Not So Fast: Flying Appropriate Approach Speeds
The correct speed to fly on final approach (Vref) on any particular approach is your published approach speed adjusted for landing weight, plus one half of the wind gust spread. Identify that speed in your approach briefing. Call it out before turning final and fly it with discipline until you cross the threshold.
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A little extra speed on final approach can add a lot of length to your landing—and that can become dangerous very quickly. Flying the final approach leg too fast is a problem that can sneak up on a pilot because the approach aim point can be spot-on and the descent rate acceptable, even when speed is much too fast. How fast is too fast? Consider that each 10% extra airspeed over the runway threshold will extend your landing distance by a whopping 21%. So 10% above Vref is certainly too fast. Rather, we should aim to remain within 5 kts of Vref on final. To avoid a long float and blowing through your planned landing distance, know your Vref and be on-speed for the final approach.
Scenario. Today’s featured fictional pilot is Stu.
Stu flies out of a small airport nestled onto a tiny peninsula barely large enough to contain the airfield. On the airport’s western side is a well-developed city with some nearby high-rise buildings, residential areas, and university and hospital buildings. On the other three sides, the airport is snugly hemmed in by a large saltwater bay.
Stu has loaded a Piper Arrow II full of fuel and passengers—just shy of its 2,650 lb max gross weight—for a short sightseeing flight. Everyone enjoys the flight. As Stu returns to base, the wind is calm and RWY 07 is in use. That approach is over the city, over buildings on short final, over a city street on very short final, and over a blast fence immediately at the airport property edge. Beyond the blast fence lies a displaced threshold, then 2,849 ft of usable landing runway, then a few jagged rocks and a whole lot of saltwater. The airplane’s published approach speed is 90 mph, which is 1.4Vso (64 mph). At a typical September density altitude of 2,000 ft, the calculated landing distance over a 50 ft obstacle is 1,400 ft. That is half of the usable runway.
It's been a while since Stu did any proficiency training, and he’s become a bit lackadaisical about flight planning. Stu is comfortable that he can get that airplane in and out of the airfield at max gross weight, so he doesn’t bother reviewing any approach or landing performance figures. He knows that the published approach speed is 90 mph. As Stu turns onto the final approach leg, he sees the large buildings and trees below growing larger in his field of view. Stu’s perception of his final approach corridor begins to constrict because of the buildings below, the city street crossing uncomfortably close to the runway’s approach end—and most pressingly, the trees and blast fence that seem to protrude up into his flight path.
Having no desire to snag a wheel on a tree or the tallish blast fence, Stu yields to the emotional pang within pleading him to keep the speed up just a tad. He is comfortable adding 9 or 10 mph, for good measure. Well clear of the blast fence, Stu zooms down toward the displaced threshold. His aim point quickly disappears under the Arrow’s nose, as does several hundred feet of runway. Stu does an admiral job of holding the airplane in a slow flare to dissipate airspeed. However, beads of fear-fueled sweat begin to form on his brow as the saltwater beyond RWY 07 grows nearer and nearer. He’s stuck in a very long float.
Stu’s Arrow finally touches down and he immediately initiates maximum-effort braking in hopes of making a taxiway exit instead of going off the runway end. Stu is pretty sure that should’ve gone better, and he doesn’t care to repeat that performance. What happened?
Stu was not diligent about flying his approach speed, which is particularly important on an approach over obstacles where runway length is limited. His landing weight was 2,500 lbs, warranting a weight-adjusted final approach speed of 87. Stu had the number “90” in his mind, which is close. But he allowed visual cues below his final approach to prod his speed up to 99 mph on final. That’s only 9 mph faster than he planned, but it is 10% above what he had planned and would’ve added 21% to his landing distance. And 99 mph is 14% above the weight-adjusted Vref for that landing, which added about 30% to Stu’s landing distance. To compound that problem, Stu was also 20 feet too high crossing the runway threshold because he feared getting too close to the obstacles on final; that extra 20 feet of altitude added another 400 feet to his landing distance (every 10 ft above 50 ft adds 200 ft distance).
