span wise flow. More common solutions to the problem of spanwise flow is the use of a wing fence or the related dogtooth notch on the leading edge of the wing. This disrupts the flow and re-directs it rearward, while also causing the buildup of stagnant air inboard to lower the stall point. This does have an effect on overall airflow on the wing, and is generally not used where the sweep is mild. To address the problems with spanwise loading, a wider variety of techniques have been used, including dedicated slats or flaps, the use of washout or automated control of the ailerons. An unusual solution tried on the XF-91 Thunderceptor prototype fighter was to give the wingtips a wider chord than the wing roots. The idea was to increase wingtip efficiency and cause the wing roots to stall first. Angle of attack sensors on the aircraft can also detect when the angle of attack approaches the attitude known to result in pitch-up and activate devices like the stick shaker to warn the pilot, and the stick pusher which overpowers the pilot and forces the nose of the aircraft down to a safer angle of attack. Twist or washout built into the wingtips can also alleviate pitch-up. In effect, the angle of attack at the wingtip becomes smaller than elsewhere on the wing, meaning that the inboard portions of the wing will stall first. A commonly used solution to pitch-up in modern combat aircraft is to use a control-canard. Another modern solution to pitch-up is the use of slats. When slats are extended they increase wing camber and increase maximum lift coefficient. Pitch-up is also possible in aircraft with forward-swept wings as used on the Grumman X-29. With forward-swept wings the span wise flow is inboard, causing the wing root to stall before the wingtip. Although at first glance it would appear that this would cause pitch-down problems, the extreme rear mounting of the wing means that when the root stalls the lift moves forward, towards the tips. Sabre dance When a swept wing starts to stall, the outermost portions tend to stall first. Since these portions are behind the center of pressure, the overall lift force moves forward, pitching the nose of the aircraft upwards. This leads to a higher angle of attack and causes more of the wing to stall, which exacerbates the problem. The pilot often