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Ice Contaminated Tailplane Stall
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| Category: | Weather | 
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| Content source: | SKYbrary | 
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| Content control: | EUROCONTROL | 
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| WX | |
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| Tag(s) | Icing |
ICTS
Contents |
Definition
The term "Ice Contaminated Tailplane Stall", or ICTS, refers to those events that involve flow separation from the horizontal stabilizer due to ice accretion.
Horizontal Stabilizer Function
Normally, the aircraft centre of gravity is located forward of the center of lift. This generates a nose down pitching moment. The horizontal stabilizer generates a lifting force in the downward direction relative to the airplane, which serves to balance the pitching moment at the main wing. If this stabilizer lift is compromised or destroyed, the pitching moment will dominate and the aircraft will nose over.
The horizontal stabilizer is essentially an inverted wing. The angle of attack at the stabilizer is influenced by a number of factors. It is usually at its highest when the maximum wing flaps are selected, because the stabilizer is strongly influenced by the downward flow of air off the wing trailing edge. Further, when flaps are selected the main wing center of lift generally moves aft, increasing the pitching moment and therefore demanding greater downward lifting force from the tail. The stabilizer angle of attack is also influenced by the airspeed; at higher speeds, near the flap extension limit, the stabilizer angle of attack will be greater than it is at lower speeds.
The lift generated by the stabilizer is also largely a function of the elevator position. The elevator, which is essentially a full span flap, serves to alter the camber of the stabilizer airfoil. A trailing edge down deflection of the elevator reduces the lift generated by the stabilizer, and the pitching moment at the main wing briefly dominates and pitches the nose down. Conversely, a trailing edge up elevator deflection increases airfoil camber and increases the lift generated by the stabilizer. In this case, the lifting force generated by the stabilizer, multiplied by the moment arm of the aft fuselage, overpowers the pitching moment and drives the nose upward.
In many simpler designs, the elevator is an aerodynamically balanced control surface. It is moved by direct linkage to the control column in the cockpit. It may be assisted by such devices as spring tabs or servo tabs. Nonetheless, the aerodynamic forces across the stabilizer surface must remain distributed in a manner that produces normal, manageable control forces at the cockpit control column. In more complex designs, the elevator is fully powered by hydraulic actuators. These systems possess enough power to overcome any aerodynamic disturbances across the stabilizer.
It is a fundamental tenet of design that the control force, or “stick force” used to move the elevator must behave in a rational and predictable manner. Powered controls accomplish this through artificial feel systems. Unpowered controls, often known as “reversible” controls, meet this requirement by careful aerodynamic balancing so that increasing elevator deflection requires increasing, but manageable, control force. If the controls are released, the elevator trails to the minimum force position. A flight control is considered “reversible” if movement of the control surface while standing outside the parked aircraft results in control wheel movement in the cockpit.
The horizontal stabilizer is designed so that it never truly approaches the stalling, or critical, angle of attack across the full range of normal flight operations. Thus, tailplane stall is not seen in uncontaminated conditions. However, when ice contamination occurs, the critical angle of attack at which stall occurs on the stabilizer is changed. Even so altered, normal flight requires little lift from the tail and it will operate conventionally. However, when flaps are extended, particularly near the flap limit speed, the critical angle of attack may be approached and then exceeded.
Characteristics of ICTS
The initial behaviour of the stabilizer as the ice contaminated critical angle of attack is approached reflects early flow separation. A separation bubble, or void, forms behind the separated boundary layer and is closed by the flow re-attachment further along the chord line. The flow separation pattern is similar to a thin-wing stall, and does not initially result in completely separated flow The separation bubble causes a redistribution of pressure across the surface, leading to several possible effects. These effects are not necessarily sequential.
The flow separation may lead to a change in the restorative forces acting against elevator deflection. This is known as “stick force lightening”, meaning that it requires less force to deflect the elevator. This will be exhibited when deflecting the elevator toward the nose down direction. As the configuration, speed and/or angle of attack are changed in ways that require more performance from the tail, the characteristics of the restorative force, or hinge moment, may change for the worse. Thus, less force is required to deflect the elevator trailing edge down, and comparably more force is required to deflect it trailing edge up. In more advanced cases, the force required to deflect the elevator trailing edge down will actually decrease with increasing deflection. In other words, the farther the pilot pushes the pitch control, the easier it gets to push it even farther. As the separation bubble develops further, it may extend across the elevator and cause an elevator “snatch”, or full nose down deflection. This condition can bring stick forces of several hundred pounds, which may exceed the pilot’s physical strength.
The flow separation leads to a degradation of lift produced by the stabilizer. This is countered by increased nose up elevator deflection, causing an increased camber of the stabilizer. At the same time, flowfield disturbance around the movable trim tab may compromise the trim tab’s ability to generate the force required to maintain the elevator in the deflected position. The pilot may have difficulty trimming, or actually need to maintain a positive pressure on the control column.
