Deadly Ice

By Bill Korbel

The crash of Flt 3407 ended an extraordinary streak of air safety for US airlines which saw them transport 1.5 billion passengers over two plus years without a fatality.  The NTSB has not yet discovered the cause of the crash but at least a major factor would seem to be icing.

 

Airplanes can run into many forms of hazardous weather including turbulence, strong winds, wind shear, heavy precipitation, hail, lightning,  reduced visibility and, perhaps the strangest and most difficult to forecast: icing.

Clouds are made of droplets of water or crystals of ice.  Which is present is dependent on temperature and other factors.  What is really weird about water in the atmosphere is that it can exist in liquid form even at temperatures below freezing.  Believe it or not, water can still be liquid at temperatures approaching zero! It’s called super-cooled water and can be super bad.  When a plane flies through super-cooled water droplets, they will quickly freeze on contact.  The ice that builds can affect the aerodynamics of the airplane by decreasing lift produced by the wings and increasing drag.  If allowed to build beyond a certain point, the plane will no longer be able to stay in the air and the change from controlled to out of controlled flight can be very, very sudden. 

 

 

To keep this from happening, all airliners and many smaller planes are designed to either keep ice from forming or remove it when it forms before it becomes a problem. As good as these syetms are, there are times when ice can form faster then a de-ice system can remove it. When that happens, the pilot must get out of the icing area fast.  Usually a change of altitude of a few thousand feet will suffice.

Thanks to improved technology and better forecasting, in-flight icing accidents have become very rare in airline flying.  The last major one was in 1994 when an American Eagle twin engine turbo-prop, in an eerily similar accident, ran into a rare form of heavy icing over Indiana and spun out of control from 8,000 feet, killing all on board

                       

 http://en.wikipedia.org/wiki/American_Eagle_Flight_4184

So why did 3407 go down when similar airplanes just before and after it had no problems with the ice (or at least nothing the airplane couldn’t handle)?  Until the NTSB completes it’s investigation we won’t know exactly what happened but it brings into the harsh light of reality something I have known since I first got into meteorology as an Air Force weather officer:  The atmosphere is full of surprises and pilots and forecasters can never relax their guard.

 

Comments

[John]

That's a fair assessment. I just (thought I) remembered an NTSB statement suggesting that it wasn't significant. I may be thinking of something else or could have misinterpreted, though.

Ah! I see now. It's the autopilot story that the NTSB rejected. Too many stories appeared in a short span for me to keep it straight.

[Bill Korbel]

As far as I know, NTSB has not ruled out icing. Several aircraft coming into Buffalo reported moderate ice below about 8,000' and the accident crew talked about ice. They also had the deice system operating. If not a prime cause I am sure NTSB will say it was a major factor.

Bill Korbel

[John]

I know it's still in the AP reports, but didn't the NTSB already rule out ice?

There was no mention of freezing on the recorder (the Chicago Tribune has a transcript at http://www.chicagotribune.com/news/chi-flight3407-transcript-feb13,0,7376989.htmlstory), other than a small buildup from the start of descent, which both the pilot and tower seem to have considered normal, at 25:03.