Many of us may consider short-field operations in the context of an arrival or departure from a runway that we don't normally operate from. For a number of years, I was based at an airport with a 2,440x70-foot runway. I flew a single-engine retractable and, except for unusually hot days at gross, 2,440 feet was more than sufficient. But, this airport supported a variety of aircraft up to and including King Airs. There was even an early-model Citation that was based there for a while.
Most of the larger aircraft had short-field operations down pat, and they had to if for no other reason than, in addition to the relatively short runway, there was a penalty for landing long: an eight-foot-high dike only 300 feet from the departure end of the runway. And, in an irony that only a pilot can appreciate, it wasn't unusual for the windsocks at opposite ends of the runway to point in
Watching aircraft landing on a typical weekend was enlightening. It wasn't for nothing that there were many sets of long rubber-tire marks near the ends of the runway. So the obvious question was: Why could larger aircraft operate with seeming impunity, while many of the Bonanza and Baron class of aircraft had problems? The answer has far-reaching implications that can affect safety in other realms of flight, as well.
A Weighty Story
While watching some pilots make max performance landings while others rolled out and turned off the runway with little fanfare, I eventually realized what the one major difference was.
The accepted procedure is to fly final approach at 1.3 times the aircraft stall speed in landing configuration. Yet, the aircraft making no-nonsense arrivals were almost always substantially slower and touched down with much less float than those that left skid marks and taxied away with flat-spotted tires.
Why the difference? Most likely, the longer-landing pilots used their aircraft's POH stall speed for their approach speed calculation. Some may even have added a couple of knots "for the winds and family." The simple fact is that their approaches were much faster than necessary; hence, the longer-landing distances.
If there's only one stall speed in the POH—which is true of most older aircraft—it's determined at the aircraft's gross weight. But, stall speed actually varies in direct proportion to the aircraft's actual weight. The lower the weight, the lower the stall speed. When an aircraft is lighter, it can safely fly at a slower approach speed and stop in a shorter distance. For example, according to my Columbia's POH, the short-field approach speed is 82 KIAS at 3,420 pounds, but it decreases a substantial nine knots to 73 KIAS at 2,700 pounds.
In addition to a shorter rollout, flying at the correct approach speed based upon actual weight means that the float before touchdown is minimized. Don't underestimate how much floating in ground effect can affect the total landing distance.
One rule of thumb states that a 1% change in approach speed will cause a 2% change in stopping distance. The nine- knot decrease in stall speed example is an 11% difference, which equates to a 22% decrease in the landing distance. For a cross-check, there's another rule of thumb that says that a 10% decrease in landing weight will cause a 10% change in stopping distance. Using the Columbia example again, the 720-pound difference in weight works out to be a 21% decrease in stopping distance. So, the two rules reinforce each other. Again, those are both ground-roll values and don't take float into account.
Reversing the analysis and examining the result of a higher-than-necessary approach speed is more indicative of real-world risks. A too-fast approach at 82 KIAS means the aircraft needs 12% more runway than if it were flown at the correct 73 KIAS. Assuming a calculated landing distance is 900 feet at 2,700 pounds, flying at the faster approach speed increases the landing distance to 1,008 feet. That's for an experienced test pilot. If you factor in the additional float resulting from a too-fast approach speed and normal piloting skills, the landing might take 1,100 to 1,200 feet more.
It's More Than Just Stalling Around
Let's examine some of the other V-speeds that are affected by aircraft weight. But before we do, it's important to identify those speeds that are sacrosanct. Simply, with two interesting semi-exceptions, no structural speed is affected by different aircraft weights. That includes maximum flap or landing gear operation or extended speeds, maximum aircraft operating speeds and any other speed that's structurally related.
As far as V-speeds that are affected by weight, the aerodynamics that affects a Boeing 787 also affects a Cessna 182. Transport category aircraft are required to calculate Vr before every takeoff. Part of the calculation includes the weight of the aircraft at takeoff. As you might expect, the lower the weight, the lower the rotation speed. I've found that simply setting the yoke to the proper pitch attitude and letting the aircraft lift off when it's ready works best for normal takeoffs. For short-field takeoffs, I use a positive rotation at the appropriate airspeed.
