Journal Alioth's Journal: Buffalo plane crash 2

As soon as I heard about it on the radio yesterday, after the words "was flying through heavy rain", I just knew that it would turn out to be icing related. My first thought was Roselawn.

Rain in NY this time of year, at altitude, probably will only have one state: freezing rain. This is where the water droplets are supercooled - liquid water below freezing point. As soon as they hit something, they freeze into solid clear ice. You may have seen it - what we call an ice storm at ground level is caused by freezing rain.

Now imagine that two inch thick shell of clear ice on a flying aircraft.

It's heavy. But it's worse than that, it can build up into formations that change the aerodynamics of the aircraft. In the Roselawn case in the 1990s, the aircraft was on autopilot in a holding pattern. Large droplet icing at the time was poorly understood - the crew didn't realise the danger. They had the anti-icing systems on, and after all the ATR that they were flying was certified for flight into known icing conditions. However, the ice was building up in places where their de-icing systems didn't reach - behind the de-icing boots on the leading edges of the wings. Most ice builds up on the leading edges, and de-icing boots can be inflated to break the ice off. The ice was forming a ridge behind the de-icing boots, and it was causing changes to the aircraft's aerodynamics. Since the aircraft was on autopilot, the crew didn't feel the changes in the flight controls - that is, until the autopilot disengaged itself and the plane rolled inverted. They ended up performing a "Split-S" - a high-G aerobatic manoevre, in the clouds, covered in ice. The plane broke up in flight from the aerodynamic loads.

From what I have heard since the initial report of the NY crash, icing again once was the final link in the chain. However, this time, from the information I've seen, they lost control when they extended the flaps for landing. The problem is with an iced up aircraft it can change the aerodynamics such that the tail can stall - and once this happens you lose control of pitch, and the aircraft nosedives towards the ground. I read a few years ago that if you suspect icing on the tail surfaces, you should not extend the flaps. Unfortunately, this seems to be just what happened. The news report I read suggested that the crew tried to retract the flaps but sadly it was too late.

Ice and airplanes don't mix. Ice and helicopters is worse.