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Feature Article

Matt Debski, Aircraft Owner WVFC tdebber@alum.mit.edu

Landing Energy

At some point in our flying careers, most of us have been told or have thought about energy management in flying. There are constant transformations between potential energy (altitude and chemical being most prominent types) and kinetic energy.  That is, at the same power setting and over short periods of time, if you pull back on the yoke you will gain altitude at the expense of airspeed.  Or, you can maintain the same airspeed and increase altitude by increasing the throttle, converting the stored energy in the fuel to altitude.  (These ideas and many more are expounded upon in the web site "See How It Flies." http://www.av8n.com/how/)

One of the places where this transformation is most crucial is during landing.  The Pilots Operating Handbook or Approved Flight Manual for the model of an airplane gives a speed or range of speeds at which to fly the final approach.  In the absence of this, the FAA recommends 1.3 * Vso (the power-off stall speed in landing configuration).  Where does this number come from?  Since we want the airplane to stop flying just before we touchdown, why don't we come in at a slower speed, so that there's less airspeed to "bleed off" and we don't use up as much of the runway?  The reason for this is the energy conversion taking place during the roundout and flare.

On final approach, the airplane is descending at a few hundred feet per minute.  Given the weight of the airplane, the airplane has a lot of kinetic energy, moving toward the ground.  As the airplane nears the ground, the movement toward the ground needs to be stopped.  The energy that will be used to stop the airplane from descending is its forward kinetic energy.  As the angle of attack is increased in the roundout and flare without adding power, the airplane gives up some of its airspeed to induced drag in producing the lift to stop the airplane from descending.  If all goes well, the airplane rounds out a foot or two above the runway with a vertical speed of 0 feet per minute.  Airspeed continues to be lost and the airplane eventually stops flying.

So, what happens if the airspeed is too low?  If airspeed is too low when the roundout begins, the airplane does not have enough total energy to arrest the descent.  As the roundout continues, the plane ends up in a stall (or very near a stall) before the vertical speed is zero.  Without steps to mitigate this, the result is a very hard landing or a bounce.  At this point, the best option is to go around, lest the plane begin to porpoise.

If the lower airspeed is noticed on final, lowering the nose will restore the airspeed to the desired value.  If the plane was flying the correct glideslope at the too-slow airspeed, an addition of power will be necessary.  If the pilot had set a correct power but was flying too slowly, the airplane may be above the glideslope and lowering the nose will increase airspeed and restore the correct downward path.  There is no way to increase airspeed and not increase the rate of descent without converting some other source of potential energy; that is, adding power.

As I mentioned last month in my article on learning to fly tailwheels, this energy management comes to the forefront when doing wheel landings.  As the airplane nears the runway, the desire is to keep the same flight attitude yet arrest the descent.  The only way to do this is by adding power.  In this case, instead of converting airspeed to stop the descent, power is added and the airspeed remains the same.  Then, airspeed is bled off during the rollout and the lowering of the tailwheel.

When you're wondering why it's important to be going faster on final and it's not enough just to be flying above stall speed, remember the reason for that extra speed.  It's energy that will be used to stop your descent and make the squeaker of a landing that will impress your passengers.

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