After rotation, pilots might climb at Vx or Vy depending upon the departure requirements. By this point, you undoubtedly know that Best Angle and Best Rate of Climb are both affected by aircraft weight in the same manner as stall speed. But, there's an interesting wrinkle regarding these two airspeeds. You typically won't find this information in older or sometimes even newer POHs, but both speeds also change with altitude. The rule of thumb is to add one knot per thousand feet density altitude to Vx, but counterintuitively, to subtract ½ knot per thousand feet from Vy.
If the POH doesn't contain a weight to V-speed chart, you can make your own.
Shake, Rattle And Roll
If you find those speeds interesting, the effect on maneuvering speed should give you pause. Flying at or below maneuvering speed (Va/Vo) is intended to help prevent structural damage to the aircraft if the pilot moves a flight control rapidly from stop to stop. My Columbia is a utility category aircraft, which means it's designed to stronger structural standards than a normal category aircraft, but the placarded maneuvering speed varies from 158 KIAS at 3,600 pounds down to 135 KIAS at 2,600 pounds. That's a 23-knot difference!
However, there's another speed that's rarely discussed and is critical for flight in turbulent conditions. Contrary to what most pilots believe, maneuvering speed (Va/Vo) isn't turbulence penetration speed (Vb), although many manufacturers use Va/Vo for that purpose. Vb is the maximum airspeed at which an external gust won't overstress an aircraft's airframe. An accepted rule of thumb for calculating Vb is 1.7 times the stall speed for the actual aircraft weight. For the Columbia, the stall speed at 3,600 pounds is 72 KIAS. Calculating Vb based upon that rule of thumb works out to 122 KIAS at gross weight.
Consider being extremely conservative if your aircraft doesn't have a table containing maneuvering or gust penetration speed for the aircraft's weight range.
Best glide speed, Vg, is another important speed that's affected by aircraft weight. According to the Columbia's POH, the best glide speed at the 3,600-pound gross weight is 108 KIAS. It decreases 12 knots to 96 KIAS at 2,700 pounds. In an engine-out emergency, flying the correct airspeed could mean the difference between a safe arrival or not!
Beware If Light
We've highlighted the fact that many reference airspeeds in POHs are only accurate when the aircraft is at full gross weight. The instant the engine is started, the aircraft is burning fuel and becoming lighter. An airplane like the Columbia burns on the order of 36 gallons per hour during the departure climb. It doesn't take a calculator to recognize that not long after takeoff, no gross weight-based airspeed is accurate. At the end a long cross-country, both the maneuvering and stall speeds are substantially less than gross weight values. You can see why pilots should take aircraft weight into account during the entirety of every flight. As an added day-to-day benefit, the aircraft can land comfortably in shorter distances, saving both brakes and tires.
Interestingly, if you don't adjust some of the V-speeds, you're sometimes just being inefficient. The airplane will land longer than optimal, for example, and climbing might be less than optimal. But maneuvering/gust penetration speeds are typically used in critical situations, so you want them to be accurate. There are also times when Vx) and even Vy) might be critical, as well.
If that doesn't convince you, in an engine-out emergency, the combination of flying at the appropriate best glide and approach speeds for the actual aircraft weight may make the difference between a safe arrival or not.
Make Your Own
|If your POH doesn't include a breakdown of the various V-speeds by aircraft weight, you might consider making your own chart. I've done that for two of my previous aircraft. What I discovered is that the changes in the various V-speeds aren't equally relative to aircraft weight changes. For example, the Columbia's stall speed and any v-speed related to it changes 1.25 knots per 100-pound change in aircraft weight. Maneuvering and Best Glide speeds change 2.3 knots and 1.3 knots respectively per 100 pound change.
You can use the following formula to calculate Maneuvering Speed: Va = Va √ (WNew/WMax-Gross), where WNew is the current aircraft weight.
If you want to create a chart and your POH doesn't have the information you're looking for, start by contacting the manufacturer or the appropriate aircraft type club. You can also empirically determine many of the speeds by doing your own flight tests. Whatever you end up doing, make sure the results are conservative. I've found it's challenging to fly these kinds of tests precisely, but it's a confidence-building experience and a lot of fun. Do yourself a favor, and bring someone along to watch for traffic and record the data while you fly.