Nose Wheel Steering (NWS) system architecture. For training purposes only скачать в хорошем качестве

Nose Wheel Steering (NWS) system architecture. For training purposes only

The Nose Wheel Steering (NWS) system architecture is based on a computer acquiring cockpit inputs to calculate the steering order. This order is compared to the actual position of the nose wheels (nose wheels’ angle measured by the Rotary Variable Differential Transducer (RVDT)) to calculate and generate a current to the servo valve. Aircraft veering phenomenon is described as the aircraft’s deviation during taxi operations. To correct these deviations, Airbus has over the years implemented solutions for each of the three main causes of abnormal veering: • Steering demand offset • Servo valve offset • Rotary Variable Differential Transducer (RVDT) offset Veering is a progressive tendency of the aircraft to steer to the right or left of the taxiway (or runway) centreline, without steering inputs from flight crew (i.e. steering handwheels, rudder pedals and rudder trim all at neutral position). It is most noticeable during taxiing at low speeds. Note: Veering is not a sudden swerve movement of nose wheels. Instances of aircraft veering have been reported, requiring sometimes extensive trouble shooting actions to fix the issue. Flight crew may need to correct using the steering handwheel or rudder trim to taxi the aircraft in a straight line. When the correction exceeds the threshold, a maintenance action is required to find the cause of this veering tendency. Steering-By-Wire The use of ‘Steering-By-Wire’, compared to the former hydro-mechanical architecture using mechanical cables and pulleys, has made it possible to reduce aircraft weight whilst ensuring a better integration of the steering system with other aircraft systems such as Auto-Pilot for Auto Land and improving maintenance through the on-board Central Maintenance System. ‘Steering-By-Wire’ was first introduced on A320 aircraft and has evolved over the past decades for continuous reliability and operation enhancement. From the above architecture it can be determined that three typical offsets can lead to abnormal aircraft veering tendency: 1 Steering demand offset generating unwanted steering order while in neutral position 2 Servo valve offset generating hydraulic flow and steering the nose wheels away from neutral position 3 RVDT* offset shifting the electrical zero position vs. zero mechanical position of nose wheels In the event of aircraft veering tendency exceeding the tolerance, and irrespective of aircraft configuration, the troubleshooting manual provides instructions to isolate the cause of the deviation (internal or external factor). To this end, it may be necessary to verify if the adjustment of the RVDT is aligned with the aircraft centreline. Correct and accurate mechanical adjustment of the RVDT is a key factor to prevent abnormal veering tendency due to large offset between the actual position of the nose wheels and the feedback sent by the RVDT. Basically, the adjustment of the RVDT is to be carried out when the nose wheels are aligned with aircraft centreline. Several procedures can be used to align the nose wheels prior to adjusting the RVDT. Operators can adapt their maintenance depending on the aircraft programme, tool and time available to do the RVDT adjustment. Maintenance and operational procedures have been adapted to account for small and acceptable offset whilst providing clear instructions for maximum allowable limits and troubleshooting activities.