Friday, March 11, 2016

Short and Soft Field Take Offs, great reprint from Mike Hart



Short-field landings are all about using excellent technique to get your airplane into a tight spot. That same technique, however, can put you in an even tighter spot when it's time to leave.
Most general aviation aircraft land shorter than they leave. This performance disparity can be subtle at sea level, where the two numbers might be equal. As altitude and temperature increase, however, the gulf between them grows and it often can take twice as much runway to depart than it does to land. Airspeed control gets you into a short field, but horsepower is what gets you out, and available horsepower drops as altitude increases.
From a risk-management perspective, takeoffs have significantly greater consequences than landings. While you are much more likely to have an accident during the landing phase of flight, you also are much more likely to walk away from it. According to the AOPA Air Safety Institute's 22nd Nall Report on general aviation accidents in 2010, there were more than twice as many GA accidents during the landing phase than takeoff, 361 versus 142. However, there were less than a third as many fatalities during landing than takeoff, eight versus 28. The higher fatality rate for takeoffs should get any pilot's attention, particularly when considering a challenging short or soft field.
The reason for the disparity can almost entirely be explained in two words: stall/spin. It doesn't happen on landings as frequently as it does on takeoffs. For short- and soft-field takeoff accidents, it is one of the single most common factors linking fatalities.

Preventable



The worst accidents to read about are the ones that stand out as obviously preventable. This is what makes reading short- and soft-field accident reports so painful. The one thing they seem to have in common is the fact that the majority seem to be obviously preventable.
Given the constraints associated with short and/or soft fields takeoff, good aeronautical decisions are paramount. That means you need to know with 100 percent certainty that your proposed takeoff is within the performance envelope of the aircraft, given the conditions. It's not difficult; you simply run the numbers. But it is shocking how often this is not done, with predictable results. Many short-field accidents could have been easily avoided by actually checking the POH and asking the obvious questions about the factors affecting takeoff performance. There is a reason we are taught this stuff.
The basic questions you need to ask and answer: How long is the field? What is the wind? What is the temperature? What is the altitude? How much weight is in the plane? Your POH should give you some convenient tables or a graph allowing you to determine the theoretical distance needed for takeoff. If the calculated length of the field is less than the number calculated from the POH, don't even think about turning your prop. An obvious accident is avoided.

Double-Check Assumptions

If the calculated theoretical takeoff distance is at all close to the runway length, you definitely want to check both your math and the generosity of your assumptions. Little things—like errors in math—matter a lot when you are shaving the runway length close to the limits of aircraft performance.
For example, the FAA's Pilot's Handbook of Aeronautical Knowledge (PHAK, FAA-H-8083-25A, dated 2008) contains errors (errata items 36 and 37, updated November 19, 2013) that in the real world could have been fatal: "Wind component (knots) column; the red line is incorrect…. Change the ground roll distance to be 700 feet, not 600 feet. Change the total distance over a 50-foot obstacle to be 1400 feet, not 1200 feet." If the calculation was being done for a 1300-foot runway, the outcome might not be pretty.
This FAA mistake was only about the effect of wind components, but a lot of other factors can become links in the accident chain when the runway is short. And, as the sidebar above explains, these factors can add up.

Acceleration Thieves

Of course, not every factor affecting takeoff performance will have a table in the POH. According to the PHAK, "In addition to the important factors of proper procedures, many other variables affect the takeoff performance of an aircraft. Any item that alters the takeoff speed or acceleration rate during the takeoff roll will affect the takeoff distance." I try to think of short- and soft-field conditions together because this is where the next round of preventable short-field departure accidents come from: failure to adequately consider runway conditions that reduce acceleration.
There are a myriad of highly variable factors that can extend your takeoff ground roll. You won't find a convenient table giving calculations for snow depth, amount of sand or height of grass, but some rules of thumb are provided. These are collected in the table above. Keep in mind, however, that there is such a thing as an impossible surface, one that has more friction than your airplane has horsepower.


Another significant acceleration thief is runway slope. If you can see one end of the runway is higher than the other, you will likely want to make depart downhill unless there is a significant wind. Some POHs will include slope in takeoff performance calculations; others don't.
A good rule of thumb is to add 10 percent to your takeoff distance for each percentage of slope (a one-foot rise over 100 feet of runway results in a one-percent slope). The runway at Challis, Idaho (KLLJ), for example, has an elevation of 5000 feet at the north end and 5072 feet at the south. That 72-foot rise over the 4600-foot runway yields a 1.6-percent uphill slope to the south. Think about that: To depart uphill, you are asking your plane to climb a seven-story building beginning at an altitude of roughly a mile above sea level. A lot of pilots wouldn't want to do that. Unless you have a stiff tailwind, you are going to want to depart to the north, which is why the FAA's Airport/Facility Directory suggests it.

