On aircraft with tricycle configuration landing gear, the nose wheel is either free castoring or, by some mechanism, steerable to facilitate directional control during takeoff and landing and to allow the aircraft to manoeuvre whilst on the ground.
The undercarriage, more commonly referred to as the "landing gear", of the vast majority of currently in service aircraft types is of a tricycle configuration. In its most simplistic configuration, the landing gear is comprised of a single nose wheel mounted near the front of the aircraft and two main wheels mounted on either side of the aircraft with attachment points either on the fuselage or the wing. The landing gear may, or may not, be retractable whilst in flight. The design of larger aircraft may incorporate multiple wheels on each gear strut or include additional strut/wheel assemblies. In all cases, the nose wheel(s) must, by some mechanism, be steerable to allow the aircraft to manoeuvre and to maintain directional control during takeoff and landing.
In the most simplistic of installations, the nose wheel is free castoring; that is, it swivels with no mechanism incorporated with which to physically steer the wheel. Steering can be effected aerodynamically by using rudder input, by utilising differential braking or, in multi engine aircraft, with use of differential thrust. For the most part, free castoring nose wheels are only found in some light general aviation aircraft and in significantly older large aircraft designs such as the Grumman Mallard. A centering mechanism, activated when the weight comes off the nosewheel on takeoff, is incorporated into the design to ensure that the wheel is properly aligned for the subsequent landing.
More sophisticated, but still basic, designs have bungees or a mechanical linkage from the rudder pedals to the nosewheel and the the aircraft steering is effected by applying pressure to the appropriate pedal - left pedal to turn left, right to turn right. A combination of rudder pedal deflection and differential brake input can be used, as necessary, to tighten the turn. In virtually all cases, nose wheel steering has a maximum deflection angle limitation and pilot controlled movement will cease when the weight comes off of the wheels during takeoff at which time the nose wheel will self-centre. In larger, heavier aircraft, a hydraulically assisted steering mechanism is incorporated.
In larger aircraft, a nosewheel "tiller" is also often added to the design to facilitate ease of steering whilst on the ground. The tiller is, essentially, a small steering wheel which is most often mounted on the left side console, or side wall, of the cockpit for use by the pilot occupying the left seat. In some aircraft types, there is a second steering tiller mounted on the right side of the cockpit. The tiller is connected mechanically or, in newer aircraft, electronically to the steering mechanism. In electronic, or "steer by wire", installations, the nose wheel response to tiller input is not always linear. In these cases, a small tiller input results in a proportionally small movement of the nose wheel whereas slight increases in that input will result in an ever increasing, non proportional nose wheel deflection.
In early designs, nose wheel steering was accomplished either with the rudder pedals or with the tiller. For aircraft with steering tillers, this meant that the tiller had to be used during the initial portion of the takeoff roll and the final portion of the landing roll when aerodynamic control of the aircraft was not possible. Consequently, it fell to the second pilot to "fly the wing" until sufficient aerodynamic control was available to cease use of the tiller for directional control. In newer aircraft, tiller equipped designs also incorporate nose wheel steering via the rudder pedals, albeit with reduced steering authority. This feature means that there is no longer a requirement for both pilots to simultaneously make flight control inputs during the takeoff or landing phases of flight. Many aircraft types incorporate a further refinement in that nose wheel steering sensitivity through the rudder pedals is inversely proportional to aircraft ground speed.
Pushback and Towing
Many aircraft are subject to pushback procedures in the course of day-to-day operations and virtually all are routinely towed from place to place whilst on the ground. As normal towing or pushback protocols have the potential to exceed maximum nose wheel deflection limitations or to damage hydraulically actuated steering components, features are often incorporated in the nose wheel steering design to ensure safe towing operations. In many cases, the mechanical steering linkage can be physically disconnected allowing the nose wheel to freely castor. In others, a hydraulic bypass mechanism, which can be "pinned" to disable the steering actuator, is incorporated. In all cases, the aircraft must be properly configured prior to commencing towing or pushback operations. It is also important that the bypass pin be removed or that the steering link be reconnected before the aircraft commences taxy.
Nosewheel steering mechanism failures are relatively rare. Mechanical steering components will occasionally break or becomed jammed and, in steer-by-wire installations, electronic component failures have led to loss of steering capability. However, the loss of nosewheel steering is most often associated with the failure of its associated hydraulic system.
In the event of a steering failure, crews should stop the aircraft and follow the guidance of the Aircraft Flight Manual (AFM) or Quick Reference Handbook (QRH). If there is no guidance specific to the situation, the flight crewmembers should use their best judgement to minimise the risk and achieve a safe outcome. Although steering might still be achievable via differential braking or differential thrust, in large aircraft, crews are generally not trained to manoeuvre the aircraft in this fashion. Consequently, it is often most prudent to stop the aircraft and have it towed to the ramp. If circumstances allow, crews with an inflight problem that will affect steering capability after landing should attempt to arrange for a tow crew to be in place at the time of landing to minimise the operational impact of a potentially blocked runway.
Accidents & Incidents
- B734, Barcelona Spain, 2004 On 28 November 2004, a KLM B737-400 departed laterally from the runway on landing at Barcelona due to the effects on the nosewheel steering of a bird strike which had occured as the aircraft took off from Amsterdam
- A320, Los Angeles USA, 2005 On 21 September 2005, an Airbus A320 operated by Jet Blue Airways made a successful emergency landing at Los Angeles Airport, California, with the nose wheels cocked 90 degrees to the fore-aft position after an earlier fault on gear retraction
- A320, Tehran Mehrabad Iran, 2016 On 13 August 2016, an Airbus A320 departed the side of the runway at low speed during takeoff from Tehran Mehrabad and became immobilised in soft ground. The Investigation found that the Captain had not ensured that both engines were simultaneously stabilised before completing the setting of takeoff thrust and that his subsequent response to the resulting directional control difficulties had been inappropriate and decision to reject the takeoff too late to prevent the excursion. Poor CRM on the flight deck was identified as including but not limited to the First Officer’s early call to reject the takeoff being ignored.