Combined, Stu’s extra speed and altitude—a mere 9 mph and 20 ft, respectively—turned what should’ve been a 1,400 ft landing distance into something more than 2,200 ft—leaving him only 650 ft of remaining runway. It’s no wonder Stu felt as though he was going to run out of tarmac on the landing rollout.
What should Stu have done differently? He should’ve known his Vref and flown it diligently down the final approach corridor—and he should’ve have flown his approach too high. If Stu is uncomfortable flying appropriate approach speeds and paths (it happens), then he might want to grab a CFI and go do some proficiency work on minimum controllable airspeed, the region of reverse command, and practice normal and short-field landings. Then Stu can get perfectly comfortable flying appropriate approach speeds with discipline—and extracting proper landing performance from his aircraft.
Note: If Stu realized that he was high and fast crossing that blast fence, he probably should’ve gone around. Go arounds are free.
The Takeaway. Know your approach speed and fly it with discipline. If you find yourself too fast or too high on short final, you can always go around.
-APA
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Here is more discussion on the specific topics addressed above.
VREF. As you’ll recall from the FAA’s Airplane Flying Handbook, we want to fly about 1.4VSO on the base leg. Then we slow to a published approach speed (or 1.3VSO if none is published) on final approach. Ideally, we adjust that speed to reflect our actual weight if the published speed is only for max gross weight (larger or multi-engine airplanes often publish several weight-adjusted approach speeds). We adjust the max gross weight approach speed by multiplying it by the square root of the fraction created by dividing the landing weight by the max gross weight. Then, if the wind is gusty, we add half of the gust spread to the approach speed to account for any sudden loss of airspeed during a gust lull. The resultant speed (approach speed, weight-adjusted, and adding half the gust spread) is VREF: the speed you’ll fly down to the runway threshold.
10% More Speed = 20% Longer Landing. Every 10% increase in airspeed over the threshold increases landing distance by a whopping 21%. Here’s why.
The kinetic energy of a non-rotating object (e.g., an airplane) with mass m and velocity v is 1/2mv^2 (one half times mass times velocity squared). Thus, an airplane’s kinetic energy increases with the square of its velocity. If mass and velocity go unchanged (1.0m and 1.0v), then energy remains unchanged. But if velocity increases by 10% (1.1v), then energy increases by 21% (the square of 1.1v, which is 1.21v).
Landing an airplane is essentially a process of reducing the airplane’s kinetic energy such that velocity decreases to landing speed (at or just above stall speed) just at the runway surface (because we want to fly the airplane down to the surface, but we don’t want it to continue flying thereafter). We then further dissipate the airplane’s kinetic energy via aerodynamics and friction (braking) before rolling off on a taxiway at a very low velocity. We really don’t begin this kinetic energy dissipation in earnest until short final (over the “fence” or threshold). Up to then, we are content to reduce gravitational potential energy (altitude) while maintaining a constant kinetic energy (via stabilized airspeed).
Considering the above physics, it should be no surprise that a 10% increase in velocity (airspeed) over the threshold equates to about 21% added landing distance; after all, energy has increased by 21%. To help keep the energy dissipation exercise (landing) under control, we fly “stabilized” approaches. An approach is stable if we are configured for landing, on a correct flight path (usually a 3° glidepath), on airspeed (Vref +10/-5), at no more than 1,000 fpm sink rate, with all briefing and before landing checklists complete, and thereafter make only small changes in heading, pitch, and power. In visual flight conditions, those criteria must be established no less than 500 ft AGL (in IMC, no less than 1,000 ft AGL).
