POSITION & HOLD
an editorial opinion
Tailplane icing—survival knowledge for pilots
Guidance and training vital for appropriate crew response.
By Don Van Dyke
ATP/Helo/CFII. F28, Bell 222
NASA’s icing research aircraft is this modified de Havilland Canada DHC6 Twin Otter. Flow probes and a clear-ice simulation casting are attached to the leading edge of the left horizontal stabilizer.
Pressures of modern operations, advances in technology—which often dilute airmanship skills—and improved cockpit facilities all encourage penetration into weather conditions far worse than our predecessors would have attempted.
The dangers are still there—we have just learned to navigate them differently. Icing conditions have been regarded as hazardous virtually since the beginning of flight. Ice buildup (accretion) results in degraded aircraft performance, flight characteristics and systems operation.
Interestingly, modern aircraft designs, in achieving greater efficiency, may also be better icing collectors than previous designs.
Aerodynamic, stability or control events resulting from structural icing include stall, loss of control, high sink rate, loss or degradation of performance, and flight control degradation. Outcomes include ground or water collision, hard landing, inflight breakup/structural failure, landing short or precautionary landing.
The especially insidious ice-contaminated tailplane stall (ICTS) is identified as causal in at least 16 corporate and air carrier mishaps, involving 139 fatalities. Since critical icing conditions occur infrequently, crewmembers may become complacent about the potential for critical ice accretion in certain operating areas or conditions.
Each atmospheric icing condition is different, and flightcrews may occasionally encounter severe conditions beyond the capabilities of the aircraft protection systems. However, totally effective anti-icing systems are—and will remain—beyond economic realization for the foreseeable future, and the threat of tailplane icing will remain a major cause for concern among flightcrews.
Developed in the 1990s, the landmark NASA/FAA Tailplane Icing Program (TIP) improved understanding of tailplane (empennage) icing but yielded comparatively little in the way of assessing design susceptibility to ICTS or developing related detection or unambiguous mitigation strategies of use to corporate and regional pilots.
Regardless of the final report findings, the recent crash of a Bombardier DHC8-Q400 near BUF (Buffalo NY) renewed widespread interest in the dangers of inflight icing and, more particularly, ICTS.
Ice adheres to all forward-facing surfaces of an aircraft in flight, often accumulating with surprising speed. The tailplane, generally having a sharper leading-edge section and shorter chord than the wings, can accrete ice before it is visible on the wing—and at a greater rate.
Pilots have reported ice accretion on the empennage 3–6 times thicker than ice on the wing and about 2–3 times thicker than on the windshield wiper arm. On turboprops, propwash cooling effect may further encourage ice formation on the tailplane.
The aerodynamic effect of a given thickness of ice on the tail will generally be more adverse than the same thickness of ice on the wing. This is due to the ratio of thickness to chord length and leading-edge radius.
Ice-induced stall In worst cases, ice allowed to accumulate will disrupt airflow over the wings and tail, causing a stall and loss of control. In some cases, only a few seconds elapse between normal flight and ground impact.
Tailplane counters the nose-down pitching moment caused by the center of gravity (CG) being forward of the center of pressure.
Wing stall normally results from flow separating from the top surface. This usually starts at the inboard wing trailing edge or at the wing/fuselage and wing/nacelle junctions. Stall identification is notified to the pilot either through the inherent aerodynamic characteristics of the airplane or by a stick shaker/pusher incorporated in the elevator control circuit.
A stick pusher induces an abrupt nose-down pitch change. ICTS occurs when, as with the wing, the critical angle of attack (AOA) is exceeded. Since the horizontal stabilizer acts to counter the natural nose-down tendency caused by the wing lift moment, the airplane reacts by pitching down—often abruptly—when the tailplane is stalled.
Flap extension can initiate or aggravate the stall. With flaps extended, the center of wing lift moves aft and downwash is increased, requiring the horizontal tail to provide greater downward lift.
Similarly, as the center of gravity (CG) moves forward, the tail may be near its maximum AOA, meaning that a small amount of ice contamination could cause it to stall. In either case, the result may be a rapid and unexpected loss of control with little or no margin for recovery.
A significant number of events occur during the landing phase, resulting in a hard landing. This may be associated with a loss of performance during the approach, forcing descent below the glidepath.
If the stabilizer is not visible from the cockpit, pilots may be unaware of ice accretion and may fail to operate deicing equipment correctly. ICTS factors are complex and exhibit symptoms unique to aircraft type and configuration.
These factors can cloud crew recognition and obscure appropriate recovery actions. It is important that symptoms of ICTS are recognized correctly and not confused with those of the more familiar wing stall.
Perhaps the most important characteristic of a tailplane stall is the relatively high airspeed at the onset and, if it occurs, the suddenness and magnitude of the nose-down pitch with the control column moving toward the forward limit.
ICTS is more likely to occur when flaps approach full extension or during flight through wind gusts. In general, the combination of factors favoring tailplane stall is ice accretion of critical shape, roughness and location, maximum flap extension, forward center of gravity, high power and nose-down elevator control inputs.
Symptoms of ICTS include:
• Elevator control pulsing, oscillations or vibrations
• Abnormal nose-down trim change
• Other unusual or abnormal pitch anomalies (possibly resulting in pilot-induced oscillations)
• Reduction or loss of elevator effectiveness
• Sudden change in elevator force (control would move nose-down to the limit if unrestrained)
• Sudden uncommanded nose-down pitch