1. A method of monitoring an operating parameter of a system of an aircraft, the method performed by one or more processors processing signals generated by one or more sensors associated with the system, comprising: with respect to each of one or more parameters of interest represented by signals generated by one or more sensors associated with one or more systems of the aircraft: using data representing a first point (P 0 ) associated with a first parameter value (V 0 ) and a first time (T 0 ), and data representing a second point (P 1 ) associated with a second parameter value (V 1 ) at a second time (T 1 ): generating data, including data representing an upper boundary slope, defining, as a function of time, an upper boundary passing through the points ((V 0 plus a tolerance), T 0 ) and (P 1 ), generating data, including data representing a lower boundary slope, defining, as a function of time, a lower boundary passing through the points ((V 0 minus a tolerance), T 0 ) and (P 1 ); generating data defining an estimated parameter function line passing through P 0 and having a slope equal to the average of the slopes of the upper and lower boundaries; reading data representing a subsequent point (P 2 ) associated with a third parameter value (V 2 ) and a third time (T 2 ), and if the third parameter value V 2 is greater than a value (VUB 2 ) associated with a point on the upper boundary at time T 2 , generating data, including data representing the upper boundary slope, redefining the upper boundary as a function of time passing through the points ((V 0 plus a tolerance), T 0 ) and (P 2 ), and if the third parameter value V 2 is less than a value (VLB 2 ) associated with a point on the lower boundary at time T 2 , generating data, including data representing the lower boundary slope, redefining the lower boundary as a function of time passing through the points ((V 0 minus a tolerance), T 0 ) and (P 2 ); and if the slope of the upper boundary is greater than the slope of the lower boundary, store data representing an estimated parameter value having a value Vest equal to the value of the estimated parameter function line at time T 2 in a stored parameter data set in persistent, computer-readable memory.