The video link shows a textbook example of Dutch roll instability in a Tupolev 154 aircraft which was supposedly flown last month (April 2011) after 10 years.
Let’s sum up very quickly what we observe in the video: (1) the aircraft rolls and yaws periodically, (2) roll and yaw are clearly out of phase (3) the aircraft has a vertical tail and the rudder does not appear to be moving significantly, and (4) the aircraft seems to be at a moderate angle of attack.
Background: typically, the dynamic behaviour of an aircraft (or any other system for that matter) can be decomposed into several modes, i.e., the the dynamic behaviour is a weighted sum of the different modes. The Dutch roll (DR) is one such mode.
Although the name suggests that it is a roll-based mode, DR primarily involves aircraft yaw. The rolland yaw oscillations that characerise a DR instability have a moderate frequency (typically 0.5 – 1Hz) and the rolling and yawing motions are out of phase. It is primarily the side-to-side (yawing) motion that is a source of piloting difficulties.
DR instability is encountered at moderate to high angles of attack, and under some circumstances, is a precursor to another instability called wing rock (where the dominant yaw motion gives way to rapid and disorienting oscillations in roll).
Forensics: For an aircraft with a tail like the Tu-154′s, it is hard to imagine that the DR was destabilised at all. As I said, it is usually stable at low angles of attack and even when it is destabilised, it can be stabilised by the aircraft’s control system. This leads us to three possible causes for the observed behaviour:
(a) The control system failed. This would explain the onset of instability, but not why it persisted.
(b) The steering mechanism was locked. This may explain why the pilots struggled to check the instability once it was triggered.
(c) The elevator was rendered partially ineffective. This prevented them from pitching down to low angles of attack where the DR would have stabilised itself.
Incidentally, one way to attenuate unstable DR oscillations is to deploy the flaps so that the increased drag on the wings damps out the oscillations. This may not always damp out the oscillations altogether, but it certainly makes them a lot more benign. In fact, flap deployment also creates a pitch-down moment which would be favourable under the circumstances.
References:
Text: Etkin and Reid “Dynamics of Flight: Stability and Control”
Paper: Ananthkrishnan, Unnikrishnan and Shah, “Approximate Analytical Criterion for Aircraft Wing Rock Onset” Journal of Guidance, Control and Dynamics, 2004.
