A method of measuring the level of a fluid in a tank of an aircraft, the method comprising: moving an ultrasonic transducer to a predetermined measurement position; measuring the level of the fluid by: transmitting ultrasound from the transducer into the fluid via an external surface of a wall of the tank, receiving an ultrasonic echo at the transducer, and deducing the fluid level based on the time of arrival of the ultrasonic echo; and removing the transducer from the measurement position.A fluid tank comprising a bottom wall having an internal surface which is inclined to the horizontal; and an ultrasound guide surface which is located in the interior of the tank and oriented to direct ultrasound substantially vertically.