There are reports that incorrect flight technique associated with spoilers extension during touchdown were contributing factors to pitch increase after landing, causing tail strikes in jet transport aircraft. Why does the spoiler extension causes a pitch up moment in the aircraft after touchdown?
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2 Answers
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Pitch-up upon spoiler deployment would occur if the spoilers are positioned aft of the center of lift of the wing when in its landing configuration. In this case, the spoiler kills off lift on the aft portion of the wing while the leading side of the wing (in front of the spoilers) is still generating lift. The lift generated by the portion of the wing in front of the spoilers is then not balanced by lift generated by that portion of the wing behind the spoilers, producing a net upwards pitching moment.
answered Jun 3, 2020 at 6:20
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$\begingroup$ Does this come from extending the spoilers before the nose is lowered? $\endgroup$– GdDCommented Jun 3, 2020 at 9:30
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$\begingroup$ After touchdown, aircraft doesnt pitch around center of gravity anymore, but main landing gear wheel axis. So theres different lever arm for spoilers to act with. $\endgroup$– user21228Commented Jun 3, 2020 at 10:46
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Spoiler deployment on a routine landing using published technique does not cause pitch up on a typical big jet. The moments and forces at play do not cause any alarming nose up tendency.
Generally the published technique recommends that (jet) thrust be brought to idle to coincide with touchdown which means Thrust Levers/throttles be at the idle position a couple of moments before touchdown. The problem arises when the thrust is not at idle during touchdown, and this scenario may be found mostly in handling strong/gusty winds, or as a result of poor piloting technique.
Typical logic for automatic spoiler deployment requires speedbrake lever or 'ground spoilers' armed + thrust at idle + wheels on ground.
In the case where auto deployment does not take place, the spoilers extend once the thrust levers are brought to idle or when the reversers are selected in the case of the system inadvertently not being armed for auto deployment.
Spoiler deployment causes a breakup of the airflow over the wings thus 'dumping lift' and putting weight on the wheels which is vital for brake effectiveness and traction - and also, without this, the airplane is still very much subject to aerodynamic forces and producing a good amount of lift. A late deployment, even by a second or two gives a considerable nose up pitching moment causing an alarming nose up which, if not actively controlled, can result in a tail strike or with a wing that is still flying, it can even cause the airplane to skip or bounce, or worse, the airplane can get unintentionally airborne again in an extremely low energy state due to the temporary spike in lift due to increase in the angle of attack.
