A Story...

The news has been covering the Air France flight 447.
Like all modern aircraft crashes the crash was the result of many causal events chained together.

Its clear that the root cause is a failure of both pitot static systems. Probably caused by super cooled freezing rain. One could reasonable argue that the root cause was the failure to replace these sensors, as that particular model had a history of freezing rain problems.
I believe that the static ports clogged and the pitot ports remained open.


We pick up our story shortly before this happened.

The experienced captain has retired to the crew rest area leaving two more junior crew to watch the airplane. Its four hours into a very long flight, its dark and cloudy flying between layers one cant really see the sky or the surface of the ocean. The cruise flight is usually uneventful and almost fully automated. The airbus flies as high as it can given the fuel load, it seeks thin low drag air for maximum efficiency. The margin between the cruise speed and stall speed where the aircraft stops flying is thin. They fly as high as they can given their weight. Maybe as little margin as 15%. In this warm cocoon with dim cockpit lights one struggles to stay alert.
Its hard to stay alert your human clock has no idea what time its it's dark, quiet peaceful.
Little has prepared you to be sharp and focused, you are about to loose the fight for your life.

The plane flies though a mist of super cooled water. This immediately makes a smooth solid sheet over the airframe. The various heaters and bleed air system make short work or removing this ice from most of the important surfaces.

The Pitot static system has two ports. One is used to measure the ambient "static" pressure. The other port is used to measure the pitot or impact pressure.
The absolute pressure on the static port is used to measure altitude.
the difference in pressure between the pitot and static pressures measures the indicated airspeed. IE dynamic air pressure. At these altitudes the airspeed might read 150mph even though the airplane is going 600mph. The air is thin here so the impact/airspeed pressure is
not adjusted for density as its real use is to tell what aerodynamic conditions the airplane sees, not the real speed over ground.

The static port heaters aren't quite up to the job. They seal shut under this icy glaze.
Since the system is basically flying in equilibrium there is no sudden change. After some time,
maybe quickly, maybe many tens of minutes the computer system on the aircraft determines that it can't tell what speed or altitude its flying at. The autopilot is officially confused. So it does what its supposed to do when confused, it shuts itself off and tells the pilots I have no clue you figure it out. The junior pilot is awakened from his day dreams with an alarm the basically says you've got it. So he is now been jarred into action to fight some flaw or failure of unknown origin and type. He can't really see outside he is now hand flying the airplane focused on the instruments. Maybe he sees the airspeed is a little bit high, so the natural reaction is to pull the nose up and slow down. When he pulls up the airspeed slows down as it should, as the plane climbs the pitot pressure decreases and the indicated airspeed slows down. So his first reaction seems to be correct, nose comes up speed slows down. Its kind of strange the altitude did not change. No he's confused. But his brain says nose up slow down it worked. Airspeed is ok.
Back to messing with the computers to see what error code caused this problem.
Yet maybe he keeps applying a little back pressure the plane climbs and slows down to the point it stalls. Now he has multiple different alarms, stall warnings, computer alarms and confused autopilot he must choose what problem to address. Scared and confused he goes back to basics, fly the dammed plane. At this time the aircraft is stalled and descending rapidly in the dark clouds. The outside air pressure is increasing so the pitot pressures is increasing showing a rising airspeed. Yet in the real world the airplane is slow REALLLY slow, falling from the sky.
Yet the pilot saw the airspeed respond to his first inputs when he took control, that must be working. soon the airspeed is really climbing, maybe even approaching redline as the pressure increases. The Altimeter shows lots of altitude, the view out the window is dark confusing and useless. The more the pilot tries to pull up to slow down the airplane the faster the airspeed shows.... he now fears that he's going to rip the wings off..... His brain screams your screwing up something is wrong, yet noting seems to behave correctly, trying to just do the basics and its not right. Its not clear that the pilot ever figured out what was wrong before the plane smacked the water killing everyone on board.



When I was a 100 to 150 hour pilot in Alaska I almost had the identical crash.
My college girlfriend had come up to visit at the end of the summer and I wanted to take her flying to show her where I grew up. The airplane I was used to flying, a super cub on floats, was down for maintenance. So we were flying my Dads Turbo 206. It was completly tricked out with all the slow speed stol slats, big engine amphibian floats (not regular floats like the cub)
I Probably had 6 to 10 hours in the 206. It was a bit much for me. We were going from Ketchikan to Craig Alaska, typical Alaskan day raining dreary 1500 ft ove cast. Getting through the pass across prince of wales island was tight but not unusually so. Getting near Craig there was patchy fog around and the water was glassy. Glassy water landings are a challenge in a sea plane because you can't judge height over a mirror surface. So to land you set up an attitude and rate of descent and just hold it till you hit the water. I set up fro a straight in landing heading directly at Craig, I'm all configured, wheels up, flaps down, cowl flaps, etc... I check the gear twice. All the check list things are done, but the plane is not flying right. The airspeed is way high and I just can't get it to slow down. I'm trying to hit the target glassy water speed to set the right approach angle. Its not working the nose is way up, the airspeed is climbing. With all the STOL gear on this plane the stall is mushy not sharp so I don't notice a buffet or stall break.
I'm fully stalled and descending to the water at a high rate of speed. My brain is screaming something is wrong fix it. I can't figure it out something is obviously wrong. If it had been the super cub I had a lot of experience in I would have realized what was wrong by the feel of the airplane. I did not have enough time in the 206 to realize this. I eventually realized that I was in trouble and decided to abort the landing, full power (this was the souped up plane so full power would climb at a 45 degree angle) Stop thinking about landing start thinking about going around.
I started looking outside exclusively as I did not want to fly into the town of Craig, I needed to go around. When I stopped focusing on the airspeed and started flying the airplane I lowered the nose. At that moment we hit the water. I'd just powered my way out of a stall into minimum controllable airspeed by pure power. We hit firmly but not hard enough to break anything.
(remember this is glassy water so there is no height clue) I immediately pulled the throttle back and we were bobbing on the water, not moving at all. I looked down and the airspeed said 130 knots. I reached down and flipped the alternate static port valve and altimeter and airspeed suddenly read the correct values.


Here I was flying by visual flight rules in a simple non automated airplane in the day time while being on high alert due to the newness of flying this plane and a frozen static port almost killed me. I can close my eyes and imagine the terror as the pilot struggled to figure out what was wrong, his brain screaming your screwing up something is wrong fix it, and being unable to figure it out. I've been there it was not fun. Its been 29 years since that flight and retellign the story still makes me feel terrified.



A side note on multiple causes:
With the STOL gear the plane stalls 10 or 15 mph slower than the factory stock version. The bush pilots that flew that plane on a regular basis found the continuous stall warning horn on approach annoying so they put a chunk of tree branch in the stall warning vane so it would not go off. If I'd heard a stall warning I might have figured it out sooner...the cub I usually flew did not have or need such things.