A system for detecting aircraft structure damage, the system comprising: an aircraft structure; and a structural health monitoring system configured to monitor structural health of the aircraft structure, the SHM system comprising: an actuator bonded to the aircraft structure and configured to: generate a reference vibration signal (202; 318) having a reference amplitude that propagates through the aircraft structure at a first time, and generate a comparison vibration signal (208; 320, 322, 324, 326, 328, 330) having a comparison amplitude that propagates through the aircraft structure at a second time after the first time, a sensor (104b, 104c) bonded to the aircraft structure and configured to receive the reference vibration signal and the comparison vibration signal that propagate through the aircraft structure along a path (112, 114) between the actuator and the sensor (104b, 104c); and a processor configured to compute for said path (112, 114) a gain damage index DI as a function of the reference amplitude multiplied by the comparison amplitude, divided by a square of the comparison amplitude, the gain damage index DI enabling the SHM system to monitor the structural health of the aircraft structure independent of temperature, the reference vibration signal and the comparison vibration signal being collected as amplitude samples over a period of time, referred to as time series, said gain damage index DI for a reference time series and a comparison time series, each having N samples, being represented by the following algorithm: Gain ′ = ∑ n = 0 N − 1 Ref n Comp n ∑ n = 0 N − 1 Comp 2 n where Ref represents the reference time series, Comp represents the comparison time series, and n is an index into the time series. an aircraft structure; and a structural health monitoring system configured to monitor structural health of the aircraft structure, the SHM system comprising: an actuator bonded to the aircraft structure and configured to: generate a reference vibration signal (202; 318) having a reference amplitude that propagates through the aircraft structure at a first time, and generate a comparison vibration signal (208; 320, 322, 324, 326, 328, 330) having a comparison amplitude that propagates through the aircraft structure at a second time after the first time, a sensor (104b, 104c) bonded to the aircraft structure and configured to receive the reference vibration signal and the comparison vibration signal that propagate through the aircraft structure along a path (112, 114) between the actuator and the sensor (104b, 104c); and a processor configured to compute for said path (112, 114) a gain damage index DI as a function of the reference amplitude multiplied by the comparison amplitude, divided by a square of the comparison amplitude, the gain damage index DI enabling the SHM system to monitor the structural health of the aircraft structure independent of temperature, the reference vibration signal and the comparison vibration signal being collected as amplitude samples over a period of time, referred to as time series, said gain damage index DI for a reference time series and a comparison time series, each having N samples, being represented by the following algorithm: Gain ′ = ∑ n = 0 N − 1 Ref n Comp n ∑ n = 0 N − 1 Comp 2 n where Ref represents the reference time series, Comp represents the comparison time series, and n is an index into the time series. an actuator bonded to the aircraft structure and configured to: generate a reference vibration signal (202; 318) having a reference amplitude that propagates through the aircraft structure at a first time, and generate a comparison vibration signal (208; 320, 322, 324, 326, 328, 330) having a comparison amplitude that propagates through the aircraft structure at a second time after the first time, generate a reference vibration signal (202; 318) having a reference amplitude that propagates through the aircraft structure at a first time, and generate a comparison vibration signal (208; 320, 322, 324, 326, 328, 330) having a comparison amplitude that propagates through the aircraft structure at a second time after the first time, a sensor (104b, 104c) bonded to the aircraft structure and configured to receive the reference vibration signal and the comparison vibration signal that propagate through the aircraft structure along a path (112, 114) between the actuator and the sensor (104b, 104c); and a processor configured to compute for said path (112, 114) a gain damage index DI as a function of the reference amplitude multiplied by the comparison amplitude, divided by a square of the comparison amplitude, the gain damage index DI enabling the SHM system to monitor the structural health of the aircraft structure independent of temperature, the reference vibration signal and the comparison vibration signal being collected as amplitude samples over a period of time, referred to as time series, said gain damage index DI for a reference time series and a comparison time series, each having N samples, being represented by the following algorithm: Gain ′ = ∑ n = 0 N − 1 Ref n Comp n ∑ n = 0 N − 1 Comp 2 n where Ref represents the reference time series, Comp represents the comparison time series, and n is an index into the time series.