Spanair Md-82 crash

Posted on 05. Sep, 2008 by in Featured

According to the Wallstreet Journal the flight data recorder revealed that the flaps and slats were not extended on the Spanair MD-82 during take off. Modern swept wing aircraft require leading edge slats and trailing edge flaps to produce enough lift on a small wing to allow take off as well as landing. Aviation is about trade offs; to get an aircraft to go high and fast the wing needs to be as small as possible and swept. Wings produce lift and a by product of lift is induced drag, this drag slows an aircraft at altitude and cruise speed where lift requirements are less. Small wings do not allow an aircraft to get slow enough to land or produce enough lift on take off to get into the air.

Flaps and slats actually change the shape of the wing (camber) by curving it. When two molecules of air hit the leading edge of a wing, by laws of physics they have to arrive at the trailing edge together. A wing (in general) has a flat bottom and curved top; thus the molecule traveling over the top of the wing has to travel faster to arrive at the trailing edge with the molecule that zipped accross the flat bottom. That action produces a low pressure zone; lift. More curve equals more lift. Flaps and slats temporarily change the curve of the wing allowing safe speeds for take off and landing; then are raised once airborne to allow fast cruise speeds. There are various positions of flaps/slats for take off and landing; these positions (amount of droop) are determined by weight and runway length. By optimising the flaps/slats it allows very large aircraft to fly into very small airports.

If you rotate (pull the nose up) at a normal take off speed with flaps/slats up the aircraft simply is not generating enough lift to fly. It will hover in ground effect then stall if enough power is not generated to accelerate the aircraft rapidly or the pilot pulls it out of ground effect. (Ground effect is a cusion of air between the airborne aircraft and the actual ground. The cusion is generally the same height as the wing is long.) To generate lift a wing must have smooth flow over the top; if the speed is too low to ensure a smooth flow the air will become turbulent not allowing for a low pressure zone. That break up of air flow is a stall; in short the wing stops flying.

As an MD-82 pilot when I heard the initial reports from witnesses and survivors: after gaining just a few feet (in ground effect) the wings began to rock (classic pre-stall indication) then the aircraft slammed back onto the runway (stall); I assumed it was a wing configuration problem. However an investigation is complex and I hate it when “experts” jump the gun. There were all sorts of other rumors of why. My suspitions grew when I read the initial problem was that a temperature prob was heating on the ground when it should only heat in the air. This is pertinent because; a possible cause for that is a failed switch that causes the aircraft to think it is still in the air. If that was the case then the normal aural warning “FLAPS, SLATS” (when not in correct position) would have been disabled. The MD-82 would have thought it was in flight so the normal position of the flaps/slats would be up.

An accident is always a chain of events. A link of mistakes, fate and failures that over come the safety devices and experience/skill of the crew. With this accident a possible chain appears to have been: a. the failure of a minor instrument. b. a return to the gate. c. the cause was not recognized or all aspects taken into account. d. confusion and rushing/pressure to get back on schedule. e. failure to properly position flaps/slats. f. no aural warning due to initial problem.

When I was a youngster learning to fly as a Naval Aviator a salty old flight instructor (he was probably 30 years old) told me. “Son if you set take off attitude and nothing happens; somethings wrong and that airplane dosn’t want to fly. Don’t force it into the air, let it fly itself off or try to stop it on the runway. Which to do will be up to you and will come with experience.” Sage advice it saved my life once. chip

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