With the winter months just around the corner in the Northern Hemisphere, the NTSB has published Safety Alert SA-097 to remind operators of the risks of flying in icing conditions, including the adverse effects of airframe and propeller icing on aircraft performance.
“As little as one-quarter inch of wing leading-edge ice accumulation can increase the stall speed by 25 to 40 knots and cause sudden departure from controlled fight,” warns the NTSB. In addition, ice accumulation on pitot tubes can cause flight instruments—particularly airspeed, altimeter, and vertical speed indicators—to stop functioning or give incorrect readings.
The notice concedes that some pilots have been taught to wait for a prescribed accumulation of leading-edge ice before activating deice boots because of the threat of ice bridging. “However, performance degradation could develop if the deice boots are not activated as soon as icing is encountered.” The notice also cautions that continuous use of the autopilot in icing conditions can “deprive the pilot of the opportunity to detect the buildup of ice on the airframe through changes in longitudinal trim requirements and control forces.”
The fatal crash of an Embraer Phenom 100 on Dec. 8, 2014, was one of several icing accidents cited in the notice where the NTSB said pilots “did not adequately comply” with instructions in their pilot operating handbooks or aircraft flight manuals leading to in-flight loss of control. While descending from 23,000 feet, the Phenom 100 pilot confirmed to ATC that he had the destination airport’s current weather observation that indicated conditions favorable for structural icing during descent and approach. About three-quarters of a mile from the approach end of the runway, the twinjet stalled and collided with three houses. The pilot, both passengers, and three people in one of the houses were killed.
Before beginning the descent, “the pilot incorrectly used the normal (non-icing) checklist, which did not call for activating the wing and horizontal stabilizer deice system,” the Safety Alert says. “Further, he set the landing reference speed at 92 knots rather than 126 knots he should have used to account for the icing conditions.”
It concludes: “Had the pilot used the correct checklist…the airframe likely would not have accumulated ice, and the pilot would have used appropriate landing performance speeds for the conditions. Thus, not using the correct checklist led to an aerodynamic stall at an altitude at which recovery was not possible.”