Ice Contaminated Tailplane Stall
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The term "Ice Contaminated Tailplane Stall", or ICTS, refers to those events that involve flow separation from the horizontal stabilizer, due to ice accretion, which leads to an aerodynamic stall of the tailplane and results in a nose-down pitch upset of the aircraft.
The procedures for recovery from a tailplane stall are essentially opposite to those for recovery from a wing stall; misidentifying the type of stall and applying the wrong procedures will make the situation significantly worse.
Some aircraft types are prone to a nose-down pitch upset, referred to as a "tailplane stall", due to ice contamination of the horizontal stabiliser. Aircraft with reversible (unpowered) elevator control surfaces are more prone to tailplane stall but aircraft with irreversible control systems can also suffer tailplane stall due to ice accretion, though without the elevator overbalance phenomena described below.
In most aircraft, the Centre of Gravity(CG) (cg) is somewhat forward of the wing or mainplane Centre of Pressure. The exact distance between the cg and the Centre of pressure will depend on aircraft loading, configuration, thrust setting and drag. However, cg forward of the Centre of Pressure produces a nose-down pitching moment. The horizontal stabilizer, or tailplane, then provides a downward force to overcome this normal, nose-down, pitching moment.
The tailplane behaves as an ‘upside down’ wing and operates with negative Angle of Attack (AOA) as shown in Figure 1
If the horizontal stabiliser becomes contaminated with ice, airflow separation from the surface can prevent it from providing sufficient downward force or negative lift to balance the aircraft and a nose-down pitch upset can occur.
When compared to an aircraft's mainplane, the horizontal stabiliser normally has a thinner aerofoil with a sharper leading edge. Differences in the ice collection efficiency or catch rate between the two surfaces means ice accumulates faster on the horizontal stabiliser and may form before any ice is present on the aircraft's mainplane.
Tailplane stall can occur at relatively high speeds, well above the normal 1G stall speed of the mainplane. Typically, tailplane stall induced by icing is most likely to occur near the flap limit speed when the flaps are extended to the landing position, especially when extension is combined with a nose down pitching manoeuvre, airspeed change, power change or flight through turbulence. Aircraft stall warning systems provide warnings based on an uncontaminated mainplane stall so during a tailplane stall induced upset there will be NO artificial stall warning indications, such as a stick shaker, warning horn or the mainplane or flap buffeting normally associated with a mainplane stall.
Tailplane Stall Aerodynamics (Simplified)
- The horizontal stabiliser, or tailplane, of an aircraft is an aerofoil that provides a downward force to overcome the aircraft's normal nose-down pitching moment. The further forward the Centre of Gravity is from the Center of Pressure, the greater the nose down moment and, thus, the greater the amount of down-force that must be generated by the tailplane. This, in turn, requires a greater negative tailplane angle of attack (AOA). angle of attack (AOA). [As shown in Figure 1, The tailplane is effectively an upside down aerofoil so an increase in negative tailplane AOA occurs with UP elevator movement or when the aircraft is pitching nose down.]
- Accumulation of ice on the tailplane will result in disruption of the normal airflow around that surface and will reduce the critical (or stalling) negative AOA of the horizontal stabiliser.
- Ice can accumulate on the tailplane before it begins to accumulate on the mainplane or other parts of the aircraft.
- Flaps extension usually moves the mainplane Centre of Pressure aft, lengthening the arm between the Centre of Pressure and the cg and increasing the mainplane nose down moment. More down force is required from the tailplane to counter this moment, necessitating a higher negative tailplane AOA.
- Flap extension, especially near the maximum extension speed, increases the negative tailplane AOA due to the increase in downwash, as shown in Figure 2.
- Increasing the power setting on a propeller driven aircraft may, depending on aircraft configuration and flap settings, increase the downwash and negative tailplane AOA.
- When the critical negative AOA of the horizontal stabiliser is exceeded causing it to stall.
- Tailplane stall drastically reduces the downward force it produces, creating a rapid aircraft nose-down pitching moment.
On aircraft with reversible (unpowered) elevator, tailplane airflow changes caused by ice accretion may lead to an aerodynamic overbalance driving the elevator trailing edge down and pitching the aircraft nose down. This can occur separately from or in combination with the nose down pitching moment caused by tailplane stall. The yoke may be snatched forward out of the pilot’s hands and the control force required for the pilot to return the elevator to neutral or to a nose-up deflection can be significant and potentially greater than the pilot can exert.
Tailplane Stall Indications
Indications of an impending tailplane stall, as determined under test flight conditions, include:
- Difficulty in trimming in pitch.
- Reductions in elevator force required for control, especially in the forward sense.
- Elevator control oscillation or pulsing with forward movement of the yoke much lighter than a corresponding aft movement. This can often lead to pilot induced oscillation (PIO)
- Reduction in elevator effectiveness
If tailplane stall occurs it will result in an abrupt and/or un-demanded nose down pitching manoeuvre that may be preceded or accompanied by sudden forward control column movement or snatch.
- NO indications of an approaching stall such as airframe buffet or activation of stall warning systems
- Flying with the autopilot engaged will mask the symptoms of an impending tailplane stall.
- In all cases, indications might not become apparent until flap extention.
Tailplane Stall Prevention
Note that in ALL cases, manufacturer's procedures and Aircraft Flight Manual (AFM) guidance take precedence over any recommendations of this article
If, in an aircraft type susceptible to tailplane stall, an approach in icing conditions cannot be avoided, the pilot should:
- Avoid use of autopilot when flying an approach in known icing conditions.
- Plan to fly the approach and landing at the minimum allowable flap setting for the conditions.
- Be ready to ‘undo’ any flap selection.
- Avoid abrupt pitching manoeuvres and power changes. Nose down pitching motion and power changes in propeller aircraft can increase the tailplane AOA.
- Fly the approach "on speed" for the configuration. Avoid extension of the flaps near the flap limit speeds.
- If indications of an impending stall occur during or shortly after flap extension, immediately retract the flaps to the previous setting and increase the airspeed to the minimum manoeuvring speed for the reduced flap setting.
- Make any nose down pitch changes gradually, even in turbulent conditions.
Tailplane Stall Recovery
Note that manufacturer's procedures and Aircraft Flight Manual (AFM) guidance take precedence over the following recommendations
Note also that the procedures for recovery from a tailplane stall are essentially opposite to those for recovery from a wing stall. Misjudging the type of stall and applying the wrong procedures will make the situation significantly worse.
If a sudden, un-commanded nose-down pitch occurs in combination with the symptoms of an impending tailplane stall, consider the following recovery actions:
- Disengage the autopilot (if engaged)
- Manually resist any nose down elevator movement
- Immediately retract flaps to previous position if the pitching moment occurred during flap extension
- Return power to the previous setting if the pitching moment occurred during a significant change in power setting. Note that in some aircraft types, an immediate power reduction may be appropriate as part of the initial recovery actions. Refer to manufacturer's guidance.
The following articles provide further information on the important topic of tailplane ice contamination and stall. The NASA tailplane icing video is particularly descriptive.
- "Tailplane Icing - too much data, not enough knowledge" - an article published in the September 2009 edition of Professional Pilot magazine, by Captain Donald Van Dyke.
National Aeronautics and Space Administration (NASA)