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Builder Spotlight

July 2012 Issue

Build It Better

Where’s Your Margin?


I am a strong believer in margin—that bit of extra space that you give yourself to make life a little more comfortable. What do I mean by comfortable? In an aviation context that means I am not sweating bullets or screaming silently because I am worried about the outcome of whatever it is I am doing. Margin is not always mental. It can be designed into airframe structure, systems, equipment and the way we operate all of the above. Margin is the extra strength built into your spar, that handheld radio you have in your flight bag or the extra fuel you keep in the tanks in case the headwind gets a little stronger.

When aircraft are designed and built, certain assumptions are made about the strength required, the capability of the systems, and the limits of endurance and range. Building and providing margin should be a goal of every aircraft designer and pilot because we never know when our assumptions about what the limits should be might just be wrong.

Structured Learning

Structural margin is usually represented as a factor of safety above the expected loads to be put on the structure. A very common design load factor is 1.4, meaning that the structure is capable of withstanding 40% more load than what it is rated for before bending or breaking. If the designer intends the airplane to be routinely operated up to 3.0 G for instance, and he is using a factor of safety of 1.4, then the structure shouldn't break until a G-load of 4.2 is exceeded.

This is more than a creative way to take a group photo. It is also intended to convey that there is a good deal of margin in the wing structure.

The truth is that most (but not all) light airplanes are overbuilt in this regard. Because a designer's goal is to have a 1.4 factor of safety, he will shave more off structure if that margin is exceeded. Sometimes he will shave if the goal is to produce the lightest structure possible. If you are trying to build a world-record setting airplane where ultimate performance is important, then you will want to save weight wherever you can. In the aerospace world, weight is always detrimental when trying to get a vehicle into orbit, so margins are carefully controlled, and when they tell you that you have 1.4, you should never assume that you have 1.5.

There's a great magazine ad from the 1970s that shows a Mooney with a staggering number of employees standing and sitting on the wing, from tip to tip. By actual count, there are 30 on that wing, which makes me wonder how high they pumped up the tires. The point of the advertisement, of course, was to illustrate the strength in that wing—far more than any reasonable pilot would ask of the airplane during normal operations. When you consider that the airplane is not certified as aerobatic, what the manufacturer is telling you is that there is a great deal of margin in that structure.

Each pilot has a unique limit, and that limit may vary. Evaluating the aircraft condition, mission and weather determine how much margin is needed in a given situation.

All aircraft have a breaking point; some far exceed others. Generally, the shorter the span, the stronger the structure is likely to be, because you will have less bending moment where the wing attaches to the fuselage. Most wings are built with the capability to sustain higher positive G than negative, frequently by a significant amount. But it is interesting to note that in most cases the wing is not going to be the first part of the airplane to fail.

More often than not, the tail fails under overload first, and once it breaks, the aircraft pitches violently forward, instantaneously develops a high negative-G loading, and the spar fails. Structural design is more complex than many people realize, and the loading at which a wing may fail depends not just on the straight G pulled with a pitch maneuver, but rolling moments as well. Without going into the details of V-G curves and charts, suffice it to say that when things get bumpy you should slow down below maneuvering speed and avoid high-G pulls while rolling. The point we want to get across with this discussion is that most aircraft have sufficient structural margin as long as the pilot operates within the specified limits.

Backup to the Backup

Systems design is the next area in which we find margin, and it is closely tied to discussions of backup plans, redundancy and even operational margin. (This is a subject near and dear to me, which is why it's woven into this series at several points.) We can add margin to our aircraft systems during the design phase, or we can incorporate it later on by adding equipment. It's important to know what you want your airplane to do, and also to know what it is honestly capable of doing. It is as unreasonable to use a short-range, two-place trainer for serious IFR cross-country work as it is to try and fly aerobatics in a four-place traveling machine.

Either one can probably be equipped to do those jobs, but that doesn't make them ideal (or necessarily safe) for that purpose. For want of a better word, the "criticality" of the mission often drives the amount of margin built in to the systems. By criticality, I mean the importance of completing the particular mission on a given day. For instance, a fun flier whose sole purpose is to run around the local area to see the sights probably doesn't need to fly on any given day. If you go out to fly and something isn't working, you might be disappointed, but it wouldn't be a disaster. At the other end of the spectrum, a medical airlift aircraft's mission may be extremely critical; if it doesn't fly now, then someone might well die.

