FOG HORN

This Friday’s shoot-the-shit topic is prompted by recent media coverage of the Air Asia tragedy. If you’ve been watching the news, you’ve gotten 24 x 7 information on the Air Asia crash. Part of that coverage was a discussion of concept referred to as an aerodynamic stall. So, I’m going to touch on it here, hopefully with greater clarity.

Any fixed wing and rotary aircraft relies on lift to achieve flight. To achieve lift, we design surfaces that look like this…

If you look at the cross section of a wing on an airplane, or the rotor blades on a helicopter, this is what it looks like.

In addition to shape, we need to have airflow over the lift surface. In a helicopter, a motor turns the rotor, and as the rotor turns it generates lift. In an aircraft, we use thrust to push the wing through the air.  Because of the shape of the lift surface, air flow at the bottom of the wing surface exerts greater pressure against the wing pushing the airplane up.

An aerodynamic stall is created when the airflow over the wing surface is disrupted to the point where lift is no longer generated.

You’ve also heard about something called the angle of attack, which is a technical term to describe the relationship of the centerline of the wing to the level horizon. when the angle of attack increases lift increases, when the angle of attack decreases lift decreases. This is an envelop with min max parameters influence by several factors.

One last concept that I want to discuss is speed. There are three types of speed that we can discuss. The first is indicated airspeed. This is the speed indication that’s most relevant to flight. It is the speed detected by the pitot tubes. I’ve included a figure of a typical pitot system below.

The second is true airspeed which is a computed value that takes into consideration density altitude.

The last speed value is speed over the ground. This is the same as the speed indicated by your car’s speedometer. It is relevant, principally, for navigational purposes. For example, a few years ago, I flew from Virginia to Elizabeth City, NC., I had an indicated airspeed of 140 knots but I was able to catch a 40 knot tail wind. So, my speed over the ground was about 180 knots. Had the tail wind been a headwind, my speed over the ground would have been 100 knots but my indicated airspeed remains 140 knots.

Airplanes operate on indicated airspeed. There is a slew of airspeed parameters applicable to flying; they are dependent on the type and model of aircraft, and its aerodynamic configuration – meaning weight, flaps down, amount of flap used, landing gear extended, etc.

When a pilot climbs he or she applies full power and increases the wings’ angle of attack to achieve a specific indicated airspeed. Even with full power applied, as the angle of attack is increased, indicated airspeed decreases. If the angle of attack continues to increase, indicated airspeed continues to fall, and when the indicated airspeed drops below a certain value, as specified by the manufacturer, the wing loses lift and the airplane falls. This is an aerodynamic stall.

Forgetting for a moment mechanical and electronic stall detection systems, a pilot knows he’s  in a stall because the airplane shakes and starts buffeting; control surfaces don’t respond. To recover from a stall, the pilot needs to push the nose of the aircraft down. As indicated airspeed increases you’ll feel the airplane start to level out and control surfaces begin to work again.

Aerodynamic stalls are critical at low altitudes; for example,  during landing or takeoff simply because you don’t have time to recover. At high altitudes, a pilot has enough time to recover.

In the case of Air Asia, one possible scenario is this…

Pilot is straight and level, at flight level 320, with an indicated air speed of 450+ knots. To avoid thunderstorms he initiates a left hand turn. So, he lowers the left wing reducing its angle of attack and increasing the angle of attack on the right wing. He hits a strong up draft pushing the right wing up even further so the right wing stalls completely. The pilot tries to correct and in the process completes a full aerodynamic stall. If the aircraft drops flat, like a pancake, with little forward motion, all of the control surfaces would fail to respond. So you’re probably looking at a combination of weather and pilot error as to the cause of the crash. It’s speculation on my part but it is one possible cause. What do you think?

Be sure to let’s us know and as always… No rules apply

Have A Great Weekend Everybody!