A method of estimating an air mass flow (WTAP) of air flowing in a bypass duct of a gas turbine engine comprising: providing a plurality of instruments (50; 80) operable to measure properties of the air flowing in the bypass duct of the gas turbine engine, the properties comprising each of static pressure P s and total pressure P t, the plurality of instruments (50; 80) being disposed in the bypass duct and arranged within a common measurement plane; assigning a stream tube to each instrument (50; 80), wherein each stream tube represents a region of space in the common measurement plane within the bypass duct and each stream tube surrounds one of the plurality of instruments (50; 80), wherein the stream tubes together correspond to the cross-sectional shape and area of the bypass duct in the common measurement plane; measuring the properties of the air using the instruments (50; 80) to obtain measured values for each stream tube; using the measured values for each stream tube to estimate the air mass flow in an annulus of the common measurement plane from the measurement for each stream tube according to the equation: WTAP = W 1 A × T t 1 A P t 1 A × A 1 A = P s 1 A P t 1 A 1 γ × 2 γ R γ − 1 1 − P s 1 A P t 1 A γ − 1 γ × cosα Where: W=mass-flow; γ = the ratio of specific heats of air; R = the gas constant; A=the area of the stream tube; Tt= total temperature and α is the angle between the longitudinal direction through the annulus and the flow vector; and summing the air mass flow values across all of the stream tubes. providing a plurality of instruments (50; 80) operable to measure properties of the air flowing in the bypass duct of the gas turbine engine, the properties comprising each of static pressure P s and total pressure P t, the plurality of instruments (50; 80) being disposed in the bypass duct and arranged within a common measurement plane; assigning a stream tube to each instrument (50; 80), wherein each stream tube represents a region of space in the common measurement plane within the bypass duct and each stream tube surrounds one of the plurality of instruments (50; 80), wherein the stream tubes together correspond to the cross-sectional shape and area of the bypass duct in the common measurement plane; measuring the properties of the air using the instruments (50; 80) to obtain measured values for each stream tube; using the measured values for each stream tube to estimate the air mass flow in an annulus of the common measurement plane from the measurement for each stream tube according to the equation: WTAP = W 1 A × T t 1 A P t 1 A × A 1 A = P s 1 A P t 1 A 1 γ × 2 γ R γ − 1 1 − P s 1 A P t 1 A γ − 1 γ × cosα Where: W=mass-flow; γ = the ratio of specific heats of air; R = the gas constant; A=the area of the stream tube; Tt= total temperature and α is the angle between the longitudinal direction through the annulus and the flow vector; and W=mass-flow; γ = the ratio of specific heats of air; R = the gas constant; A=the area of the stream tube; Tt= total temperature and α is the angle between the longitudinal direction through the annulus and the flow vector; and summing the air mass flow values across all of the stream tubes.