Earlier this year, Norris Hibbler, the Insurance Director of the American Yankee Association and the head of the insurance brokerage that handles the AYAs group insurance plan, asked me to take a look at nearly five years of insurance claims information. He was hoping I could identify some things AYA could do to reduce losses, and thus lower premiums and improve its chances of continued insurability.
Insurance company records reveal a lot of surprises, including the fact that a lot of aircraft damage incidents that looked like reportable accidents never make the NTSB files. This suggests that insurance records are really a more complete cross-section, as folks who are loathe to let the government know about something that could result in an FAA enforcement action will still report it to their insurance company in order to get money to fix the thing. But what was most striking is that out of the 105 claims, the top reason for the claim was aircraft damage due to pilot error on landing.
Keep in mind were not talking about student pilots in training here. These are all members of an aircraft owners association – folks youd think were the most fastidious about their airplanes and how to fly them.
These are all people who spend the money to own their own planes. These are all people who are flying planes they should know well. And yet, the number one reason their airplanes are damaged is that they failed to bring them safely to earth.
We are also not talking about landings in general. This is strictly pilot error – no engine malfunctions, no mechanical failures, no 50-knot gales. And all of the airplanes involved were tricycle gear airplanes, which were developed to make landings easier than in tailwheelers (maybe too easy, in fact).
Every one of these landings involved a perfectly good airplane landing at an airport on which it should have been able to land safely and in conditions that were well within the airplanes capabilities. Why then did the insurance company pay out nearly half a million dollars – a quarter of all claims money paid – to take care of accidents that have no good reason for occurring?
Anyone who reads the accident reports and summaries realizes that pilots bend a lot of airplanes on landings for no discernable reason other than failure to control the airplane properly. As Sam reminded Rick and Ilse, The fundamental things apply, as time goes by.
Yet, as important as those fundamentals may be, pilots seem to be missing the ones that apply to landing an airplane on a runway. While there is some theory involved, preventing these kinds of accidents is primarily a stick and rudder issue. These accidents do not generally occur due to bad judgment, such as picking an unsuitable field or landing in conditions beyond the aircrafts capabilities.
Runway overruns are an interesting class of runway accidents because they simply involve poor energy management. The rest of the aircrafts control tends to be within acceptable limits.
When you really get down to it, theres only one reason why an airplane goes off the end of the runway – failure to dissipate all its energy before it uses up the pavement.
There are several ways this can come about. First, the airplane may have so much energy when it crosses the approach threshold that theres no way to dissipate it all within the distance available. Second, the pilot may fail to use all the means at his disposal to deplete all that energy. Third, the pilot may add to the energy level by adding power to go around when there isnt sufficient distance remaining to get airborne and climb out safely over whatever obstructions may reside at the end of the runway.
Recall from high school physics – you did take that, right? – that there are two types of energy the airplane has which must be dissipated in order to come to a stop on the ground. The first is potential energy, the energy due to the aircrafts weight and height above the ground.
Remember that weight and mass are different concepts. Mass is the quantity of matter in an object and weight is the force the object exerts on a solid surface due to the pull of gravity. Thats why an object has the same mass no matter where it is, but its weight changes when the gravitational constant is different. Thus, an object that weighs 100 pounds on Earth weighs about 17 pounds on the moon even though it still has the same mass.
Weight is measured in pounds and mass in slugs, where one pound is the weight of one slug of mass at earth-standard gravitational acceleration of 32 feet/second/second. The formula is simple – height times weight equals energy.
The second type is kinetic energy, or the energy due to the aircrafts motion. Kinetic energy is a function of mass and velocity, where the energy equals half of the mass times the square of the velocity.
Thus, a 2,000-pound airplane that is 1,000 feet agl has 2,000,000 ft-lb of potential energy. That airplane also has a mass of about 62 slugs (2,000 pounds divided by the gravitational constant). If its moving at 75 knots, or 127 ft/sec, then that plane has a kinetic energy of about 500,000 ft-lb. Thus, this airplane has a total of 2,500,000 ft-lb of energy to get rid of before it comes to a stop on the runway.
Being high or fast at the approach end increases the amount of energy you need to dissipate. The airplanes book landing performance is based on starting at 50 feet right on some specified approach speed (usually the short-field, not normal approach speed, unless you have separate short-field charts).
Lets take a hypothetical 2,000-pound airplane with landing distances of 1,200 feet over a 50-foot obstacle and a ground roll of 400 feet, and an approach speed of 60 knots. Thus, theres a window at 50 feet above the ground 800 feet from the touchdown point through which you must fly at 60 knots in order to land in these distances.
At that point, you have 100,000 ft-lb of potential energy, and 319,000 ft-lb of kinetic energy, or a total of 419,000 ft-lb of energy to get rid of before you stop.
