Definition
Spoilers on the upper surface of each wing that are deployed by unequal amounts to assist or replace ailerons in producing roll. The spoiler on the wing the pilot wants to lower extends further (or extends alone), reducing lift on that wing and rolling the aircraft in that direction.
Plain English
Panels on top of each wing that pop up by different amounts on the left and right sides to roll the aircraft. The wing whose panel pops up more loses lift and drops, turning the aircraft that way.
Context Anchor
Seen in high-speed flight control discussions, especially on airplanes where spoilers help or replace ailerons for roll control.
Derivation
‘Differential’ comes from the Latin differre, meaning ‘to carry apart’ or ‘to differ.’ It describes spoilers that operate by different amounts on each wing, rather than together. ‘Spoiler’ is plain English: a surface that spoils (disrupts) the smooth airflow over the wing.
Why Pilots Care
Maintains effective roll control at transonic speeds where ailerons alone may produce reversal or insufficient response.
Intuition Check
Do not read “spoiler” as something that gives information away, and do not read “differential” as a special separate device. Here, “differential” means unequal left-right movement, and “spoiler” means a panel that disrupts airflow over the wing.
Example Sentence 1
At cruise speed the flight control system uses differential spoilers for roll, since the outboard ailerons are locked out.
Example Sentence 2
At high altitude, differential spoilers supplemented the ailerons to maintain roll response without adding twist to the wing.