One of the stall warning indicators the airplane is equipped with is the stick shaker. It helps provide a ‘WAKE UP’ warning to an impending stall. It literally creates vibration in the yoke control that simulates a car on a dirt road feeling. The mechanism that creates it is loud, while holding the yoke with it active it will shake your entire arm.
There is no mistaking what is occurring when you feel and hear the shaker activate.
Recently during a departure from a hub airport, we were cleared for take off immediately following an Airbus 319. You’d be surprised how close they clear us for take off normally, however during this departure they were stacking us pretty close.
This specific hub airport utilizes RNAV(Area Navigation, more commonly known as GPS) departure procedures for all RNAV capable aircraft. This means that the flight path for 99% of the traffic departing the airport is within one tenth of a mile of each other. With adequate lateral and vertical separation it’s never really a problem. During this departure we had significantly less lateral separation, however still beyond the minimum required.
Around 1500 feet above the ground we encountered the wake turbulence from the Airbus.
Wake turbulence is defined as a byproduct of induced drag. As airflow over the wing passes over the tip of the wing, it rolls over as it’s mixed with the air from under the wing. There is a small amount of span-wise flow from the air under the wing that creates a rotational vortex. These vortices flow outward and down from the wing tips of the aircraft. Since these are created as a byproduct of induced drag, the heavier the aircraft, the more intense the vortices. There are other factors that create more wake turbulence such as clean and slow aircraft. Clean referring to lack of high lift device deployment such as flaps or slats. Without those devices employed a higher angle of attack is required for flight which increases lift production, which increases induced drag. Slow aircraft require more angle of attack as the amount of lift generated is directly proportional to the indicated airspeed of the wing.
Put all of these factors together and you can see that during take-off, you have the highest amount of wake turbulence creation after take-off.
Small vibrations of what normal light turbulence feels like first. The captain is hand flying the aircraft throughout the climb. One of the signs of wake turbulence is the rotation force exerted on the aircraft that requires aileron input to keep the wings level. That happened next.
Another stall warning and protection device our aircraft uses is a pitch limit indicator. It shows how close to the stalling angle of attack the aircraft it currently at. It immediately showed that we were less than 3 degrees from stalling angle of attack. Simultaneously the stick shaker activated.
The entire event lasted less than 5 seconds.
I spent years teaching students about the dangers of wake turbulence and the techniques to avoid it. This made me realize I need to be more vigilant about it. Almost every take off and landing we hear ‘caution wake turbulence’ because we’re either landing behind or taking off behind a heavy airplane. Perhaps that dumbs it down a little or dilutes the seriousness of it.
Land above and beyond, rotate before and climb above.