A computer-implemented method for determining Auxiliary Power Unit health indicator, the method comprising: receiving, by one or more processors of a processing server, a transmission of field data of an auxiliary power unit, wherein the field data includes data measured during a main engine start and optionally also during auxiliary power unit idle; transforming, by the one or more processors using an operating conditions correction, the received field data of the auxiliary power unit into corrected field data, wherein the operating conditions correction involves automatic adjustment of coefficients used for operating conditions correction to capture manufacturing variability of the specific auxiliary power unit; filtering, by the one or more processors, the corrected field data to obtain filtered field data, wherein the importance of the filtered data is determined by the variance of the data; determining, by the one or more processors, offset field data at least based on the filtered field data and one or more offset correction parameters to define a specific threshold of performance indicator that corresponds with known state of auxiliary power unit health, wherein offset correction parameters are estimated by investigating the influence of a plurality of factors on the value of performance indicator corresponding to the state when the auxiliary power unit was removed; determining, by the one or more processors, a health indicator of the auxiliary power unit based on the offset field data, wherein the health indicator provides a representation that the health of the APU has deteriorated and that maintenance work may be performed; determining, by the one or more processors, a maintenance procedure for the auxiliary unit based on the health indicator; and providing the determined health indicator to be accessed by personnel via an online web portal, and/or to be sent to personnel via weekly emails. receiving, by one or more processors of a processing server, a transmission of field data of an auxiliary power unit, wherein the field data includes data measured during a main engine start and optionally also during auxiliary power unit idle; transforming, by the one or more processors using an operating conditions correction, the received field data of the auxiliary power unit into corrected field data, wherein the operating conditions correction involves automatic adjustment of coefficients used for operating conditions correction to capture manufacturing variability of the specific auxiliary power unit; filtering, by the one or more processors, the corrected field data to obtain filtered field data, wherein the importance of the filtered data is determined by the variance of the data; determining, by the one or more processors, offset field data at least based on the filtered field data and one or more offset correction parameters to define a specific threshold of performance indicator that corresponds with known state of auxiliary power unit health, wherein offset correction parameters are estimated by investigating the influence of a plurality of factors on the value of performance indicator corresponding to the state when the auxiliary power unit was removed; determining, by the one or more processors, a health indicator of the auxiliary power unit based on the offset field data, wherein the health indicator provides a representation that the health of the APU has deteriorated and that maintenance work may be performed; determining, by the one or more processors, a maintenance procedure for the auxiliary unit based on the health indicator; and providing the determined health indicator to be accessed by personnel via an online web portal, and/or to be sent to personnel via weekly emails.