ArgentoSystems wrote:You don't understand what "control lightening means". Normally if you are pitching up and you let go of elevator control, the airplane will return to level flight. Control lightening means aircraft loses this tendency to return to level at higher angles of attack. It will just stay at this high AoA if you let of of the controls. This new position becomes it's local stable point. Is it stable at that point? Yes. Is that a good stability? I'll let you figure out the answer.
The condition you are describing is an airplane that is neutrally stable.
For a stall handling demonstration, the airplane is trimmed by adjusting the horizontal stabilizer at 1.3 times the stall speed. Speed is then decreased by pulling on the yoke. The airplane must exhibit a steadily increasing pull on the stick as speed is reduced. Let's say the pull is 3 lbs for a 5 kt speed. reduction. If the stick is released at any point, a stable airplane will pitch down and will recover near the original 1.3 times stall speed point.
In the example above, if the stick force decreased from 3 lbs for 5 kts to 2 lbs for 5 kts at some point during the stall approach, that is "stick lightening". If the stick is released at the point the stick force gradient changes, the airplane will pitch down and recover near its original trim speed, 1.3 times the stall speed. The airplane is still stable but the stick force gradient break means it all not pass FAR 25.203.
If the stick force goes negative (the pilot has to push) for a 5 kt speed decrease during the stall approach, the airplane is unstable.