A method of obtaining aircraft frequency-response from transient response data by Fourier analysis is currently being investigated. This report describes progress that has been made between the commencement of the contract (1st December, 1964) and the time of writing (October, 1965). The dynamic response characteristics of a Hawker Siddeley Dore' aircraft are being determined from flight measurements using the Fourier method of analysis. At present attention is centred on the longitudinal response as the short-period mode of the aircraft is well damped and should be defined by a simple transfer function. The aircraft transient responses to various pilot-applied control inputs are recorded and the Fourier analysis of these transients is being carried out on a Ferranti Pegasus digital computer. Three development flights have been completed to date, and some preliminary results have been obtained, although the detailed analysis of the flight data is awaiting the incorporation of the instrumentation calibration into the computer programme. This last procedure will speed up the analysis of future data.