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small amounts of ice can dramati- cally decrease thrust. This condition can be prevented by the immediate application of prop heat (turn it on when entering the clouds below 5 Celsius).
Jet pilots aren’t off the hook. Fan blades are not any better with ice than props, though they are somewhat more resistant to accumulation. In a jet, the presence of ice will manifest as an abnormal vibration on the affected engine. Cycling the thrust lever is typi- cally enough to shed the offending ice. As always, follow the type-specific guidance on your particular aircraft. It is a good idea when gearing up for the winter months to review cold weather guidance contained in the operating manuals. Don’t get caught by surprise.
Ice in the Real World
In 2009, I was a brand new captain flying a Beech 1900D between Denver and Cheyenne. Snow showers were present at both airports. It was a short hop, and we were in icing conditions the entire f light. Descending into Cheyenne, the clouds became thick enough that I had a hard time see- ing the leading edge of the wing. The airplane felt sluggish. I suspected that we were trucking around a substantial amount of ice. I flew the aircraft faster than was technically allowed, adding 10 knots to our approach speed.
The aircraft stalled a few seconds into the flare. There was no buffet or aural warning. We were 30 feet above the runway when the bottom simply dropped out. We left a divot in the touchdown zone following the hardest landing I ever had. Once it was shut down, I inspected the airframe. The wings were caked in a couple inches of ice. The ice was in the shape of an inflated deice boot. During the last few minutes of flight, the deice boot had been inflating inside a hollow block of ice. This particular phenomenon is referred to as bridging.
Bridging is a touchy subject. The NTSB dismisses it, contending in 2007 that “It [has been] established that ice bridging does not occur.” The FAA is more nuanced, noting that bridg- ing can occur with older deice boot systems. Both the NTSB and FAA are
unanimous in their conviction that waiting for a predetermined amount of ice to accumulate before “blowing the boots” (a technique ostentatious- ly used to avoid bridging) does more harm than good. Their position stems from this: “Since 1982...43 icing oc- currences involving turbine-powered airplanes...have resulted in 201 deaths and 16 serious injuries.” None of the accidents were attributed to bridging. Several were associated with the fail- ure to activate deice boots in a timely manner. The FAA encourages pilots to activate deice boots as soon as ice begins accumulating.
In truth, the operation of deice boots – while important – is secondary to maintaining sufficient airspeed. I had been reluctant to fly 10 knots faster than the published approach speeds (which already included a few knots to account for ice accumulation). In hind- sight, I should have added 20 knots. A month after my event, a crew made an approach into Salina, Kansas. They encountered severe ice on final. The pilots reported windshield wiper arms
Rosen
November 2021 / TWIN & TURBINE • 11