by Thomas P. Turner
Last month I described the first presentation made in the National Business Aircraft Association’s 2014 Single-Pilot Safety Stand-down, held in October. That first session focused on single-pilot resource management. Additional sessions, one on loss of control and upset/emergency maneuver training by Paul “BJ” Ransbury of Aviation Performance Solutions, another by Dr. Scott Shappell of Embry Riddle Aeronautical University on spatial disorientation, were excellent reviews of these important topics. This month, however, I’ll focus on a particularly interesting presentation: “Airframe Icing: Filling in the Details,” by Scott Dennstaedt.
Scott Dennstaedt is an FAA-certificated instrument flight instructor and former National Weather Service research meteorologist. He specializes in teaching pilots how to minimize their exposure to adverse weather. In the last 15 years, Scott has authored over 100 articles published in various aviation magazines. He is now employed by ForeFlight LLC as its weather scientist—this hints of interesting and perhaps unique new weather features in future updates of the market-cornering ForeFlight Mobile flight planning application. Scott also owns and operates AvWxWorkshops.com, a subscription-based aviation weather training website.
To prompt your consideration of airframe ice detection, avoidance and escape, let me present a few of my many notes from Scott’s NBAA presentation:
Airframe Icing: Filling in the Details
The presentation focused primarily on operating aircraft certificated for flight in icing conditions… the so-called FIKI, or Flight Into Known Icing, aircraft certification. Scott asked a question: “How many feel that your primary training did not adequately prepare you from an icing perspective?”
The classroom participants, packed primarily with business pilots of light turbines and piston twins, unanimously agreed that primary training does basically nothing to prepare a pilot for ice flying. Addressing the ice-certificated crowd, Scott noted that: “We’re taught how to plan a flight to avoid ice, not plan a flight through ice.”
Is gaining icing experience the answer? Just get out and fly more often, that’s all you need to do, right? Well, no. Scott agrees with the conclusions of the National Transportation Safety Board’s report Risk Factors Associated with Weather-Related General Aviation Accidents (September 2005). On page 38 of that report the NTSB states:
- It appears that pilots generally require formal training to obtain weather knowledge, and cannot be expected to acquire it on their own as they simply gain more flight experience.
- “There are no ‘great’ rules of thumb with respect to airframe ice,” Scott reports. “You need to understand the science behind airframe ice:”
- Airframe ice is directly related to how supercooled liquid water is realized in the atmosphere.
- In a pure vapor environment (no condensation nuclei), condensation won’t occur until the relative humidity exceeds 300%.
- Condensation occurs when adequate volumes of cloud condensation nuclei, or CCN (“dirt”) exist.
- As relative humidity increases CCN become “wetted” and visibility becomes obstructed.
- A relatively clean environment (fewer CCN) will typically increase the drop size.
- Larger drops represent a greater icing hazard.
- Where are the clean environments? Over large bodies of water, over snow covered ground, in clouds above temperature inversions, near the tops of clouds.
- “Cloud top temperatures warmer than -12°C means the cloud is likely dominated by supercooled liquid water.”
- “The only way to guarantee there will be no ice [above the freezing level] is to fly at colder than 40° below [zero],” Scott advises.
Supercooled Liquid Droplets
Even airplanes certificated for flight into icing conditions (so-called “FIKI” approval) are not approved nor permitted to operate in Supercooled Liquid Droplet (SLD) conditions.
SLD Conditions: An environment containing drops with a median volumetric diameter (MVD) greater than 50 microns.
1,000 microns = 1 millimeter
50 microns = Half the width of a human hair
If you can see water droplets or distinguish drops in the mist on the windscreen or leading edges before takeoff, or as you fly through clouds or precipitation, it’s SLD conditions. Flight Into Icing Conditions approval is not valid in such an environment; FIKI equipment will not protect you. The worst SLD condition is freezing rain. In freezing rain, the rate of ice accumulation is so great that no amount of ice protection equipment is going to prevent the dangerous build-up of aerodynamics-destroying ice in very short order.
We’re taught the almost universal presentation that conditions resulting in freezing rain involves a shallow band of freezing air near the surface, perhaps only a few hundred feet thick, over which lies a band of warm air, with above-freezing temperatures. Far higher, a second freezing layer marks the boundary above which the air is below freezing again. The freezing rain model we all learn goes like this: Snow forms in the cold air at altitude. As snow falls through the above-freezing layer of air it melts, with water droplets coalescing into larger, supercooled raindrops. These large raindrops, upon striking surfaces chilled to below freezing by the cold air near the surface, flash-freeze onto those surfaces. This creates a thick and irregular coat of clear ice—freezing rain.
Using this model for the formation of freezing rain, one in which above-freezing air is just above the surface, suggests pilots may employ these common avoidance and escape tactics for flight in areas of freezing rain:
If freezing rain is reported at or near the surface, cross the area a few thousand feet above the height of the freezing rain and you’ll remain in ice-free, albeit potentially wet air.
If at any time you encounter freezing rain conditions, climb. Above-freezing air is just a few hundred feet above you.
The trouble is that, meteorologist and commercial pilot Scott Dennstaedt warns, this set of conditions is what’s happening in only 8% of all freezing rain events. In 92% of all freezing rain events, Scott tells us, below-freezing temperatures exist from the surface, with no warm band of above-freezing air above the lowest layer. Instead, this is how freezing rain usually forms: Above a boundary defined roughly by the height where the temperature is at -12°C, small supercooled water droplets (SLD) are suspended in the atmosphere. These droplets collide with one another and fall into the lower levels, where the temperature is still below freezing, but closer to the freezing point. In this environment, larger supercooled droplets can form without themselves freezing. These large supercooled raindrops, upon striking surfaces chilled to below freezing by the cold air near the surface, flash-freeze onto those surfaces. This creates a thick and irregular coat of clear ice—freezing rain.
Why is it so vital to know this? Because everything we have been taught about freezing rain avoidance and escape is wrong, Scott tells us, in 92% of all instances when freezing rain occurs. Since there is no band of above-freezing air overlying the freezing rain, flying at a higher altitude still exposes the airplane to SLD conditions—for which no aircraft is certificated. And trying to escape by climbing out of freezing rain conditions, hoping to melt off the ice accumulation, would only result in adding additional ice to the airframe.
The only workable strategy when freezing rain is reported, Scott tells us, is to avoid flight in clouds or precipitation anywhere near or above the freezing rain unless the outside air temperature is colder than -40°C. This applies to Known Ice airplanes as well as those not certificated or equipped for flight in icing conditions.
(Many Twin & Turbine readers may never have been exposed to information such as was presented at the NBAA’s Single-Pilot Safety Stand-Down. Yet, it’s this level of pilot education that makes the difference between the superb safety record of airlines and professionally crewed corporate aircraft, and the much less desirable accident history of business and personal IFR flying. IFR flying is a profession whether you pursue it as your livelihood or not. It takes significant continuing educational investment of time and, yes, money, to enjoy the benefits of personal aviation at an acceptable level of risk.
The 2015 NBAA Single-Pilot
Safety Stand-Down will be held November 16 in Las Vegas. I hope to see you there.
Thomas P. Turner is an ATP CFII/MEI, holds a Masters Degree in Aviation Safety, and was the 2010 National FAA Safety Team Representative of the Year. Subscribe to Tom’s free FLYING LESSONS Weekly e-newsletter at www.mastery-flight-training.com.