If you're a military or turbine pilot, you're probably familiar with an AOA indicator. It's a reliable way to keep your aircraft out of a stall and to maintain optimum performance. For students, it's also a great way to learn how weight, bank angle, and other factors affect stall speed - which brings me to the subject of this story - the accelerated stall.

Bank, Yank, Beep - The Accelerated Stall

CFIs are familiar with this maneuver; their examiner may have asked them to demonstrate the stall on their CFI check ride.

To demonstrate an accelerated stall, the CFI applicant rolls the aircraft into a banked turn and, while keeping the aircraft coordinated, firmly applies back pressure. The aircraft suddenly stalls, pitches down, and the applicant recovers. If the applicant doesn't stay coordinated during the maneuver, the aircraft can quickly enter an incipient spin.

All of this happens well above published stalling speed, which often surprises the pilot. Why? When you're in a banked turn, your aircraft is generating horizontal lift to turn as well as vertical lift to balance weight. You generate that extra horizontal lift by increasing angle of attack. So, in a turn, you generate more lift and fly at a higher angle of attack than in level flight at the same airspeed. That means you're closer to your critical angle of attack during a turn, and you can stall at a higher airspeed.

Distracted = Deadly

Accelerated stalls can quickly turn deadly if they happen unintentionally, and abrupt maneuvers at low speeds can provide the perfect setting. On February 29th, 2012, while maneuvering to avoid traffic on final, a Cirrus SR22 crashed due to an accelerated stall. The NTSB reports (ERA12FA196):

Josh Beasley / Flickr

Several airplanes and a helicopter were in the traffic pattern at the tower-controlled airport performing simultaneous operations to parallel runways (9L and 9R) around the time of the accident. The accident pilot contacted the tower air traffic controller while south of the airport requesting a full-stop landing; the controller advised the pilot to report when the airplane entered the downwind leg of the traffic pattern.

The controller subsequently cleared the accident airplane to land and expected the pilot complete a "normal" downwind traffic pattern and land behind the airplane already established on final approach for runway 9R; however, the controller did not provide sequencing instructions. The accident airplane proceeded directly to a tight right-base entry into the traffic pattern for landing on runway 9R, contrary to the controller's original expectation but permissible based on the clearance to land.

The controller radioed the accident pilot to confirm that he had visual contact with the airplane on a 1-mile final approach for runway 9R (the traffic was 300 feet below and 1 mile west). This was the first indication by the controller to the accident pilot that there was additional landing traffic sequenced to the same runway he had been cleared to land on. The accident pilot replied that he was on a "real short base" for runway 9R, and the controller responded, "no sir, I needed you to extend to follow the [airplane] out there on a mile final, cut it in tight now, cut it in tight for nine right."

The two airplanes had closed within 1/2 mile of each other, but were still separated by 300 feet altitude. The pilot of the airplane on short final for 9R maintained situational awareness throughout, perceived the conflict before the controller or the accident pilot, and responded calmly and benignly to the conflict. The accident pilot needed only to arrest his descent, at a minimum, to avoid any collision. A flight instructor and an airline pilot both described seeing the accident airplane pitch up, bank left, then roll inverted. The flight instructor stated that this action occurred as the controller was "yelling at" the pilot. Both witnesses described what they saw as "an accelerated stall."

Data extracted from the multifunction and primary flight displays revealed that the airplane pitched up and rolled inverted to the left at the same time that engine power was increased rapidly. When engine power is increased, a pilot must apply sufficient right rudder to counteract the left-rolling tendency, particularly if the airspeed is slow and the angle of attack is high, as it would be during landing. When instructed by the controller to "cut it in tight," the accident pilot over-controlled the airplane, lost control, and impacted terrain. (Emphasis and paragraph breaks mine)

In this case, the pilot was focused on the traffic. Instead of calmly arresting the aircraft's descent, the pilot banked, pitched up, and applied power abruptly. The bank and pitch most likely caused an accelerated stall, and the added power threw the aircraft out of coordination - causing the aircraft to roll inverted due to an incipient spin.

Calm, Cool and Aware

So - what's the point? Always stay aware of your airspeed, and during slow turns or abrupt maneuvers, remember that the aircraft can stall well above the published stall speed. When maneuvering for any reason, stay calm on the controls. "Yanking and banking" often makes things worse.

Take A Look

If you want to try an accelerated stall, grab an instructor. They can provide some realistic scenarios to simulate where the stalls can occur, and they'll make sure you're safe.

In the meantime, check out the video below from BruceAirFlying. At 0:20, you'll see a coordinated accelerated stall. At 1:30, you'll see an uncoordinated accelerated stall. At 2:00, you'll see one while vertical in a 1/2 Cuban 8. And, please, support your local CFI and keep out of the NTSB reports - grab an instructor before you try one!

UPDATE: After I posted this article, Bruce uploaded another accelerated stall video to YouTube, this time in an A36 Bonanza. It's a great example of how coordinated accelerated stalls appear in a non-aerobatic aircraft. For more videos, check out his webpage at bruceair.com