During some ‘hangar talk’ recently, beside all the usual war stories, came a discussion around slips and skids; the differences between them, and why skids are inherently dangerous, particularly in the circuit.
Given bush pilots often use slips to lose height quickly, and stall-spin accidents are one of the biggest killers in General Aviation, we thought it would be a good topic to write a few points about.
We’ve included a video below from Episode 48 of In The Hangar to give a visual representation of the differences below, but here are a few notes explaining the differences between slips and skids and why skids can be extremely dangerous.
To briefly describe the two, a skid is where the rate of turn is too great for the angle of bank, while a slip is where the angle of bank is too great for the rate of turn.
A slip is a safe, useful manoeuvre. It is an intentional manoeuvre where a pilot inputs aileron in one direction and rudder in the opposite direction to increase the rate of descent without increasing airspeed. It is an uncoordinated flight condition where the aircraft is moving somewhat sideways as well as forward relative to the oncoming airflow.
A skid, however, is a dangerous condition where the tail of an aeroplane moves away from the centre of a turn resulting in too much rudder input in the direction of a turn, ie turning left from base to final, and inputting too much left rudder. Skids can be an instigator of stall-spin accidents in the circuit, where there isn’t enough altitude to properly recover. Take, for example, an aircraft with a tailwind on base. The aircraft is moving quickly down the base leg and the pilot begins to execute the base to final turn but realises they may be going to overshoot. In addition to left aileron, too much left rudder can be applied, initiating a skid. If the nose begins to lower and the pilot pulls back on the yolk, the critical angle of attack is reached and the lower wing stalls. Because the aircraft is yawed left due to the skid, the aircraft enters a spin (after all, you need a stall and a yaw to create a spin). In the circuit close to the ground, there is usually insufficient altitude to recover.
Data from the US suggests that stall-spin accidents occur for 10% of all GA accidents, and 13.7% of fatal GA accidents. It is important we understand the difference between skids and slips, so we can continue to safely enjoy aviation and stop these avoidable fatal accidents from occurring.