Accuracy landings: putting the airplane down where you want it is a matter of picking the right spot, controlling airspeed and monitoring your progress.
My aviation upbringing valued many factors, but my mentors were a little like drill sergeants over accurate landings. That meant nosewheel (or tailwheel) on the centerline, inside the touchdown zone, ideally within an airplane length of the threshold paint. They preached the benefits gained when you can pick and then hit a designated spot on the runway: Precision-landing skills become handy when you need to squeeze into a field--think engine failure, fuel exhaustion or fire. As with many other operations, accurately landing an airplane can be broken down into a number of elements.
PICKING A SPOT
Picking, identifying and committing to a touchdown point, then flying the airplane to achieve that outcome is the essence of accurate landings. But we're not talking about merely tooling around the pattern and, after turning final, identifying the intended touchdown spot of that moment: That stationary spot on the ground simply growing larger in the windshield as you slide down toward the runway is a wildcard pick. It doesn't count.
Instead, identifying the intended spot from the downwind--or earlier--and hitting it with precision and repeatability is the name of the game here. All the while you need to remember: Precision landings are an all-axes, all-systems process. You'll need to vary pitch and power to adjust glidepath and maintain approach speed, use the ailerons and rudder to compensate for crosswinds and keep you on the centerline, then using gear and flaps to augment your descent control.
Aiming for that spot and remembering you'll overfly what moves toward you and down in the windshield, and you'll be short of what moves up and away from you in the windshield is part of the game. You want to keep your target in between, where it doesn't move up or down--and only grows larger.
Pilots with a commercial certificate tucked safely in their wallets may have their own opinions about spot landings, or accuracy landings, or whatever you want to call them. Non-standard and unplanned arrivals take up a significant portion of the commercial practical test standards. The polished spot-landing practitioner should have little trouble clearing those items on the checkride.
For pilots with soaring experience in sailplanes or hang gliders alike, it's an old familiar procedure. For so-called power pilots, it's something they must learn and demonstrate. The procedure embraces some of the techniques already discussed, but arranged and executed a bit differently. In this theoretical forced landing, the goal is to clear a ditch across the landing surface--but clear it by as little as possible--and stop as quickly as practical.
Some CFIs--including my private and instrument instructors--considered this exercise an opportunity to also demonstrate emergency engine-out landings. It goes this way:
* After the power loss, slow the airplane to its best glide speed to preserve altitude and options;
* While descending, search for and identify a suitable landing area and steer toward an entry into a downwind leg for it;
* While passing abeam the target spot, begin a 180-degree turn toward the spot, deploying whatever flaps and other drag devices (like landing gear) are needed;
* Slip or skid as necessary to keep the airplane on target for the spot and the centerline;
* Touch down without stalling--hopefully on the target.
In practice, as in a real-world emergency, we use ground indications--ripples on water, smoke from chimneys, etc.--to determine wind direction and velocity, and choose the field accordingly, landing upwind.
In the checkride world of most examiners, clearing the imaginary ditch and touching down within 200 feet would be passing, though I know one who'll push for within 100 feet. "They're commercial candidates, for Pete's sake--they should be good at this," he told me.
The same technique, of course, underpins any accuracy landing, but without the crisis emphasis.
PLANNING A SPOT LANDING
There's no bad time, really, to practice precision-landing skills--weather and traffic, permitting, of course. Whether you've just finished a cross country, or you're out for a $100 breakfast/burger, or simply converting fuel to motion with some bumps and circuits, every landing can be an opportunity to spot land. Let's take the technique outlined above and put it to use under normal circumstances.
You're at pattern altitude and in the downwind:
* Pick your target spot on the runway before passing abeam the arrival end--the threshold marks or any identifiable spot beyond the runway end;
* Traffic permitting, begin a descending 180-degree turn to final, keeping your eyes moving between watching for traffic, the panel and the target, without losing a clear view of that spot;
* Adjust pitch and power to maintain just below normal approach speed--my starting point when learning a new plane is usually 1.2 x [V.sub.SO]--while keeping the plane above stall speed and the target just above the stationary spot in the windshield; making the aiming point about 150 to 200 feet before the target point should get you a touchdown very close to your target in the average light airplane--adjust as necessary when flying heavier birds, or those with high wing loading.
