Are spoilers primary or secondary flight control surfaces?

Question

The literature on primary and secondary flight controls is very contradictory. My professor provided this image from which I infer that spoilers are used for primary flight control:

Also, here it says that spoilers qualify as primary flight control.

On the other hand, Skybrary lists spoilers as secondary flight controls.

What kind of flight controls are spoilers?

Answer 1

There does not seem to be a consistent definition for primary and secondary flight controls. It depends on the aircraft.

The Airbus A310 FCOM distinguishes between primary and secondary controls as follows:

The primary flight controls ensure flight path control and include:

• pitch control, using:
  • the elevators, which are hinged to the Trimmable Horizontal Stabilizer (THS), and
  • the Trimmable Horizontal Stabilizer (THS) which provides Pitch Trim.
• roll control, using:
  • 1 aileron per wing, and
  • the 5 outboard spoilers (Nos. 3 to 7) on each upper wing surface, as roll spoilers.
• yaw control, using a single rudder.

[...] The secondary flight controls are lift and drag augmenting devices and includes:

• flaps: there are one double slotted inboard flap and on single slotted outboard flap.
• slats: there are three slat sections per wing (inner, center and outer). Associated with the slats a Kruger flap is located on the leading edge of each wing, inboard of the inner slat section.
• spoilers: there are 7 spoilers (Nos. 1 to 7) on each upper wing surface which are used as speed brakes, roll spoilers or ground spoilers for lift dumping (all 7 surfaces).

^{(emphasis mine)}

Therefore, spoilers can be counted as primary controls, but only those which are actually used as roll spoilers (asymmetric deployment in the air).

As far as I could find, Airbus does not distinguish between primary and secondary controls in subsequent models.

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The Boeing 747 FCOM (-400 and -8 models) defines the primary and secondary controls as follows:

The primary flight controls are elevators, ailerons, and rudders. [...]

Secondary flight controls include a moveable horizontal stabilizer, spoilers, and leading and trailing edge flaps.

^{(emphasis mine)}

Here, spoilers are clearly excluded from the primary flight controls. Other Boeing FCOMs I found define the primary flight controls in a similar way, but do not go into further details about secondary flight controls.

Answer 2

It depends on the aircraft. For example:

• On a Learjet 45 airplane. There are spoilers that are mainly used in the air to slow down and/or descend. However, when flaps are increased, those same spoilers become spoilerons and help the airplane roll. There is even a roll disconnect feature that will disconnect the pilot side yoke to the aileron cables and solely use the spoilerons to roll the aircraft.

To summarize I would say this

• Spoilers are secondary flight controls. In the air, the help the airplane slow down and/or descend. On the ground, they are used to spoil lift and transfer the weight of the airplane to the wheels to aid in braking

• Spoilerons are primary flight controls. They help the pilot maintain roll authority by either supplementing the ailerons or being the only way to roll the airplane.