A gas turbine engine for an aircraft comprising: an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor; a fan located upstream of the engine core, the fan comprising a plurality of fan blades mounted for rotation about an engine axis, each blade of the plurality of fan blades having a leading edge and a trailing edge extending across a span of an airflow duct from a blade root to a blade tip; and a gearbox that receives an input from the core shaft and outputs drive to the fan so as to drive the fan at a lower rotational speed than the core shaft, characterized in that: the trailing edge of each blade is characterized by a metric M defined as a rate of change of an angle of the trailing edge between 0.4 and 0.8 of the span divided by an area averaged trailing edge angle, M being not less than around 4, where: the trailing edge angle at a given cross-section at a span is defined as the angle of the camber line with respect to the engine axis at the trailing edge of the blade; the metric M is defined as M = χ 0.8 − χ 0.4 0.8 − 0.4 χ ̃ where χ f is the trailing edge angle at a fraction f from the blade root to the blade tip and χ̃ is the area averaged trailing edge angle from the blade root to the blade tip, and χ̃ is a discrete area averaged trailing edge angle, defined as χ ̃ = χ 0.1 π r 0.2 2 − r hub 2 + χ 0.3 π r 0.4 2 − r 0.2 2 + χ 0.5 π r 0.6 2 − r 0.4 2 + χ 0.7 π r 0.8 2 − r 0.6 2 + χ 0.9 π r tip 2 − r 0.8 2 π r tip 2 − r hub 2 where r f is the radius from the engine axis at a fraction f of the span from the blade root to the blade tip, the radius r tip being the radius at the blade tip and r hub the radius at the blade root. an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor; a fan located upstream of the engine core, the fan comprising a plurality of fan blades mounted for rotation about an engine axis, each blade of the plurality of fan blades having a leading edge and a trailing edge extending across a span of an airflow duct from a blade root to a blade tip; and a gearbox that receives an input from the core shaft and outputs drive to the fan so as to drive the fan at a lower rotational speed than the core shaft, characterized in that: the trailing edge of each blade is characterized by a metric M defined as a rate of change of an angle of the trailing edge between 0.4 and 0.8 of the span divided by an area averaged trailing edge angle, M being not less than around 4, where: the trailing edge angle at a given cross-section at a span is defined as the angle of the camber line with respect to the engine axis at the trailing edge of the blade; the metric M is defined as M = χ 0.8 − χ 0.4 0.8 − 0.4 χ ̃ where χ f is the trailing edge angle at a fraction f from the blade root to the blade tip and χ̃ is the area averaged trailing edge angle from the blade root to the blade tip, and χ̃ is a discrete area averaged trailing edge angle, defined as χ ̃ = χ 0.1 π r 0.2 2 − r hub 2 + χ 0.3 π r 0.4 2 − r 0.2 2 + χ 0.5 π r 0.6 2 − r 0.4 2 + χ 0.7 π r 0.8 2 − r 0.6 2 + χ 0.9 π r tip 2 − r 0.8 2 π r tip 2 − r hub 2 where r f is the radius from the engine axis at a fraction f of the span from the blade root to the blade tip, the radius r tip being the radius at the blade tip and r hub the radius at the blade root. the trailing edge angle at a given cross-section at a span is defined as the angle of the camber line with respect to the engine axis at the trailing edge of the blade; the metric M is defined as M = χ 0.8 − χ 0.4 0.8 − 0.4 χ ̃ where χ f is the trailing edge angle at a fraction f from the blade root to the blade tip and χ̃ is the area averaged trailing edge angle from the blade root to the blade tip, and χ̃ is a discrete area averaged trailing edge angle, defined as χ ̃ = χ 0.1 π r 0.2 2 − r hub 2 + χ 0.3 π r 0.4 2 − r 0.2 2 + χ 0.5 π r 0.6 2 − r 0.4 2 + χ 0.7 π r 0.8 2 − r 0.6 2 + χ 0.9 π r tip 2 − r 0.8 2 π r tip 2 − r hub 2 where r f is the radius from the engine axis at a fraction f of the span from the blade root to the blade tip, the radius r tip being the radius at the blade tip and r hub the radius at the blade root.