* Deploy flaps and/or slip as needed (but only if and as approved for the airplane) to maintain that target's relative position;
* Reduce power and pitch up just before you'd arrive--you'll likely float a few feet and come down either on, or slightly past, your target.
* Mission accomplished!
First, regardless of your technique, there's that ground-effect "thing" to work out for your particular aircraft. As you remember from primary training, ground effect comes into play when the wing is within its span above the ground, effectively reducing drag, increasing glide ratio and lengthening your final glide to touchdown. Arriving with excessive speed exacerbates the problem of ground effect, which at 1.4 x [V.sub.SO] and faster adds considerably to the runway needed.
High-wing aircraft suffer less from ground effect than their low-wing brethren; similarly, low-wing airplanes with particularly short gear legs--I'm talking in particular to Comanche and Mooney drivers--suffer even more than, say, a Bonanza. But high-wing airplanes with huge Fowler flaps can float way more than we want when those flaps are fully deployed, reducing sink momentarily and adding to any tendency to float in ground effect if too fast.
As one result, many instructors and mentors teach their students to deliberately aim a few feet ahead of the desired spot; not a bad practice, as long as that aim doesn't result in being too low to overcome any unexpected sink or turbulence.
Flaps work in two directions, of course, and it's acceptable to retract as well as extend them in the name of hitting the spot. Ditto for the gear in a retractable: In a pinch, such as a true emergency, I wouldn't hesitate to keep the wheels tucked away until the last minute if it helped assure my arrival at the desired spot. Absent a real emergency, though, it's best to practice putting them out on downwind--of wherever your normal procedures dictate--before starting the 180-degree turn and leave them out, lest your focus on the spot lead you to neglect that all-important switch.
But if gear-out hurts the situation, put them away again--as long as you know that when needed you can get them back down in time. Know thy gear cycle; know it to the second.
And we mustn't neglect controllable props. In any accuracy landing exercise other than an engine-out drill, using power, pitch and rpm are fair game for keeping your target where you want it; raising rpm can act like a brake at low power settings, increasing your descent angle without increasing airspeed.
And always remember: The goal is to land as closely as possible to a target, to dust the chalk or tire-mark the paint, or however you gauge success--a goal, not a court sentence. Outside an actual crisis, continuing to pursue that goal should not continue past the point of breaking something.
That particularly goes for avoiding coming up short. If your set-up makes you fearful of landing short of the pavement, well, there's less penalty in a go-around than in losing a bet. In fact, there's no rule that says the target has to be so close to the runway threshold that failure to be long enough means being short.
At the least, crank up the power anytime you feel an approach isn't coming together safely. If low, let ground effect help save you from a fate worse than missing a spot. If too high, well, decide whether to miss long or to go around altogether.
PUTTING IT ON THE MARK
Remember: The goal of practicing is one part self-satisfaction, one part confidence-building and eight parts salvation. That's especially true if ever you truly need to hit a target without hitting something else. Learn several techniques, but don't mix them up on a single approach. Practice each one to completion and pick a new one as you turn downwind. Practicing and perfecting your accuracy will make you feel good about your flying.
Dave Higdon is a Wichita-based aviation addict who writes about and photographs aviation subjects to fund a flying habit picked up during the Disco Era.
RELATED ARTICLE: The Steep-Approach Approach
Mountain-flying spot-landing techniques help pilots negotiate challenging back-country strips, runways with little or no overrun and trees often encroaching on normal glidepaths to the threshold. You set up a steeper glidepath, keep your eye on the spot, adjust power as needed after rolling out of the turn to final, clear the trees, and get down and stopped with room to spare.
