Ice contamination of the horizontal stabiliser can cause Ice Contaminated Tailplane Stall (ICTS), formed when ice forms on a tailplane's leading edge; the resulting ice accretion leads to an aerodynamic stall of the tailplane, which may occur before the wing stall during severe icing.
Manifestations of the phenomenon are more common on high-wing aeroplanes flying in mixed icing conditions, because mixed ice provokes tailplane stall more readily than clear ice, and the rump of the tailplane experiences airflow separation. Impending stall often shows itself through vibrations and a sudden lightening of the controls, together with a detectable loss of elevator effectiveness.
If the stall progresses, the nose-down pitch upset caused by the contaminated surface becomes apparent. Tailplane stall recovery requires that the pilot pitch the nose up, thereby re-attaching flow and regaining elevator authority.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is tailplane stall?
Tailplane stall refers to the stalling of an airplane's horizontal stabilizer, which involves flow separation from the horizontal stabilizer due to ice accretion and leads to an aerodynamic stall of the tailplane. This results in a nose-down pitch upset of the aircraft, causing the aircraft's nose to pitch forward violently.
Tailplane stall occurs when airflow separates from the tailplane. It is the moment the horizontal stabilizer loses the downward force it normally produces. As the stabilizer stops providing sufficient tail-down force, the aircraft's nose pitches forward violently and an abrupt nose-down pitching maneuver begins. This aerodynamic stall of the tailplane is equivalent to rapidly removing weights on one side of a balance scale, creating a rapid aircraft nose-down pitching moment.
Tailplane stall happens when the angle of attack of the tailplane exceeds its reduced critical value. This critical negative angle of attack is reached sooner when flaps are fully extended, when airspeed is relatively high, or when ice contaminates the surface. At these moments flow separation from the horizontal stabilizer leads to loss of lift and the airplane pitches down because tail-down force is lost.
What causes a tailplane stall due to icing?
A tailplane stall caused by icing begins when ice accretion on the leading edge of the horizontal stabilizer increases the angle of attack and disrupts the normal airflow. As mixed ice builds up, its rougher surface thickens the boundary layer. Flow separation then moves aft, a separation bubble develops on the tail's lower surface, and the critical negative angle of attack at which the tail can still generate downward force is markedly reduced. Consequently, the tail stalls at a lower angle of attack, and elevator authority is lost. The most probable scenario for icing to occur is when a high-wing aeroplane flies in mixed icing conditions with flaps extended fully to the landing position, near the flap-limit speed. During this configuration the flap's energetic down-wash increases the stabiliser's effective angle of attack. The down-pitch manoeuvre adds a further increment, and any accompanying change in power, airspeed or turbulence abruptly expands the separation bubble and shrinks elevator effectiveness, precipitating a sudden nose-down upset generated by the iced, stalled tailplane.
Why is the tailplane a dangerous place for ice to form?
The tailplane is a dangerous place for ice to form because the pilot does not readily see the tailplane, so contamination accumulates unnoticed. Sharp-edged surfaces like the horizontal stabilizer collect ice more efficiently than large blunt surfaces. When the main wing is collecting ice, the horizontal and vertical fins also collect ice, often in the rougher mixed-ice form that is especially dangerous. Designs with a small radius of leading edge are more prone to collecting ice, and lowering full flaps increases its angle of attack, making any ice that has formed even more dangerous. Because the tailplane sits outside the pilot's view, mixed-ice formation builds to disruptive thickness before the crew realizes the aircraft is threatened.
What are the symptoms of tailplane icing?
Reduction or loss of elevator effectiveness is an early sign of airplane icing, often felt as the elevator feeling lighter than usual and the control wheel moving forward easily while becoming difficult to pull back. Elevator control buffet occurs when flaps are lowered at the high speed range of flap extension, and this buffet is accompanied by elevator/stabilizer control pulsing, oscillation, or vibration. Unusual pitch anomalies possibly result in pilot-induced oscillations, and the pilot feels a buffeting in the wheel unlike a pre-wing stall buffet. If the separation bubble grows past the elevator hinge point, low pressure under the elevator forces the elevator down and yoke forward, producing a dramatic uncommanded nose-down pitch. These aircraft tend to be flying in icing conditions with ice, especially mixed ice, built up on the leading edge of the stabilizer. Thicker ice accumulation increases drag up to two times and develops speedily. Flight crews encounter severe conditions beyond the capabilities of aircraft protection systems, and autopilot use during severe icing is discouraged because it masks ICTS symptoms.
How can tailplane icing be detected by a pilot?
A pilot first looks for ice where ice builds first on small-radius objects. A strip of material within sight of the pilot is used as an ice evidence probe. NASA engineers outfitted a Twin Otter with such a strip on the leading edge of the left horizontal stabilizer. The material simulated accumulation of ice and gave pilots an noticeable cue. The same ice that coats the tail raises drag. Pilots detect increase in drag as loss in airspeed or increase in power required to maintain the same airspeed. Ice accumulation not seen directly is thus betrayed by the aircraft's own performance.
How to recover from tailplane icing?
To recover from tailplane icing, activate the airplane de-icing system if it has one to dislodge ice from the horizontal stabilizer. If the airplane is not equipped with a de-icing system, exit icing conditions as quickly as possible. When the tailplane stalls, retract the flaps to the previous setting to reduce the angle of attack of the tailplane. Reduce power as part of initial recovery to unload the tail. Pull back on the yoke slowly and with caution to raise the nose and move the tailplane away from the critical angle. Landing with reduced flaps is required if problems persist. Flaps fully extended increase nose-up trim requirement and cause the nose to pitch down. Manufacturer's procedures and AFM guidance take precedence over these recommendations.
I identified evidence of horizontal stabilizer malfunction when the aircraft started experiencing large, unreactive nose-down movement. My immediate action was to decrease the inclination of attack on the horizontal stabilizer. I bypassed pulling back on the yoke and retained forward force on the steering shaft. I realized that stall recovery methods would worsen status, so I decreased motor strength and diminished airflow over the back layer. I moved into a warmer atmosphere and pulled back airfoils to their former place.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air