IFR Fix: The sig in sigmet

December 13, 2013

The forecast included a prospect of mixed icing below 8,000 feet for the proposed IFR practice flight. The briefer mentioned a pirep reporting 10 degrees Celsius at 3,500 feet.

That spelled "go" for two instrument flight instructors planning to take turns shooting VOR approaches in a Cessna single.

But the situation was a bit more complicated than that. The icing noted in the briefing had been included in a sigmet. Sig, for "significant to safety," should get your attention.

The aircraft was glitchy. On departure, the pilots discovered that one of their two radios was intermittent. They decided to continue “on the remaining one.”

Into the clouds they slipped. At some point—probably when the first pilot completed an approach and turned control over to the other in the climbout—someone accidentally switched off the comm panel, disabling the working radio. Two minutes passed before contact was restored with an unhappy Albany Approach controller.

Now things snowballed.

"I was cleared for the VOR 28 approach and was just intercepting the final approach course when I noticed structural icing," one CFII reported to the Aviation Safety Reporting System. "This was my first encounter with icing and totally distracted me."

Compounding the distraction, the pilots were surprised to see abnormally low airspeed indicated despite a high descent rate and low power.

They switched on pitot heat "and the ASI jumped 20 knots."

But the flight had descended, unsupervised, to 1,700 feet—well below the segment’s minimum and barely above a nearby obstruction, prompting a query from ATC.

"The whole situation scared me a lot. Both myself and the other CFII had under 10 hours IMC between us and neither of us had ever encountered ice before," the pilot wrote.

On landing, the aircraft was found encrusted with an inch of mixed ice.

The 420-hour pilot acknowledged failure to consider implications of the temperature inversion hinted at by the pirep. Simplified illustrations aside, a warm front’s slope "isn't a flat plane, and the vertical distribution of temperatures isn't linear, either. This means that temperatures can rise and fall with altitude" creating "several freezing levels, and alternating layers of clear, mixed, and rime icing," as discussed in this Wx Watch column.

When a weather forecast contains Airmet Zulu for icing, it provides a hazard alert. A sigmet upgrades the hazard to severe. It’s a warning, not an invitation to take a plunge.

Share your lessons learned from inadvertent encounters with icing in the comments below.

Dan Namowitz

Dan Namowitz | Aviation Writer

Dan Namowitz is an aviation writer and flight instructor. He has been a pilot since 1985 and an instructor since 1990.