Now, imagine youre 10 percent higher than you should be. Instead of flying through the window at 50 feet, you pass it at 55 feet. This increases the potential energy to 110,000 ft-lb, and total energy to 429,000 ft-lb – an increase of a bit over 2 percent. Now, lets imagine youre on altitude but 10 percent – six knots – fast. Your kinetic energy increases to 386,000 ft-lb, and total energy is up to 486,000 ft-lb, or 16 percent more energy.
If you run the numbers, youll find that being 5 knots fast is the same as being 30 feet too high crossing the treeline. In terms of glide path angle, thats about the same as being 120 feet high on the glide slope at decision height – more than enough to put the glide slope needle off the bottom of the CDI.
The message here is that being a little fast is the same as being way high. Pilots who add extra speed on final for mom and the kids, or for a non-gusty crosswind, or for any other reason, are dramatically increasing their landing distances.
As a rule of thumb, you can figure that adding 5 knots to your proper approach speed increases the landing distance in this hypothetical airplane by more than a third. The addition of 10 knots kicks it up by nearly two thirds.
Of course, those who operate off long runways might figure that extra distance is irrelevant, since they have plenty of space to roll out without smoking the tires. However, there are other direct effects of too much speed on final.
First, and perhaps most importantly, the extra speed significantly changes the pitch attitude as you enter the flare. Since airspeed and angle of attack are inextricably intertwined, being fast means being at a more nose-low attitude as you cross the threshold.
You will have to make a larger pitch attitude change from the approach to the proper touchdown attitude. In this respect, you should note that, regardless of weight, the aircraft should touch down in just about the same attitude every time as long as the configuration is the same.
Because pitch attitude is the sum of flight path angle and angle of attack – and the aircraft should be in as close to a zero descent angle as possible at touchdown to make the landing as smooth as possible – the pitch attitude should be the same every time as long as the flaps are at the same setting.
It becomes more difficult to judge the flare if the airplane is more nose-down than normal, since youll have a larger change to make. Also, when youre fast, you have greater dynamic pressure of the air over the elevator. The result of that is increased pitch sensitivity – the same amount of control pressure will produce a significantly (if not substantially) quicker pitch change and make it harder to control the rotation without overflaring into a balloon.
It is the balloon that all too often sends the pilot into an oscillating series of overcontrolled pitch excursions as the airplane floats down the runway. While this can be controlled and recovered from, the price of that recovery is increased distance down the runway, which can lead to running off the far end.
Alternatively, if the airplane is too fast and the pilot is too gentle with the stick, the aircraft may not rotate enough, resulting in a nosewheel-first touchdown. Since the center of gravity is behind the nosewheel, the airplane will now be rotated rapidly around the nosewheel by the pull of gravity, producing a very rapid increase in angle of attack and lift.
The result is that the airplane leaps off the runway at a rapidly decreasing speed. While the correct solution to this is to add power, hold the nose up, and stabilize the plane above the runway, this will add to the landing distance.
Unfortunately, many times the pilot does not choose the correct solution, but rather pushes forward to stop the pitch up. This rapid decrease in angle of attack, and a consequent elimination of lift, results in the airplane simultaneously pitching nose low and sinking – rapidly. The airplane again strikes the runway nosewheel first.
There are two possibilities at this point. The lucky pilot will again bounce the nosewheel off the runway and repeat the whole process, eating up more runway before the airplane can be stopped. The unlucky pilot will hit the runway hard enough to collapse the nosewheel or strike the prop.
Keeping Speed Down
Avoiding a too-fast approach means understanding how to determine the proper approach speed for conditions. In addition, you need to maintain that speed while keeping your attention focused outside on the runway, glide path and traffic.
A lot of study by the FAA and others has determined that there are two general rules for determining the best approach speed for light planes.
The basic approach speed should be 1.3 times the stall speed in the landing configuration. Remember that this is calibrated airspeed, so the baseline stall speed must be found in the POH/AFM, not the marks on the airspeed indicator. This provides the best balance between adequate stall margin and control authority. It minimizes landing distance and the difficulty in judging the flare.
While most pilots know what the book stall and approach speeds are, a lot of them forget that both stall speed and approach speed change significantly with weight. The pilot should compensate by reducing approach speed when the airplane is under maximum weight.
Your target approach speed should be the book speed at max gross weight times the square root of the ratio of actual weight to max weight.
The significance of the necessary speed reduction is easy to see. Take a typical light trainer that weighs 1,500 lbs empty and has a 2,400-lb max gross weight. If this trainer is loaded with half fuel and two people (say, a student and instructor returning from a training flight), the aircraft will be nearly 400 lbs under max gross.
If standard approach speed is 60 knots, the corrected approach speed is only 55 knots. The pilot who flies the standard 60 knots will be carrying the same excess energy noted earlier as the equivalent of being off-scale high at the middle marker on an ILS.
