POSITION & HOLD
an editorial opinion
Tailplane icing—survival knowledge for pilots
Guidance and training vital for appropriate crew response.
This diagram illustrates how severe nose-down pitch attitude and loss of control may become following tailplane stall.
Tailplane icing is a capricious killer, but steps can be taken to defend against its hazards, the foremost of which is to maintain vigilance and be ready to undo configuration and power changes if ICTS is suspected.
At all times, ice protection systems should be used as the AFM suggests. Certification rules for aircraft operations in icing conditions were never intended to endorse flight of unlimited duration in severe icing conditions.
The safest action is to avoid prolonged operation in moderate to severe icing conditions. Prolonged operations in altitude bands where temperatures are near freezing and heavy moisture is visible on the windscreen should be avoided. Flap extension should be limited during flight in icing.
For turboprops, the use of flaps is prohibited in icing conditions when enroute or holding. Autopilot use during flight in severe icing conditions is discouraged since (within its capabilities) it will correct anomalies and divergences that signal ICTS onset, thus almost certainly masking these symptoms by not allowing the pilot to receive tactile feedback from the controls.
FAA advises pilots to use caution when applying flaps during an approach if tailplane icing is possible. Selecting final flap earlier in the approach should be considered to use the greater height margin in case ICTS recovery is needed.
Uncoordinated flight (side or forward slips) which can adversely affect pitch control should be avoided. Landing with reduced flap setting, accounting for the greater landing distance required, is encouraged if allowed by the AFM.
Crosswind landing should be avoided, since ice accumulates not only on the horizontal stabilizer but also on the vertical stabilizer, reducing directional control effectiveness. Landing with a tailwind component should also be avoided, because of the possibility of more abrupt nose-down control inputs.
Recovery from ICTS
Available statistics show that an ICTS rarely occurs until approach and landing. However, the odds of recovery from uncontrollable nose pitch-down in this flight regime are poor, especially if the cause is misdiagnosed, since recovery procedures for wing stall will aggravate ICTS and vice versa.
For example, adding airspeed in this case may actually reduce the margin of safety. Because of the reduced maneuvering height margin available, increased stall speed and altered stall characteristics, recovery actions must be correct, immediate and aggressive.
(One ICTS event during the NASA/FAA TIP required 170 lb of elevator force to recover!) A good recovery strategy involves early detection and restoring the aircraft configuration just prior to the ICTS.
Corporate and regional crews may not receive much unusual attitude training and rarely experience full stalls and recovery in the aircraft they are flying. Without this training, they may misdiagnose aerodynamic buffeting as due to other causes, such as ice on propeller blades.
Typically, flightcrews are trained down to stick shaker and taught to power out of the stall warning with minimal altitude loss. These pilots may not recognize an ICTS that occurs before stick shaker activation and may not be sufficiently aggressive in recovery action even if they do recognize the situation.
A hard way forward
|WING STALL||TAIL STALL|
|• Full power||• Immediate hard pullback|
|• Relax back pressure or lower nose||• Retract flap to previous setting|
|• Actions i a w AFM||• Apply power judiciously and maintain precise control|
Some stall symptoms may not be detected by the pilot if the autopilot is engaged.
The likelihood of a flightcrew experiencing a full stall is much lower than the probability of a stick shaker encounter. As useful as a stick pusher is in avoiding wing stalls, it offers no comfort regarding an ICTS.
Dangers of ICTS begin with the absence of visible or tactile stall cues for which pilots are usually trained. The empennage giving rise to the ICTS event is not usually visible from the cockpit. Recovery from the event—if allowed to develop fully—is counterintuitive to conventional pilot training and may require physical strength exceeding the pilots’ capability.
The certified primary means of ice detection is visual inspection of the airframe by the flightcrew, including observation of areas such as the windshield, windshield pillars, windshield wiper bosses, wing leading edges, and propeller or engine fan spinners.
The admonition against autopilot engagement in icing conditions requires a subjective assessment of current weather conditions in which the empennage may already have been contaminated.
For pilots unfamiliar with stick pusher action beyond the classroom, it may be difficult to distinguish a valid wing stall warning from an ICTS-initiated elevator snatch. A decision to land, in which a pilot elects to divert and make an unscheduled landing due to ice accretion, is effective in fewer than 25% of cases classified as either an accident or incident.
More rigorous operating criteria and training requirements are needed to prevent ICTS-related accidents. Aircraft permitted flight into known and forecast icing conditions are only approved and certified for flight in supercooled water droplet conditions, as defined in FAR/CS/ JAR-25 Appendix C.
Reportedly, the current Appendix C chart and standard come from 1951–52 USAF icing research data showing that a Douglas C54 (DC4) could survive for approximately 8 min while descending 6500 ft, covering a distance of 20 sm at 150 mph.
It was adopted by FAA’s predecessor—the Civil Aviation Administration—in 1955 following the 1950 crash of a Northwest Airlines DC4 into Lake Michigan. This may be an appropriate time to revisit this design standard in light of technological advances and improved understanding of icing since its promulgation.
In the same way as training and recovery techniques were developed for jet upset recovery, current tailplane icing data must be distilled into a universal system for detection, avoidance and recovery.
Finally, operators are encouraged to give pilots the realistic training they need. Don’t require them to become test pilots each time they encounter ice.
Don Van Dyke is an 18,000-hr TT pilot and instructor with extensive experience in charter, business and airline operations. A former IATA ops director, he has served on several ICAO expert panels and is a Fellow of the Royal Aeronautical Society.
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