Airliners are usually well enough equipped that they can complete their missions even with failed equipment. Down-checked (unavailable to fly) aircraft can wreak havoc with a complex flight schedule, so redundancy is built in, and minimum equipment lists are written to allow flight with known defects under certain conditions.

General-aviation homebuilts fall into categories depending upon how their owners and pilots think about them, but a generalization would be that rarely must a mission be completed or life and limb will be at risk. While our upcoming vacation in the Bahamas might be ruined if we can't fly, the risk to life and limb when we fly without specific equipment could be much greater. An honest assessment of the aircraft's suitability for "all-weather" operation needs to be made before spending tens of thousands of dollars to provide a capability that might be an illusion—or worse, a temptation to do something unwise.

Many Experimental airplanes today are better equipped than the light jets of a few years ago, and even better than some military and commercial aircraft. Highly integrated EFISes, WAAS GPSes, redundant power systems and sophisticated autopilots all make today's airplanes much more capable for IFR flying. Note, however, that I did not say all-weather flying. My own aircraft is superbly equipped for IFR flying in low-visibility situations thanks to precision approach capability, highway in the sky (HITS) guidance and fully coupled approach technology. Yet the basic airframe can't carry ice, and it certainly won't survive a thunderstorm.

We all used to fly IFR with steam gauges and lots of needles that we had to mentally transpose into an image of where we were relative to the established navigation aids and surrounding terrain. Today's systems give us a margin by drawing a map, complete with a little airplane following a purple line. In fact, many airplanes have two such maps, so there is even more margin to help our saturated gray matter. An extra com radio makes the communication job easier, as does the ability to monitor more than one active frequency. Fuel totalizers make the management of this important commodity easier and more reliable.

The Margin Between Your Ears

While the designer and/or builder of an aircraft may provide structural and systems margins, the pilot provides the operational margin, which is that little extra pilots leave to stay inside what they understand to be the limits of the airplane. This might be a margin in airspeed above the stall, or below Vne. It might be the distance the pilot will stay away from weather. It may be found in the amount of fuel the pilot insists on having in the tanks upon landing—or once the decision is made that the destination weather isn't going to cooperate, and it is time to head to the alternate. Operational margin is usually the margin that has been exceeded when we read an accident report that includes running out of fuel, attempting to fly aerobatic maneuvers below ground level or continued VFR into IFR conditions.

Every pilot has limits. They are different for different people, and even different for the same person at different times. It is important to always evaluate current conditions of aircraft, pilot, mission and weather to decide how much margin one needs.

I'll share a few examples—not intended to be adopted by others, but merely to illustrate the concept—that I have used in the past. When I am flying IFR in a single-engine GA airplane, I like to leave myself a great deal of operational margin. In fact, I generally won't fly IFR in a "sensitive" aircraft without an operating autopilot. By sensitive I mean those designs, including many homebuilts, that have sprightly handling. The autopilot is for redundancy and to ease pilot workload under normal conditions.

I am also extremely paranoid about fuel. I constantly keep an eye on weather (using onboard satellite METARs, TAFs and NEXRAD radar) to make sure that at all times I have enough fuel to reach someplace with at least 1000-foot ceilings or better—and that I am sure will stay that way. Because ceilings and visibilities frequently take a nosedive at dusk, I generally won't trust forecasts that indicate marginal VFR at that time of day. Frankly, I won't fly IFR in a single at night because it just stacks one too many straws upon the camel's back.

As a general rule, if my destination is going to require an instrument approach, I want to always have two methods of completing an approach—or have the ability to retreat to a field where I can reasonably be assured of completing a landing. I will file for a destination that is predicted to require a precision approach, but only if I have an ironclad alternate. Those who know me know that I cancel flights when I don't feel good about them.

Margin, in all its forms, is a breath of fresh air in an activity that can sometimes be filled with tension. In fact, lack of margin is often directly related to that feeling you get in the pit of your stomach when you are out of options, and everything needs to go exactly right. An aircraft that always operates on the razor's edge can be exhausting to fly, and while at times that can be a thrill, operating at that point all the time is not the way to a long and enjoyable life. Look for ways to build margin into your airplane, your equipment and your operations, and know what that margin is and how it can be grown. Then relax a little, knowing that you have a cushion to protect you if the next worst thing happens.

Paul Dye is an aeronautical engineer and multi-time builder. He flies a Van's RV-8 and, along with his wife, Louise, recently completed an RV-3.

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