My take on AirAsia 8501

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http://www.nbcnews.com/storyline/airasia-plane-crash/airasia-flight-8501s-tail-section-found-divers-java-sea-n281171

Unfortunately AirAsia 8501 is terrible airliner accident. My heart goes out to the families. As a pilot all crashes hit close to home.

I’m not claiming to be up on all of the evidence. As an ex Air Force pilot, aeronautical engineer with 4,500 hours, ATP rating and 727 Type rating I do have some experience. There are some possible scenarios that may have occurred and I’m explaining some of them.

Icing can effect an airplane typically in three ways. The pitot static system, the aerodynamics and engine performance.

THE PITOT STATIC SYSTEM

The pitot static system is how the airplane measures its speed. It is a pretty simple and reliable system. It measure two pressures. Dynamic pressure which is what you’d feel when you put your hand outside a car window. The second is the static pressure which is a little harder to imagine. Try this, take a piece of paper and hold it so it bends like a ‘c’. Now blow over it. The paper will rise as you blow. That is the static pressure and it is negative (Relatively speaking). In the atmosphere these two pressures add up to the total pressure. When you go faster the dynamic pressure increases and the static pressure decreases (negative number). Adding these two together yield the total pressure (positive dynamic pressure reduced by the static pressure). As the speed increases the total pressure increases.

Ice will usually cover the static pressure sensor and because of the math that causes the total pressure to increase. The pilot and auto-pilot see an increase in speed and either of them may pull the nose up and/or reduce throttles trying to correct. As it progresses the angle of attack (angle of the flight deck vs. direction of travel) will increase and the actual speed will continue to decrease until the airplane stalls. The entire time the pilot or auto pilot is seeing increasing airspeed.

AERODYNAMICS

The aerodynamics are affected as ice forms over the wings, tail or other parts of the fuselage, the plane gains weight and it doesn’t fly as well. At some point either the weight causes it to stall, or the lift created by the wing is reduced and the plane stalls. A stall occurrs when the plane isn’t creating enough lift (up) to counteract the weight (down) and the airplane loses attitude.

Small airplanes with straight wings stall differently from airplanes with swept wings like the airliners. The straight wing will tend to be sharper and the nose will drop more. A swept wing will tend to mush and the nose won’t drop very much but the airplane will start to lose altitude and stay in that position. Straight wings will tend to roll to one side when the stall occurs and a swept wing will tend to stay level.

If ice is added to the stall a swept wing might act more like a straight wing and roll to one side when it occurs. Depending on the amount of ice it can be violent and the airplane can roll inverted. As it rolls inverted the nose can drop to near vertical. In this situation where an airliner is near vertical and stalling it will lose a lot of altitude quickly. From 32-38,000 feet it will still take a couple of minutes for it to hit the ground but during the period of time the cockpit would be a very hectic place and it would be easy for they crew not to make a radio call. Some news broadcaster have discounted this because they say that a stall results in a oscillation where the nose drops, the air speed increase and the nose tends to come back up. That is true except when there is a asymmetrical stall.

ENGINES

The engines are generally pretty insensitive to ice unless the amount of ice is excessive. There are normal (emergency) procedures to start them again. During the restart the airplane is under control and it would be easy for the crew to make radio calls. In an engine out scenario they would have a long time to work the issue and make the calls.

 

CONCLUSION

High in a thunder storm there can be a ton of ice and the airplane could ice up very quickly. Add a large amount of turbulence and any ice build-up could be catastrophic. Pitot static icing, and aerodynamic icing would quickly make the situation catastrophic. During any asymmetric rolling and/or pitching the airplane configuration might have been changed either by the pilots or one of the auto systems. If the spoilers/flaps deployed the airplane could easily got into more of an asymmetric configuration (because of ice) which would make the situation worse. More asymmetry could cause some structural failures and it might have stayed in a fairly tight spiral into the ocean.

This type of uncontrolled decent would explain limited structural damage, small debris field and a tail separation. Recognizing pitot-static icing is tricky and if the proper anti-icing weren’t turned on or failed the icing can grow very quickly.

 

Once again, this was terrible and I’m sure they’ll figure out the cause. I’m merely putting forth a possible scenario so I can explain some of the possible contributing factors.

 

Ray Jay Perreault

https://rayjayperreault.wordpress.com

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