Most aircraft accidents occur during the take-off or landing phase of the flight. Collisions with obstacles during climb out, runway overruns on landing occur every now and then. In this section of the site we will take a look at the various factors contributing to the performance of the aircraft in this part of the flight.
A number of aircraft have the tendency to drop a wing or roll at the stall which could result into a spin if left unattended. We will take a look at its cause and the actions a pilot can take to minimize the wing drop preventing a spin.
When both wings on an aircraft stall at exactly the same angle of attack (AOA) the aircraft just noses over. But as there are always slight differences between both wings, one wing will stall before the other. Thus the wing drop has begun.
There are a number of causes for a wing to drop. Below a list of the most common reasons:
Unequal amounts of thin ice (ice or snow should be avoided at all times), other contamination or damage can and will result in uneven air flow separation between both wings and on the wing itself too.
Particularly so with low winged aircraft, as they usually have some dihedral and during a slip or skip one wing has a higher effective AOA resulting in that wing stalling before the other.
As the downward deflection of an aileron increases the AOA on that section of the wing, that wing will stall before the other. So, a turn to the left means downward right aileron and at the stall the right wing will drop and the aircraft will roll right although the ailerons say otherwise... Which is one more reason NOT to use ailerons during initial stall recovery, as it will make matters much worse.
In a turn, climbing and descending too, the wings each have a different angle of attack. Thus, if the stall is approached during turning maneuvers one wing will stall before the other.
Climbing turns: the higher wing will stall first. Decending turns: the lower wing stalls first. The last one is a common occurrence during traffic pattern operations as the turn from downwind to base and final are decending left (usually) turns, close to the ground with no room to recover from the wing drop.
When flaps are not properly adjusted or build to the same dimensions and form, the angle of attack between the wings will be different the moment the flaps are extended. You will notice a roll tendency too. If the aircraft wants to roll to the left, the right flap has a greater deflection and creates a larger AOA on the right wing or vice verse on the left wing.
When flaps are extended the curvature of the wing increases, due to this, the wing will stall at a lower angle of attack as the air will not flow around the wing so easily. With flaperons the whole wing will stall compared to normal flaps, resulting in an early wing drop.
The application of engine power during the approach to stall results in an increased airflow over the inboard sections of the wing and this causes a delayed separation of the airflow compared to the outboard sections where the ailerons are located. The outboard section stalls before the inboard does and if an asymmetry exists between the wings the wingdrop will be even more pronounced.
A stall should develop from the wing root to the tips so that if a wing drops, it will be relatively benign and the ailerons will be effective enough to help counter it (combined with coordinated rudder, of course). Most General Aviation wing planforms are rectangular which display the aforementioned characteristics.
If a wing has a more tapered planform the stall is more prone to occur nearer to the wingtips. Wing washout (were there wing angle of incidence is higher at the root) or stall strips are therefore used to make sure that the stall starts at the root.
When a wing does drop, its downward movement increases the AOA even more, thus bringing it deeper into the stall. Using ailerons at that moment would not be of any help at all as picking up the wing (downward aileron) also increases AOA but then at the wingtip. The stall is now developed from wingroot to the tip.
Ailerons should therefore NOT be used when a wing drops, which can be difficult to do as the pilot learned during training that during 'normal' flight its the way to pick up the wing.
At the wing drop the only correct action is to apply opposite rudder to stop the yaw/rolling motion and then push the stick/yoke forward to reduce the angle of attack of the wings. Both actions can be applied at the same time. As the airspeed increases, ailerons coordinated with rudder can be used and power applied to bring the aircraft back to straight and level flight.
If the nose of the aircraft was below the horizon then the application of power maybe delayed or else the aircraft will be going downhill fast and the recovery will take much more altitude (which you may not have).