Posted by: harrisonjones | August 2, 2011

The concrete dilemma…

Over-run accidents are rare, but when they occur, it’s usually a combination of factors that lead to the problem.

On one side of the equation, you have a fixed amount of concrete available for landing and on the other side you have all the variables. The weight of the airplane which dictates the approach speed, the type of approach available for the runway, the slope of the runway, any inoperative equipment on the airplane, and of course the weather. How much wind is reported and is it steady or gusting? Is the surface wet? Is the runway grooved for drainage? 

The landing weight of the airplane is a bigger variable than you might think. It would seem simple to just subtract the weight of the fuel burned from the takeoff weight to get an accurate number, but therein lies the mystery. The takeoff weight is determined by the number of passengers, but nobody weighs them. Passenger weight is calculated by assigning an arbitrary number, usually 170 pounds, for each passenger. Who knows how much the airplane really weighs? The CG is also estimated by assigning a moment for each first class, business class, and tourist class passenger.  With such fuzzy math, you might understand a captain adding a couple of knots to the approach speed. 

Approaches are usually flown at 1.3 VS plus five knots (30% above the stall speed) and reduced over the threshold to touchdown speed. However this number is adjusted for wind conditions. The formula is to add half the steady wind and all the gust, up to 20 knots maximum. The required landing distance is actually computed in pounds rather than feet and expressed as the maximum allowable landing weight for a given runway. A 15% penalty must be taken for a wet runway. Penalties must also be taken for inoperative equipment, such as the anti-skid system for the brakes, and the ground spoilers that kill lift after touchdown. The reverse thrust of the engines is never considered in computing landing distance.  

Once all is said and done, the FAA requires that you must be able to stop within 60% of the available runway length. Considering that we’re talking about short runways, that’s not as big a cushion as it might sound. Sixty per cent of nothing is nothing.  

My purpose here is not to analyze a particular accident, but to provide insight into all the variables involved in making a decision. Everyone wants to complete the assigned mission, but sometimes the goal is viewed through partly to mostly cloudy skies.

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