Flight Control System Gain

James Albright

Updated: 2020-07-15

I think of gain as the "power steering" given to you under various conditions. Think about how little effort it takes on the stick to move a large control surface. If you had too much gain, you could break something. So gain is actually programmed to react differently at various airspeeds and control laws.

[PAS, p. 6-11] Gain and control deflection scheduled with airspeed; amplification of signal to achieve desired aircraft response, lower gain means less control deflection and vice versa.

In the Normal Flight Control Law, "gain" is normal. That means control deflections asked for by you (though the active control stick) or the autopilot are "scheduled" by airspeed. The amount of flight control deflection decreases with increasing airspeed. You also have a number of protections, such as low speed protection.

There are three other modes: Alternate Mode, Direct Mode, Backup Mode.

[PAS, p. 6-16] Fixed set of gains (2)

• Flap and gear handles up → Based on 340 KCAS (low gain)

• Flap or gear handle down → Based on 250 KCAS (high gain)

  • Extend flaps to at least 10° < 200 kts (Improves response due to high gain at lower speed)

In these modes, the variable gain is reduced to two steps. When the gear and flaps are up, gain is set to 340 KCAS. So at slower speeds, your flight controls will be comparatively sluggish. If either the landing gear or flaps are extended, the gain is set to 250 KCAS. Once again, the slower you go the more sluggish the flight controls will be.