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A fuel system (12) comprising a vapor trail detection sensor (20) configured to generate a first signal (28) which indicates the optical depth of a vapor trail (35). A control unit (40) is provided responsive to the first signal (28) and configured to generate a second signal (80) in dependence upon

A fuel system (12) comprising a vapor trail detection sensor (20) configured to generate a first signal (28) which indicates the optical depth of a vapor trail (35). A control unit (40) is provided responsive to the first signal (28) and configured to generate a second signal (80) in dependence upon the first signal (28). The second signal (80) defines a percentage of at least one of a first fuel composition and second fuel composition required to produce a resultant fuel composition. At least one regulator (42) is provided configured to receive and be responsive to the second signal (80) and regulate the percentage of first and second fuel composition required to produce the resultant fuel composition.

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1. A fuel system comprising: a vapor trail detection sensor configured to generate a first signal which indicates an optical depth of a vapor trail;a control unit responsive to the first signal and configured to generate a second signal in dependence upon the first signal which defines a percentage

1. A fuel system comprising: a vapor trail detection sensor configured to generate a first signal which indicates an optical depth of a vapor trail;a control unit responsive to the first signal and configured to generate a second signal in dependence upon the first signal which defines a percentage of at least one of a first fuel composition and a second fuel composition required to produce a resultant fuel composition; andat least one regulator configured to receive and be responsive to the second signal and regulate the percentage of the at least one of the first fuel composition and the second fuel composition required to produce the resultant fuel composition that controls the optical depth of the vapor trail,wherein the control unit comprises a blending ratio calculator configured to calculate the required resultant fuel composition as a function of desired soot emission index determined by a soot requirement generator which communicates a value of soot emission index to the blending ratio calculator. 2. A fuel system as claimed in claim 1 wherein the control unit is operable to determine a likelihood of persistence of the vapor trail and determine the resultant fuel composition in dependence upon the determined likelihood of persistence of the vapor trail. 3. A fuel system as claimed in claim 1 further comprising a fluid blender in fluid communication with a source of the first fuel composition and a source of the second fuel composition via the at least one regulator, the blender being configured to mix the at least one of the first fuel composition and the second fuel composition to produce the resultant fuel composition. 4. A fuel system as claimed in claim 1 wherein the vapor trail detection sensor is an optical device configured to deliver a signal indicative of a presence and/or properties of the vapor trail. 5. A fuel system as claimed in claim 1 comprising a source of illumination, the vapor trail detection sensor being configured to detect electromagnetic radiation of at least one wavelength emitted and/or reflected by the vapor trail in response to energy emitted from the source of illumination. 6. A fuel system as claimed in claim 1 wherein a source of the first fuel composition is a first tank for storage of the first fuel composition; a source of the second fuel composition is a second tank for storage of the second fuel composition; andthe first tank and second tank are fluidly isolated from one another. 7. A fuel system as claimed in claim 3 wherein the fluid blender has at least one outlet for delivering the resultant fuel composition to an engine fuel injector. 8. A fuel system as claimed in claim 1 wherein the soot requirement generator is configured to calculate the desired soot emission index as a function of one or more of vapor trail optical depth and/or measured values of ambient temperature, ambient pressure and/or ambient humidity. 9. A fuel system as claimed in claim 8 wherein the soot requirement generator is in communication with the vapor trail detection sensor to receive vapor trail optical depth data and/or a sensor to receive the one or more measured values. 10. A fuel system as claimed in claim 1 wherein the soot requirement generator is in communication with a second sensor to receive a value for instantaneous measured actual values of soot emission index generated by an engine. 11. A fuel system as claimed in claim 1 wherein the soot emission index is derived from a look up table or database in dependence upon one or more of the first fuel composition and the second fuel composition, combustion characteristics of an engine and/or engine operating point. 12. A fuel system as claimed in claim 1 wherein the soot requirement generator determines a value of desired soot emission index which would generate optimal vapor trail optical depth for given ambient conditions and engine operating point. 13. A fuel system as claimed in claim 1 wherein the soot requirement generator comprises a model and/or algorithm for direct calculation of the desired soot emission index. 14. A fuel system as claimed claim 1 comprising a vapor trail modeling tool configured to first determine ambient air conditions in dependence upon actual measured values of ambient air conditions, and then to determine the likelihood of vapor trail formation and/or vapor trail persistence based upon engine operating point and the determined ambient air conditions. 15. A fuel system as claimed in claim 14 further comprising a comparator for comparing output from the vapor trail modeling tool with an instantaneous actual vapor trail observation to determine the effect of the resultant fuel composition on vapor trail formation and/or vapor trail persistence. 16. An aircraft comprising a fuel system as claimed in claim 1, wherein the vapor trail detection sensor is mounted towards a rear of an aircraft facing aft. 17. An aircraft as claimed in claim 16 comprising an engine, the vapor trail detection sensor mounted such that it has a field of view directed towards a vapor trail formation region. 18. An aircraft as claimed in claim 17 wherein a source of illumination is directed towards at least one region downstream of the engine.