A propulsion system, comprising: a gas turbine engine including a combustor section in flow communication with a turbine section; and a fuel system, characterised in that: the fuel system defines a plurality of localized nodes (F N) including a metering valve (62 A), the metering valve (62 A) operable to meter flow between the combustor section and a fuel source (F S) in response to a metering signal (L M), each of the plurality of localized nodes (F N) having a distinct set of failure parameters; and the propulsion system further comprises a fuel observation assembly coupled to the engine and the fuel system, the fuel observation assembly comprising: an estimation module (66 EM) operable to calculate an expected condition (E C) of each of the plurality of localized nodes (F N) based upon at least the metering signal (L M), at least one fuel condition relating to the fuel system, and at least one engine condition relating to the engine, wherein the expected condition (E C) corresponds to at least one parameter of the set of failure parameters; an observation module (66 OM) operable to calculate an observed condition (O C) of each of the plurality of localized nodes (F N) based on present values of the at least one fuel condition and the at least one engine condition; and a comparison module operable to determine an abnormality corresponding to a first one of the plurality of localized nodes (F N) based upon a comparison of the expected condition (E C) and the observed condition (O C) for each of the plurality of localized nodes (F N). a gas turbine engine including a combustor section in flow communication with a turbine section; and a fuel system, the fuel system defines a plurality of localized nodes (F N) including a metering valve (62 A), the metering valve (62 A) operable to meter flow between the combustor section and a fuel source (F S) in response to a metering signal (L M), each of the plurality of localized nodes (F N) having a distinct set of failure parameters; and the propulsion system further comprises a fuel observation assembly coupled to the engine and the fuel system, the fuel observation assembly comprising: an estimation module (66 EM) operable to calculate an expected condition (E C) of each of the plurality of localized nodes (F N) based upon at least the metering signal (L M), at least one fuel condition relating to the fuel system, and at least one engine condition relating to the engine, wherein the expected condition (E C) corresponds to at least one parameter of the set of failure parameters; an observation module (66 OM) operable to calculate an observed condition (O C) of each of the plurality of localized nodes (F N) based on present values of the at least one fuel condition and the at least one engine condition; and a comparison module operable to determine an abnormality corresponding to a first one of the plurality of localized nodes (F N) based upon a comparison of the expected condition (E C) and the observed condition (O C) for each of the plurality of localized nodes (F N). an estimation module (66 EM) operable to calculate an expected condition (E C) of each of the plurality of localized nodes (F N) based upon at least the metering signal (L M), at least one fuel condition relating to the fuel system, and at least one engine condition relating to the engine, wherein the expected condition (E C) corresponds to at least one parameter of the set of failure parameters; an observation module (66 OM) operable to calculate an observed condition (O C) of each of the plurality of localized nodes (F N) based on present values of the at least one fuel condition and the at least one engine condition; and a comparison module operable to determine an abnormality corresponding to a first one of the plurality of localized nodes (F N) based upon a comparison of the expected condition (E C) and the observed condition (O C) for each of the plurality of localized nodes (F N).