircraft; and determining a maintenance recommendation for the APU based on the contextual data using a predictive maintenance model associated with the first degradation group, wherein: obtaining the measurement data comprises obtaining exhaust gas temperature measurement data during operation of the APU; mapping the APU to the first degradation group comprises identifying the APU as belonging to a first degradation group of APU lifecycles from among a plurality of degradation groups based on a relationship between the EGT measurement data and historical EGT measurement data associated with the APU lifecycles of the first degradation group; the predictive maintenance model is determined for the first degradation group based on relationships between the historical EGT measurement data associated with the APU lifecycles of the first degradation group, historical contextual data associated with the APU lifecycles of the first degradation group, and historical maintenance data associated with the APU lifecycles of the first degradation group; and a maintenance schedule for the APU is influenced by the maintenance recommendation. obtaining the measurement data comprises obtaining exhaust gas temperature measurement data during operation of the APU; mapping the APU to the first degradation group comprises identifying the APU as belonging to a first degradation group of APU lifecycles from among a plurality of degradation groups based on a relationship between the EGT measurement data and historical EGT measurement data associated with the APU lifecycles of the first degradation group; the predictive maintenance model is determined for the first degradation group based on relationships between the historical EGT measurement data associated with the APU lifecycles of the first degradation group, historical contextual data associated with the APU lifecycles of the first degradation group, and historical maintenance data associated with the APU lifecycles of the first degradation group; and a maintenance schedule for the APU is influenced by the maintenance recommendation.