Thoughts on Ice and Aviation…

(Ed. note: I was going to tell you some fascinating tales of my adventures in Kenya over the past few weeks, but that will have to wait…)

Airplane crashes are almost always the result of a series of unrelated factors, decisions and failures that conspire to make a tragedy. Remove – or change the order of – any one link in the chain – and the accident doesn’t happen.

Keep this in mind as you watch the often inaccurate, nonsensical, irrelevant coverage of the crash of Continental 3407.

The NTSB “Go Team” that is heading to Buffalo will be comprised of experts in all facets of aviation – and they are hardwired to make sure they do not put blinders on as they sift through this smoldering hole in the ground.

They are seasoned professionals with the collective goal of releasing a thorough and comprehensive report – with the hope that it will make air travel safer in the future. It often takes months or even years for them declare the “probable cause” of an accident.

But of course we all want to know what happened now. And there are some telling clues and facts that you should keep in mind as you follow this investigation:

1)    This happened suddenly. The flight crew did not issue a “May Day” – or report to controllers that they had any sort of problem (ala Sully’s Hudson River splash-landing).
2)    The Bombardier (nee de Havilland Canada) Dash 8 series aircraft have a sterling safety record.  My query to the NTSB database does not return a single accident report. In the fall of 2007, all Dash 8 Q400’s with more than 10,000 landings were grounded for inspection after the landing gear collapsed on two Q400’s in the same week. But that is about it.
3)    The weather conditions were absolutely perfect for the formation and buildup of ice on the surfaces of the aircraft.
4)    It was dark – making it harder to detect ice buildup.
5)    Turboprop airplanes are more vulnerable to the threat of ice.
6)    The accident happened near the “final approach fix” – the place where the flight crew would reduce power, and slow the airplane down for its descent down the “glide slope” to the threshold of the runway.

Take a look at this icing conditions warning chart issued by the NOAA Aviation Weather Center that was applicable for the time of the crash (10:10 PM EST). At that time, the forecasters showed the right combination of temperature and precipitation to cause icing existed in Buffalo from the surface through 18,000 feet.

Also take a look at these numbers below:

METAR text: KBUF 130354Z 24011KT 3SM -SN BR SCT011 OVC021 01/M01 A2981 RMK AO2 SLP103 P0002 T00061006

This is the weather report  (or “METAR” in aviation parlance) for BUF at the time of the crash. I have pasted in the actual report for you pilots out there – the basic translation is the wind was blowing from the southwest at 11 knots; visibility was 3 miles with snow and mist. There were scattered clouds at 1100 feet, a solid overcast deck at 2100 feet. The temperature was +01C (about 33 degrees Fahrenheit) – and the dew point was -01C.

Snow, mist and a 2-degree separation between the temperature and the dew point around the freezing mark are sure signs that there is ice in the air, if you will. As you fly higher, the temperature drops. Pilots learn about something called the “adiabatic lapse rate”. While it varies with temperature and pressure, when the air is saturated with moisture, it should get 2.7 °F colder for every thousand feet of altitude gained.

The Instrument Landing System approach to Runway 23 at BUF requires the aircraft to maintain 2300 feet of altitude above sea level (1600 feet above the ground) before beginning the decent down the “glide slope”.

Using the lapse rate as a rule of thumb, the temperature at that altitude would be about 4 degrees colder – or 29 degrees Fahrenheit. It is highly likely this airplane was picking up a load of ice

Now icing is a huge problem if you fly a little airplane like mine. Matter of fact, the FAA prohibits my airplane from flying into that blue box – where there are “known icing conditions” because it is not equipped with the necessary de-icing equipment. No sane pilot would thumb his or her nose at this regulation – as ice can bring an airplane down with frightening efficiency: it reduces engine performance, adds weight and changes the shape – reducing the lift – of aerodynamic surfaces.

But this airplane was certified to fly in icing conditions – and had all the necessary equipment. It should not have been a problem. Was the de-icing equipment not working properly? Did the crew use it properly?

The Dash 8 manual says when the plane is operating in icing conditions, engine intake by-pass doors must be open, engine ignition switches must be set at manual, and airframe de-ice must be set to slow or fast. The first two rules are designed to insure the turbine engines maintain required power and the latter is the system that keeps ice from building up on the leading edges of the aerodynamic surfaces.

They are called “boots” – and they consist of a rubber membrane covering the leading edge of the aero surfaces. When in use, the boots inflate repeatedly with air to break off ice at it forms. There is a myth among many pilots that is it wiser to wait for ice to build-up a bit before activating the boots. The fear is that constant use of the system will create a gap between the ice and the boots – a so called “ice bridge” – rendering the boots impotent. The experts now say boots should be on the minute a pilot sees the slightest bit of ice on the wings. But it was dark and the crew was focused on flying the approach. Did they check for ice?

This accident hearkens back to the crash of flight American Eagle flight 4184 on Halloween night 1994. It was an ATR-72 – also a twin-engine turboprop. The flight was en route from Indianapolis to O’Hare when bad weather forced controllers to put the aircraft into holding pattern at 8,000 feet – where it flew through so called supercooled water droplets – liquid precipitation that is actually colder than the freezing point of water. I am not going to give you the full explanation here (it is a little complex) – check out this wiki link if you are curious.

The key point is this: a supercooled drop of water freezes instantly on contact with surfaces such as electrical power lines, trees, and roads – creating what we call an ice storm. Those same supercooled drops can cover an airplane in ice almost instantly.

As the ice built up on American Eagle 4184, it rolled – and then dove into the ground in Roselawn, Indiana – killing the 64 passengers and 4 crewmembers aboard. The full NTSB report can be found here.

The crew of 4184 did have the boots turned on – but the conditions were “outside the envelope” – meaning the equipment was no match for Mother Nature on that night.

One of the lessons of that crash however is something many pilots like me keep in mind. The crew of 4184 was using the autopilot before they lost control. Since their hands were not on the controls, they could not easily detect that the ice–laden plane was requiring a severe correction in order to maintain altitude. Finally, the control surfaces “ran out of authority” – the autopilot disconnected – and the plane rolled into an uncontrolled fatal dive. It was too late for the crew to right the craft.

Whenever I see the slightest bit of ice on my wings, I disconnect the autopilot – so I can “feel” what is happening to the airplane  (while asking the controller for a lower altitude immediately).

You have to wonder if Continental 3407 was flying on autopilot – carrying enough ice on its wings that its normal approach speed was simply too slow for it to stay in the air. So when it slowed down, it simply dropped out of the sky. You have to wonder…