Of course, the same techniques employed at back-country strips like the one depicted above work on the pavement at Regional Muni to minimize runway used and help us land close to the airplane's minimum published landing distance.
One of the keys to combining accuracy landings with obstacle approaches to landing areas where you want to touch down at minimum speed is discarding the standard, relatively flat three-degree glidepath we've gotten used to--thanks to the ILS approach--and steepening things up. Using an approach angle of at least four to five degrees can also improve your view of the runway, particularly when coupled with a descending turn from downwind or with a bit of a slip as part of your final.
To rough in your glideslope angle, these formulas have some popularity: multiply ground speed on final by five to find the descent rate for a three-degree glidepath; for a 4.5-degree approach angle, make the multiplier eight times your groundspeed.
So, a 60-knot groundspeed would need a 300-fpm descent to hold a three-degree glidepath on approach; to make the angle about 4.5 degrees, set up for a 480-fpm descent at that same 60-knot groundspeed. If your airspeed dial shows only miles per hour, multipliers of four and six, respectively, get you near the same descent angles. You can also use the table above, found on the inside back cover of every FAA/AeroNav terminal procedures book.
Remember: Arriving at the steeper angle means you'll see a different picture through the windscreen--the arrival target will be at a different spot in the windshield, lower, as befits the steeper angle. You can maneuver to move the spot up in your windshield--but you'll be setting up a different glide angle when you do, with all the changes that can bring.
RELATED ARTICLE: LANDING'S A DRAG: GET BEHIND THE CURVE TO IMPROVE SPOT LANDINGS
One of my personal favorites for getting into tight fields and onto short runways--or to simply help make early turn-offs at busy air-carrier airports--involves mimicking airline arrivals and dragging it in. You've surely noticed how most jets seem to fly final with their noses pitched up with power pushing them along.
One of the reasons jets employ this type of approach is the time it takes for turbine engines to spool up from idle to full thrust. Rather than fly the approach at a near-idle power setting and risk getting into the weeds while waiting for the engines to accelerate from idle to full power in the event a sink rate develops, jets deploy their flaps, slats and other drag-producing controls to limit airspeed while the engines' power setting is high enough to allow obtaining full thrust on command.
While piston-powered airplanes don't have the spool-up problem some jets do, we can emulate their approach, dragging it in a little behind the power curve, but controlled. For me, the set-up means flying at about 1.15 x [V.sub.SO], using engine power as needed to adjust glidepath and progress toward the runway spot.
Not only does this method support a steeper arrival angle than normal, it also assures a touchdown right at stall--presuming the flare is properly coordinated with cutting power. Miss on the pitch-up when you cut power--or cut power too soon--and the airplane's sink rate will increase rapidly, making for a firmer arrival than you wanted and risking a prang of the nosewheel in airplanes so equipped.
There are two downsides we can think of when flying this type of approach in a piston-powered airplane. The first is the reduced margin above stall if a gust exceeds the wing's critical angle of attack. The second involves power failure. In both events, the nose needs to come down immediately.
RELATED ARTICLE: UNDERSTANDING GROUND EFFECT
When an airplane in flight comes within a wingspan of the ground or water, a change occurs in the three-dimensional flow pattern around it due to restrictions imposed on the vertical component of the airflow around the wing. This alters the wing's upwash, downwash and wingtip vortices. Among other effects, the reduction of the wingtip vortices due to ground effect alters the spanwise lift distribution and reduces induced angle of attack (AoA) and induced drag. Therefore, the wing will require a lower AoA in ground effect to produce the same lift coefficient ([C.sub.L]). If a constant AoA is maintained, [C.sub.L] increases.
The induced drag reduction is greater the closer the wing is to the surface. When the wing is at a height equal to its span, the reduction is only 1.4 percent. However, when the wing is at a height equal to one-fourth its span, the reduction is 23.5 percent. Because of the reduced drag and power-off deceleration in ground effect, any excess speed brought into the flare may result in considerable float. Maybe that's why your accuracy landings aren't so accurate.
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|Date:||Mar 1, 2011|
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