Its easy enough to sit down with a calculator and come up with a little reminder card of the proper speeds for a reasonable range of weights, but for those who need one in a hurry, reducing approach speed 1 knot for each 100 lbs under max gross works reasonably well for most light singles.
Also, some aircraft manufacturers pad approach speeds a bit, apparently believing that this will prevent fatal stall/spin accidents on final. While they may be correct on that point, a study of landing accidents suggests that the reduction in risk is offset by the increase in negative outcomes due to overruns and gear collapses.
A typical training aircraft with a 46-knot stall speed in landing configuration might have a manufacturer-recommended approach speed of 65 knots at max gross weight. The FAA-recommended 1.3 times stall speed is 60 knots, which means you are 5 knots faster than optimum on final at max weight, and possibly 10 knots or more too fast if landing well under gross.
The practical application of all this is actually easier than the theory and calculations might suggest because the pilot can accomplish it without computations or closely monitoring the airspeed indicator.
Since the angle of attack will be the same at 1.3 times stall speed regardless of weight, the pilot need only learn the correct aircraft attitude on final, and then maintain it on each approach.
In fact, other than as a crosscheck or confidence builder, the pilot need not even look at the airspeed indicator. Instead, by focusing on the relationship of the nose to the horizon (using either outside references in visual conditions or the attitude indicator when the outside horizon is not usable), the pilot can easily maintain the proper speed without having to shift attention and possibly eye focus to and from the airspeed indicator.
For example, in the Cessna 172, I teach students to maintain a pitch attitude very slightly below level with full flaps. This can be seen both forward by gauging the amount of ground between the top of the nose and the horizon and out the side by paralleling the bottom of the wing with the horizon.
Another reason pilots cite for carrying extra speed is a crosswind. This does not appear to have any rational justification, since approaching at 1.3 times stall speed provides plenty of control authority to handle any crosswind up to a gale in which no one wants to be flying.
Extra speed for wind conditions is useful in gusty winds, regardless of whether the wind is 90 degrees across or straight down the runway. The FAA recommends padding the airspeed by half of the gust factor.
Thus, if tower calls the wind 320 at 15 gusting 25, add half of the 10-knot gust factor to the approach speed and be prepared to be a lot more careful to fly the plane onto the runway rather than flaring to land.
Another practice that can lead to runway overruns is the failure to use full flaps on all normal landings. There are plenty of arguments for partial flaps, ranging from Flaps make the plane float more to I need the extra speed to control gusts or crosswinds.
These arguments are generally specious. Flaps only make the plane float more if the landing speed is too high. If you land at the same speed flaps up and flaps down, then you will be landing at a higher percentage of stall speed with flaps down, and it will take longer to dissipate the extra speed.
On the other hand, if you approach at the proper 1.3 times stall speed, youll have less speed to dissipate from flare to touchdown, and with the added drag of the flaps, it will happen in less time and a lot less distance.
The extra speed to handle gusts is not enough to require one to reduce flap setting unless the gusts are truly phenomenal and the modified approach speed is above Vfe. In addition, reducing flaps while adding speed defeats the purpose of adding the speed (additional margin above the stall).
Another way an airplane can end up off the far end of the runway is a late go-around. The reason for the go-around in this situation is usually a high or fast approach, coupled with the pilots failure or refusal to recognize the approach was not going to result in a landing with enough runway remaining to stop.
Rather than ride it out and eat the damage resulting from the overrun, the pilot attempts a go-around too late. This results either in high-speed impact of obstacles at the departure end due to insufficient altitude, or a stall/mush impact when the pilot tries to horse it into the air or over the obstacle.
The preventive measure for late go-arounds is to know just how much runway you need to stop and add an appropriate safety margin. When it looks like you dont have that much runway in front of you at touchdown, youll need to bolt pronto.
Anyone who has flown the same airplane for a while has a pretty good idea of how much runway it requires under usual circumstances, but when you go to an airport of significantly higher elevation than normal youll need to go to the books to adjust your numbers for the increased density altitude.
If you are going long, its important to avoid the temptation to shove the nose over and dive for the runway, since that will result in a lot of extra speed – the #1 enemy of safe landings. Just take your go-around like an aviator and try it again.
One final point is that runway overruns often happen at airports where the ability to take off again would be marginal or nonexistent.
Pilots should remember that most light airplanes require about twice the takeoff distance as landing distance (both runway length and 50-foot-obstacle distance). If the runway is so short that you have to use extraordinary technique to get in, you probably cannot get out again safely other than on the back of a truck.
Also With This Article
Click here to view “Energy Management on Final.”
Click here to view “Tricks and Traps.”
Click here to view “Coping With a Slam-Dunk.”
-by Ron Levy
Ron Levy is an ATP, CFI and director of the Aviation Sciences Program at the University of Maryland Eastern Shore.