Pitch For Airspeed, Power for Altitude. As a very practical matter, pitch controls an airplane’s airspeed and its power setting controls its altitude (climb or sink rate). On final approach, this principle is a featured player. A simple exercise at altitude demonstrates it well. Climb to a safe altitude where you can afford to lose a couple thousand feet, clear the area, and do a before-maneuver checklist. Configure the airplane for landing. Trim the airplane to your calculated Vref and set the power to provide an appropriate final-approach descent rate (e.g., 400 fpm). Once the airplane is stable in airspeed and descent rate, add 100 rpm (or 1” manifold pressure). See what happens to airspeed and descent rate. While the airspeed may initially fluctuate, it will stabilize back at the original descent airspeed—and your descent rate will now be less (perhaps 300 fpm). Now cut the power back to your first setting, and let the airplane settle back into Vref and the initial descent rate. Then cut another 100 rpm (or 1” MP) and see what happens. The airspeed will stabilize at Vref, and the descent rate will increase (perhaps to 500 fpm). You’ve just seen how power—not pitch—adjusts your climb or descent rate.
Trim for VREF on final. Given that pitch controls airspeed, that an airplane can be trimmed for a pitch (and thus airspeed), and that being on speed during final approach is vital for achieving planned landing performance, then we obviously need to trim the airplane for VREF if we really want to maintain VREF on final. Otherwise, we are more likely to “chase” VREF all the way down final with the control yoke—and risk an unstable approach. Know your approach speed and trim for it on final.
Fly A Stable Approach. An approach has been stabilized when: (1) the aircraft is configured for landing; (2) all before landing checklists are complete; (3) the aircraft is on-speed for final approach; (4) the vertical speed is appropriate for the descent (and does not exceed 1,000 fpm); (5) the aircraft is on an appropriate flight path (typically a 3° glide path); and (6) minimal changes in pitch and heading are required to maintain an appropriate final approach flight path. These criteria should be met before descending below 500 ft above touchdown in a VFR approach and before 1,000 ft above touchdown in an IFR approach. If any of those criteria become unsatisfied, then the approach has become unstable and a go-around is appropriate.
The stabilized approach achieves does two key things for our energy-dissipation landing operation: it provides (1) a constant and sufficiently-low kinetic energy state (airspeed/groundspeed); and (2) a safe and consistent rate of gravitational potential energy dissipation (altitude reduction). In the final phase of landing (the roundout), the pilot begins smoothly exchanging (1) for (2) by increasing the aircraft’s pitch and the wing’s angle of attack. This commits some of the kinetic energy to making more lift via the increased angle of attack. That extra lift reduces the sink rate so the airplane approaches the runway surface less abruptly. The extra lift also converts kinetic energy into drag (mostly induced via lift creation), reducing the airplane’s total kinetic energy state (reducing its velocity). So we fly the final approach at some fixed kinetic energy state that is sufficiently high to allow some of it to be traded for lift to slow the sink rate above the runway. But if our kinetic energy (speed) is excessively high over the threshold, then we’ll have a lot of energy dissipation to do with the wing before we can stop flying and bring the rolling aircraft to a stop on the tarmac. To be more precise, we’ll have 21% more kinetic energy to dissipate just above the runway for every 10% more speed we carry into that roundout.
Don’t get slower than 5 kts below Vref. The advice not to be too fast obviously comes with a caveat: don’t get too slow. I suspect that the fear of being too slow is a lot of what contributes to carrying excess speed energy on final; that is a generally healthy idea. Notice that the FAA’s stable approach airspeed guidance is Vref +10/-5. See AC 61-98E, at 2.1.4.3. That provides less “below speed” margin than “above speed” margin. Remember, if you’re too fast, you can just go around. But if you get too slow while on final, far worse things can happen: stalls, stall/spins, or an uncorrectable excessive sink rate deep into the region of reverse command (behind the power curve).
Speed discipline, please. So, with the above "don’t get too slow” caveat, here’s the clincher: A mere 10% extra airspeed over the threshold increases landing distance by 21%. And 10% over speed is rather easy to accomplish. If your approach seed is 78 knots, just 8 knots over adds 21% to your landing distance! On final approach, know your weight- and gust-spread-adjusted Vref and be on speed.
-APA