While these effects are being observed, it is likely that an elevator buffet will be felt, resulting from increasingly turbulent flow across the elevator. It is important to distinguish this control vibration from a main wing pre-stall buffet. The pre-stall buffet is characterized by a shuddering felt throughout the entire airframe; the ICTS buffet is felt only through the elevator controls.
In all of these cases, the potential for a pilot-induced oscillation is very high. An entirely new, and changing, relationship between stick force and elevator deflection is presented to the pilot. In his/her efforts to apply normal control corrections, the control response is different, resulting in potentially severe pitch oscillations. The PIO is an indicator of ICTS, and also sets up the typically fatal pitchover.
In extreme cases, the elevator “snatches” into the separation bubble and out of the pilot’s hands. The snatch is always nose down, and it is important to remember that the elevator is still functioning. The substantial de-cambering that occurs when the elevator travels fully to the trailing edge down position causes the stabilizer to lose a great deal of lift. The main wing pitching moment then rotates the nose toward the ground.
Recovery Techniques
The proper response to these effects is always to apply full nose up elevator, using all of the physical strength necessary to get the nose back up, while retracting the flaps to the last safe position. It is thought that increasing power may be detrimental to recovery, but National Aeronautics and Space Administration (NASA) points out that this may be aircraft specific. In any event, physically re-cambering the stabilizer by deflecting the elevator trailing edge up will restore some lift to the tail, and retracting the flaps to the last position will reduce the stabilizer angle of attack.
It is also important to consider the technique used to arrest a pilot-induced oscillation. Once the elevator is re-cambered and the nose has been pitched up, any further attempts to pitch down, as in trying to continue the approach, should be abandoned. A PIO is arrested by placing the controls in a position leading to the desired recovery attitude and then essentially freezing them until the attitude is stablized. Once the nose is pitched up, and the flaps are retracted to the last safe position, a normal go-around sequence can be followed.
Typical Accident Scenario
In the typical ICTS accident, landing flaps are selected close to the ground (fairly standard for propeller driven aircraft). Nothing happens initially... until the pilot detects a need to slightly correct into a "fly down" glide slope indication. He gently nudges the elevator nose down... and gets a substantially larger elevator deflection than he expected for the push he applied. The nose tucks, and he corrects with the obvious, and proper, response: a strong pull to the nose up elevator deflection. The stabilizer is re-cambered, the separation bubble contracts, the pilot gets normal force and response, and the nose rises rather dramatically. Now the pitch attitude is too high, and the aircraft is rapidly rising above the glide slope. So now he responds by applying a significant, but not reckless, nose down push to restore the correct attitude.
Unfortunately, when he does this, he radically de-cambers the stabilizer; the stick force gradient is again changed, and his not-unreasonable push force results in the elevator traveling all the way to the stops. Unless there is a lot of altitude, this second pitch down will usually be fatal.
Accident History
ICTS accidents have been recorded which involve a range of aircraft. The most prominent earlier cases involved Viscounts, DC-4s, Convairs and YS-11s. The Jetstream 31 was plagued by the problem for a period of time, as was the DHC-6 Twin Otter. Non-fatal events have occurred involving the ATR 42, the Saab 340 and the DC-9/MD-80 series.
Revised Certification
FAR 25.143 provides a requirement, during certification, for a zero G pushover manoeuvre with critical artificial ice shapes attached to the horizontal stabilizer. The requirement is for a push force to exist all the way to zero G, and that push force to continue increasing at least until 0.5 G is attained. A pull force of no more than fifty pounds should cause prompt recovery. This certification requirement is intended to identify design problems that lead to an ICTS event.
Contrast with Normal Stall Recovery
A significant threat exists if the tail stall is confused with a main wing stall, as the recovery procedures are precisely opposite. It is important to be aware of airspeed, as the ICTS event generally occurs at speeds near the upper end of the flap extension range. The buffet must be distinguished from the pre-stall buffet by the feel of the controls. This cannot be accomplished if the autopilot is engaged. Any tendency toward pilot induced oscillations in pitch following the selection of landing flaps may reflect an ICTS condition but will not indicate a main wing stall. Flaps must be retracted at the onset of a tail stall, while they are generally left in position during a main wing stall recovery. Power should be used judiciously during an ICTS event.
Stick Shaker/Stick Pusher
It is considered possible for a pilot to confuse the stick shaker with the elevator buffet condition, followed by interpreting the stick pusher as an elevator snatch. This reinforces the requirement to be aware of airspeed, configuration and pitch control forces. Misinterpreting the shaker/pusher for a tailplane stall could be a catastrophic mistake; conversely, misinterpreting the elevator buffet/elevator snatch behaviour for a main wing stall could be equally disastrous.
Related Articles
Weather/Aircraft Icing
Accident Reports
Further Reading
- "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.