A Hair’s Breadth

A Hair’s Breadth

A Hair’s Breadth

Many pilots don’t realize that an aircraft certificated for flight in icing conditions (sometimes called Flight in Icing Conditions, or FIKI, or simply “known ice” certification) are not designed, safe or legal to operate with impunity in all icing conditions. If the rate of ice accumulation is too great even “known ice” airplanes are not safe to operate. In areas of lesser ice accumulation known-ice airplanes may enter, but may not remain in icing conditions beyond a defined amount of exposure. 

Read this icing accident report from the NTSB:

Approximately 13 minutes after departure, the pilot reported the airplane was accumulating ice. He requested and was cleared to descend from 5000 to 4000 feet MSL. Subsequently, the pilot requested and was cleared to descend to 3000 feet, and to proceed direct to the initial approach fix for the RNAV (GPS) 36 approach for landing at a nearby airport. No distress call or additional ATC communications with the pilot were recorded. The airplane impacted trees and terrain approximately 17 miles south of the airport. Tree deformation, ground scars and craters were consistent with a near vertical impact. 

Instrument meteorological conditions (IMC) with low ceilings, reduced visibility, light rain, mist, and drizzle prevailed at the departure airport and along the flight route. The temperature profile in the accident area was +1° Celsius at the surface, -3°C at 3000 feet, and above freezing at 7000-8000 feet. SLD (Supercooled Large Droplet moisture) was likely present in the accident area at and below 5000 feet and produced moderate to severe clear icing on the airframe in the minutes prior to the accident. Propeller blades exhibited physical evidence (blade bending and twisting) consistent with high power (at or near the low pitch/high rpm range) and rotation (symmetrical energy) at impact. No evidence of an in-flight mechanical or flight control malfunction was found that would have rendered the airplane uncontrollable prior to the impact. 

The NTSB determined the probable cause of this accident to be: The pilot’s inadvertent flight into severe icing conditions. A contributing factor was the pilot’s inadequate preflight planning.

Two PIREPs (Pilot Reports) from the immediate accident area were filed in the hour before the accident airplane’s departure. FAA records confirmed the pilot received both of these PIREPs when he briefed and filed for his departure just before his 0918 (local time) departure. At 0838, a Beech Baron 58 at 7,000 feet MSL reported the ceiling during climb out was 1,000 feet overcast with the tops of the overcast at 6,000 feet, temperature 10 degrees Celsius, wind 221° at 39 knots, light icing at 3,000 feet to 4,000 feet during climb. At 0905, a Mitsubishi MU2 at 7,000 feet MSL reported during climb out the sky was overcast at 900 feet with the tops of the overcast at 5,000 feet, light icing 2,300 feet to 3,300 feet during climb. 

It was Christmas Eve. The airplane had a broken alternator switch, and the pilot’s final flight was an attempt a hop to nearby Jonesboro, Arkansas to have the switch replaced before
flying to meet up with his family for the holiday. Weather near the departure airport was 200 overcast, visibility 1.5 miles, and +1°C surface temperature. Jonesboro (KJBR) was reporting 700 overcast, visibility 10 miles, with a 15-knot wind, +1°C surface temperature, and rapidly falling barometric pressure. The pilot may have thought he could quickly climb through the ice (after all, PIREPs called it “light”) and into an inversion above the clouds, then descend rapidly through the clouds in the approach to his destination. The holiday may have increased his perceived stress to make the flight despite the adverse conditions. 

Many Twin & Turbine readers’ aircraft are certified for flight in icing conditions – so-called “known ice” approval. But in what conditions exactly does “known ice” approval permit you to safely operate? Most pilots don’t know that ice certification provides a relatively small amount of ice protection. When is ice accumulation too much for even a known-ice airplane?

FAA certification for flight in icing conditions requires that the airplane’s ice protection systems be adequate to prevent or remove accumulations of ice in one of two conditions: continuous exposure and intermittent exposure. Known-ice airplanes are permitted to remain in continuous icing conditions only in stratus clouds when water droplets are no more than 40 microns in diameter. That’s 0.0019 inches (0.05 mm). Even then known-ice certification assumes the pilot will exit icing conditions before traveling 17.4 nautical miles – any more exposure than that and the accumulation may exceed the system’s ability to remove accumulated ice.

In cumulus clouds, only very short and intermittent ice exposures are approved. The maximum droplet size under known ice protection is 0.002 inches (0.05 mm). Exposure to ice accumulation is limited to 2.6 nm – requiring an immediate exit from icing conditions to avoid overwhelming the protection system.

Any water droplet greater than 50 microns in diameter is considered a “large droplet.” If the water is in a liquid state and the temperature is at or below freezing, it is a “supercooled” large droplet or SLD. By definition, no ice protection system is certified for flight: 

  • in any SLD conditions; 
  • in icing in stratus clouds for
    more than 17.4 nautical miles of continuous exposure; 
  • in cumulus clouds for more than 2.6 nautical miles of exposure.

To put this in perspective, the diameter of a human hair is 90 microns, or 0.070 mm – nearly 150 percent of the maximum water droplet exposure limit. What this means is that at or below freezing temperatures if water droplets are large enough to be perceived as individual drops or “streams” on your windshield or wings, they are too big for even known ice airplanes to be protected. No matter what you’re flying, you need to exit this type of visible moisture immediately.


One of the most significant advances in aviation weather in recent years is the introduction of Current Icing Potential (CIP) and Forecast Icing Potential (FIP) charts. CIPs and FIPs allow the pilot to see the probability ice may form at a given altitude along a planned route of flight, and if it does, the anticipated rate of ice accumulation. The FIP charts also plot areas of forecast SLD. Remember that the CIP and FIP are just a few years out of the experimental phase, and they are far from perfect in their predictions. However, the FIP, especially, is an outstanding addition to a pilot’s weather briefing when freezing temperatures may exist where that pilot wishes to fly. 

To use the charts, look first at the Probability chart for the altitude(s) you might fly. Any probability of ice at all means you need to investigate further – although of course, a probability of 60 percent (for example) means ice is far more likely than if the probability is less than 25 percent, the reality that even less than 25 percent likelihood of ice means ice may indeed affect your flight. If the FIP indicates an icing possibility, next check the expected intensity chart. Any route and altitude that includes an ice probability greater than zero and more than possibly “light” ice accumulation means even the known-ice pilot must plan for an escape. If the forecast is for greater than moderate ice accumulation or anywhere the FIP predicts SLD, make other plans, whether to fly a different route, fly at a different altitude, or to fly at a different time – because even known-ice certification is not valid there, and will not keep you and your passengers safe. 

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