Hybrid Technique

One of the shortcomings of the PTS and most aircraft POHs is they contain a discreet technique for short-field takeoffs and a different one for a soft-field takeoff. The differences often can include have different lift-off and climb speeds and flap settings.
In my experience, short fields tend to be soft, and soft fields tend to be short. If my experience holds for others, you will need to improvise a blended technique that combines elements of each. While there may be good tribal knowledge in the community of people who fly a particular type of plane, for the most part, this knowledge is experiential.
If you are cutting it close to the aircraft performance envelope, you need to pick your abort point and an abort speed. The rule of thumb is to have 70 percent of liftoff speed by the runway's midpoint. Identify the spot and know the speed. You can't execute a planned abort without a plan.
It is shockingly common to read accident reports where the pilot needed maximum short-field performance and chose an inappropriate flap setting. In fact, I wouldn't be surprised if it's a check box item for the NTSB investigators arriving at a short-field mishap.
Don't conflate the full-flap setting you need to get into a short field with the setting required to get out: Chances are, it won't be full flaps. It's common for planes to require some flaps for best short-field performance. My 180 likes 20 degrees, and so did the 182 I owned before it. (My Cub doesn't have flaps, so that is a no-brainer.) The key is to follow the short-field technique in the POH.


For a truly short-field takeoff, VX always will return the greatest altitude in the shortest distance. If your short/soft-field procedure calls for some flaps, you may consider putting required flaps in part way through the ground roll in order to minimize drag and gain needed acceleration.
How your flaps are controlled is an issue, also. If yours are electric or hydraulic, they require some amount of time to extend, and may also demand attention to achieve the desired setting. Meanwhile, the Johnson Bar of my old 182 allowed me to pull in 20 degrees of flaps all at once with a single motion. Doing this after the plane had accelerated to near its flying speed literally lifts the plane into ground effect. Once you have the plane in the air, use ground effect by staying close to the ground (no more than half the wing span). It is free energy that will add to your acceleration. For a high-performance, short-field takeoff, you will want to hit VX and hold it while in ground effect. You may need to nose over a bit once the wheels leave the surface to remain in ground effect and accelerate, so be ready.

Keep Your Energy, Don't Stall

The great thing about ground effect is that you can use it for acceleration, but eventually you will need to trade that energy and every bit of power your engine can muster for a VX climb. The problem with VX is that the next lowest V number is typically one with an "s" for "stall" in it. The best angle of climb speed always is a relatively slow one and can require an uncomfortably aggressive pitch. It also can be an uncomfortable place to be while very near the ground because it is at the edge of your plane's performance envelope.
Too often, short-field accidents involve gutless planes that seem to be able to accelerate in ground effect, but once they climb beyond this free energy source, they falter and settle back to the ground. When this happens once, an abort is a smart move. If it happens twice, an abort is a really smart move. A settling airplane is one that isn't flying—it's quitting. It's better to settle back to the ground and quit than proceed beyond the airplane's ability or willingness to fly. Continuing forward is the beginning of the two words that lead to fatalities: "stall" and "spin."
If you choose to press on with an anemic climb, maintain airspeed. As the end of the runway looms closer and closer along with any attendant obstacles, it is still not too late to abort. It may result in a forward crash at VX, but that's almost always better than a stall/spin. Let me repeat that: A forward crash at VX or faster is better than a stall/spin.
Of course, VX is not a good place to be when encountering an engine issue. If you lose power, you will need to push the nose down—aggressively. That does not come naturally when you are running out of runway or the obstacle is approaching, but if you can't hold VX, put the nose down or risk that stall/spin.

What Not to Do

That make-or-break moment where the airplane isn't really wanting to fly and collision with an object is inevitable is the one when many (perhaps most) pilots make the worst possible wrong choice. The unfortunate tendency is to pull up.


Chances are you can feel pressure on the yoke or stick while holding VX. That feel of wing loading may give you the illusion that you can "pop" it over the fence or tree top. Unfortunately, that one last yank to clear the fence, tree, rock or other obstacle may put you irrecoverably behind the power curve. It is the reason why the fatality rate is so high for takeoffs when compared with landing.
Unloading the wing and dropping below VX may give you a temporary bit of energy to clear the obstacle immediately in front of you, but the next part of this Faustian deal with the aerodynamic gods is definitely going to be an ugly amount of down. There is no Bernoulli Viagra: There is no place physics can bootstrap energy to get you back up. In the words of the rapper Ice-T, "You played your self." It won't end well.
What struck me during researching this article was how preventable most short-field takeoff accidents are. In nearly every investigation I read, the contributing factors were obvious, in many cases embarrassingly so. Sure, hindsight offers great clarity, but when a simple calculation shows you need 2000 feet of good pavement and no wind, it might not be a good idea to try taking off uphill and with a tailwind from a 1500-foot grass strip.
This article originally appeared in the March 2014 issue of Aviation Safety magazine.

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