A pilot friend recently recounted how once, on a weekend trip to a well-known resort village, he watched in astonishment at what he considered to be the terrorizing antics of an unknown pilot putting a Cessna Cardinal through banks, dives and pull-ups that likely weren’t in the manufacturer’s mind as acceptable uses for the plane. Let alone the legality at which this low altitude and impromptu aerial display was taking place, he was sure he was about to witness a disaster unfold. Lucky pilot: he got away with it. The loadings hoisted onto the wings of that aircraft were abnormal (and unnecessary), figured the observer; a pointless manoeuvring accident had been beckoning to transpire.
What our observant pilot friend was aware of, in watching the theatrics above his head, was that a relationship exists between stalling speeds and Gs. The pilot of the Cardinal, perhaps, had forgotten the lessons from his ab initio training and had not, in all likelihood, ever read any of the regular accident reports full of examples of pilots stalling their aircraft when close to the ground. What follows is a reminder of what the pilot should have been taking heed.
Increase the angle of bank of your aircraft and the amount of lift required to sustain level flight also increases owing to the greater load factor created by the bank you’ve initiated. To obtain this greater level of required lift, the pilot must increase the angle of attack of the wings. Increasing the angle of attack of the wings increases the aircraft’s induced drag. As you bank, the end result is a stall angle that is reached at a higher airspeed than is the case in level flight. Any airplane can be stalled at any airspeed – so long as the wings don’t get peeled off first in an ill-advised and catastrophic manoeuvre. In unaccelerated level flight, the load on the wings is equal to lift and to the weight; consequently, the load factor equals 1G. The stall speed of the wing is equal to the normal stalling speed at 1G multiplied by the square root of the load factor imposed upon the wing.
When you increase the steepness of the bank, the load factor goes up too – and starts doing so exponentially beyond a bank of 45 degrees. For example, an aircraft with a normal stall speed of 50 knots may, at 30 degrees of bank, produce a load factor of 1.15G. Its stall speed at that angle of bank will now become 54 knots (50 x sqrt of 1.15 = 54). This is known as an “accelerated stall”. The same aircraft, at 60 degrees of bank and generating a load factor of 2G, will now stall at 70 knots (50 x sqrt of 2 = 70). Reach a load factor of 4, and watch this aircraft’s stall speed increase to 100 knots – double its 1G stall speed. If the airplane’s speed is not above this revised stall speed, the wing will stall. Have that happen close to the ground and you won’t likely recover. It’s no wonder, upon observing the Cardinal pilot executing low altitude banks at close to 90 degrees, that our observer feared a mishap was about to occur.
A common accident scenario is one that develops within close proximity to an airport. Overshooting a base to final turn can often result in the pilot increasing bank and rudder to pull the aircraft back onto the extended centreline of the runway. If not mindful of the consequences of the bank/load factor/stall speed relationship, the effort to save the landing could end in disaster.
High G-loading dramatically increases the stall speed. With altitude, you can always recover from the stall. Without altitude, be cognizant of the revisions to your stall speed number if you’re inducing a heap of bank within close proximity to the ground.