November 1, 2003
Twelve years ago, the FAA held the first of three conferences to discuss a sparsely documented icing hazard: tailplane icing. In this context, the term tailplane refers to either a conventional horizontal stabilizer-with-elevator arrangement, or an all-moving stabilator. While most of the research and knowledge of tailplane icing is limited to airplanes with horizontal stabilizers, in this article we'll stick with the generic tailplane term for the sake of simplicity.
Most pilots are well aware of the hazards associated with an icing encounter, and the effects a coating of airframe ice can have on performance. Lift is reduced, stall speeds increase, drag increases, and thrust falls off. It all adds up to an airplane that's unforgiving in several flight regimes, for reasons that are important to understand. It's useful to know that ice builds first — and fastest — on small-radius objects such as pitot tubes, outside air temperature probes, rivet heads — and tailplane leading edges. If you can see a trace of ice on the wing leading edges, it's a safe bet that the tailplane's smaller leading-edge radius has a larger buildup.
Symptoms of ice-induced tailplane stalls may occur immediately after flap extension, or after nose-down pitch, speed, or power increases following flap extension. Symptoms include one or more of the following:
*May not be detected by pilot with autopilot engaged.
When ice accretes on airframe leading edges, the camber, profile, and chord of the wing are changed. Even small buildups can have disproportionately adverse effects on lift. In airplanes not certified for flight into known icing, pilots of iced-up airplanes become test pilots. No test pilot has ever flown such an airplane in icing conditions before — nor has it been required by regulations. No one knows what the handling, stall speed, or stall characteristics will be in various icing situations. These speeds and characteristics vary greatly from airplane to airplane, and from configuration to configuration.
Pilots worry a lot about icing's effects on the wing, but tailplane ice can be equally dangerous. When a tailplane stalls, its loss of lift — or changed lift distribution — can cause the airplane to pitch abruptly down. Depending on the tailplane's behavior near the stall regime in icing conditions — and this too can vary from one type of airplane to another — the control yoke can be suddenly and forcefully yanked from the pilot's grip and slammed forward to the stops. Obviously, if this occurs while flying close to the ground, or on approach, the results can be disastrous.
Why would the nose pitch down, you may ask. Remember that, as airfoils, tailplanes generate lift just the way the wing does. But it's negative lift, lift that's exerted downward, and that's designed to balance the upward lifting forces of the wings. The low pressure area that creates this negative lift operates on the underside of the tailplane.
When ice begins to adhere to the tailplane, its normal airflow characteristics are changed, its negative lift is reduced, and its stall angle of attack is reduced. Any other condition that further increases the tailplane's angle of attack may cause it to stall prematurely. One of these conditions is an increased flow of air coming from wing downwash. This happens when flaps are extended. The downwash air strikes the tailplane at a higher angle of attack than in normal flight, increases tailplane lift, and pushes the tailplane closer to its critical angle of attack. This extra wing loading causes changes in pitch forces, and is what makes us — or our autopilots — dial in nose-up or nose-down trim when flaps are extended. Another way to increase tailplane air loads and angles of attack is to fly at higher airspeeds. Flying faster means greater volumes of air flowing over the tailplane, with resultant hikes in lift and wing loading.
When flying an iced-up airplane, pilots are told to keep their airspeeds up so as to avoid premature wing stalls. There's also the temptation to extend flaps so as to reduce the nose-high pitch attitudes (i.e., angles of attack) so often required to maintain level flight in icing conditions. But flap extension can cause ice to run well aft of wing leading edges, and ruin lift even more — even on aircraft with inflatable deice boots.
But add a layer of ice to the tailplane, then extend flaps at airspeeds at or near the maximum flap extension speed (V FE), and you've got the recipe for a tailplane stall. In several accident reports, witnesses described airplanes on stabilized final approaches that suddenly pitched over and dove straight into the ground. The assumption is that the pilots extended flaps, then experienced tailplane stalls.
What happens is that the airflow can separate from the leading edge and re-attach downstream on the underside of an iced-up tailplane. As the tailplane angle of attack increases due to flap extension or airspeed increase, an airflow separation "bubble" moves aft along the tailplane chord. When it moves beyond the elevator hinge point, its low pressure exerts suction on the elevator. This draws the elevator down — sometimes suddenly, which is what can pull the controls out of the pilot's hands — and pitches the airplane nose-down.
William J. Rieke, chief of aircraft operations at the NASA Glenn Research Center (where in-flight icing research is an ongoing endeavor; see " Wx Watch: Ice Flight," October Pilot), has a lot to say about tailplane icing. He should know, having flown and supervised numerous tailplane test flights in actual icing conditions in NASA Glenn's de Havilland Twin Otter.
"What happens is that the separation bubble moves all over the place — forward and aft — along the tailplane chord. We've seen that on our videos of tufted tailplanes. That's what makes the control yoke so light and sensitive in pitch. You can get into a pilot-induced oscillation if you try to chase the pitch forces...and as these oscillations become more wild, that's when a full-blown tail stall can happen.
"But there's another interesting thing going on. In some icing conditions, you can have as little as a three- or four-knot difference in airspeed between a wing stall and a tail stall. Fly too slowly and the wing stalls, fly a little faster and the tail stalls. Until we started testing, we never realized how much of a part power played in the tailplane stall regime."
According to Rieke and other icing experts, tailplane stalls in icing conditions can be avoided by keeping flaps retracted and flying at recommended no-flap approach speeds. Hand-flying is a must because autopilots prevent pilots from sensing subtle pitch and other control-feel changes. Approaches to landing should be made at near-constant pitch and power settings, and with gradual, shallow banks — which strongly suggests an ILS approach in cases where landings must be made in instrument meteorological conditions.
Confusion persists over tailplane stall recovery procedures, which are counterintuitive to the conventional stall recovery measures drilled into our heads from our first flight lessons. To recover from a tailplane stall, raise the flaps (if extended) and apply aft stick pressure. This reduces the tailplane angle of attack and reattaches the airflow to the underside lifting surface.
We'll discuss other icing issues in subsequent articles this icing season. In the meantime, I thought it useful for us to review tailplane behavior and flying techniques in icing conditions. It's a subject that's seldom discussed in aviation texts, seldom taught in ground school, and therefore a vast unknown for pilots of light airplanes. Most of the information gleaned about the phenomenon comes from research directed at commuter turboprop twins.
For more information about tailplane icing, consult Advisory Circular 23.143-1, titled "Ice Contaminated Tailplane Stalls," available on AOPA Online.
E-mail the author at firstname.lastname@example.org.
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