Understanding Flat Spins: Critical Insights into Aviation Safety Risks and Recovery Techniques

61 people are dead after an ATR-72-500 operated by Brazilian airline Voepass Linhas Aéreas crashed in a residential area near Vinhedo in the state of São Paulo, Brazil, on Friday. It is believed that the aircraft experienced a flat spin, which occurs after both wings of a plane are stalled.

A spin is a severe aircraft stall combined with a spiraling descent, often discussed in pilot training and aviation safety. Let’s break this down.

A spin happens when both wings of an airplane stall, but one wing stalls more deeply than the other. This imbalance causes the aircraft to rotate and lose altitude in a downward spiral, a key focus in flight safety.

To enter a spin, the plane must stall while also yawing. This can result from improper rudder use, uneven power in multi-engine planes, power-induced turning in single-engine planes, or environmental factors like icing, wind shear and turbulence. Understanding these factors is crucial for enhancing aviation safety.

Single-engine planes certified under normal category regulations must demonstrate recovery from a one-turn spin during the aircraft certification process. However, they don’t always have to show they can recover from fully developed spins, an important aspect in flight training.

Flat Spins in Aviation:

In most spins, the aircraft’s nose points downward as it spirals toward the ground. The wings remain stalled, with an angle of attack between 20 and 45 degrees.

In a flat spin, however, the nose stays nearly level with the horizon, and the wings are severely stalled at an angle of attack between 45 and 90 degrees. Understanding the dynamics of flat spins is essential for aviation safety training.

In this situation, the control surfaces, including the elevator, might also be stalled, making it impossible for the pilot to adjust the angle of attack and recover from the spin. This scenario is a critical consideration in pilot safety and emergency recovery techniques.

Fortunately, modern aircraft design aims to avoid flat spins, a key focus in aviation safety standards. The only way a flat spin can occur is if the plane is loaded incorrectly.

If the pilot has properly checked the aircraft weight and balance before flight, keeping the plane within its center of gravity (CG) limits, a flat spin should be impossible.

However, if the CG is beyond the aft limit, the nose won’t pitch down during a stall, potentially leading to a flat spin. This highlights the importance of proper pre-flight checks in maintaining flight safety.

For a detailed explanation of flat spin and recovery, refer to the provided link, an essential resource for those involved in aviation safety training and pilot education.

https://pilotinstitute.com/what-is-a-flat-spin

Early recognition of stall

A stall warning is not the only means to identify the onset of a stall. Other visual indications form the instruments when interpreted will also give an early indication of the onset of a stall. An example of an ATR in Spain that stalled and recovered can be used for awareness.

Incident involving an ATR72-212A aircraft, registration EC-KKQ, operated by Swiftair,
at flight level 170 in the vicinity of reporting point NARGO(Albacete-Spain) on 9 September 2017.

American Eagle Flight 4184 (1994): An ATR 72 experienced a fatal stall and crash due to icing conditions. The aircraft encountered severe icing, leading to a loss of control and a subsequent stall. All 68 people on board perished.

Causes/Contributing factors
The investigation has determined that the probable cause of the loss of control in icing
conditions was a deficient flight management by the crew and an inappropriate use of
automation.

Flight prior to the stall:

• The flight prior to the stall lasted 18 minutes and corresponded to the climb
  phase. It was made in daylight conditions.
• IAS mode (speed) was not used during the climb, which is always recommended,
  and required in icing conditions.
• The climb was made in autopilot PITCH mode, even after the aircraft entered icing
  conditions.
• Once icing conditions appeared, there was a 2-minute delay before the anti-icing
  systems were activated.
• Once the icing light turned on, the de-icing systems were turned on immediately
  (5 s).
• The procedure was not applied when the degraded performance caution turned
  on.
• The severe icing conditions were not detected and the procedure was not applied.
• The autopilot was not disengaged.
• In order to climb above the cloud layer, VS (vertical speed) mode was selected on
  the autopilot. This is prohibited in icing conditions. Thrust was changed from CLIMB to MCT.
• The thrust mode was changed from CLIMB to MCT by the captain even though
  the first officer was the pilot flying.
• The speed was maintained above the icing speed (red bug) at all times.
• In the final 35 s of the climb, the speed dropped from 174 kt to 153 kt. It was
  not kept above the severe icing speed (red bug + 10 kt).
• The correct weight was not entered into the APM system, which prevented one of
  the cautions (INCREASE SPEED) from being generated.
• The cabin crew reported that the wings were iced over, and they heard the ice
  break off from the propellers due to the centrifugal force and impact the fuselage.
• The pilot flying during the climb was the first officer.
  
• The stall:
• 18 min after takeoff and 8 min after flying in icing conditions, the aircraft stalled.
• The stall began with the left wing, causing a descent of 1661 ft. The minimum
  speed reached was 151 kt.
• The maximum bank angles reached were 58º left and 41º right.
• The maximum pitch angles reached were 6º nose up and 11º nose down.
• The maximum angle of attack was 19.6º.
• During the stall, the autopilot and yaw damper disconnected.
• The stall warning was activated once, and the stick pusher three times.
• The captain provided a nose-up command to his stick on four occasions, countering
  the effect of the stick pusher.
• The first officer pushed down on his stick on two occasions.
• The stall recovery procedure was not used.
• The stall recovery lasted 33 s.
• The pilot flying during the stall was the captain.