A Margin Of Safety

I once heard a story about a pilot flying at 13,000 feet in his Cessna while playing a game. His airplane's high-altitude performance was so miserly, he was trying to see if he could stretch his fuel supply by "just one more airport." Then the stupidity of what he was doing dawned on him, and he realized why it was happening. Oxygen starvation had eaten into his judgment, and he was violating his own rules on fuel margin. Had he not had that margin firmly planted in his under-oxygenated brain, we might still be looking for him in the mountains.

The concept of a margin of safety is central to the larger concept of aviation itself. Nothing is pushed or designed right to the edge. An airplane consists of safety margin layered upon safety margin. This layering of safety margins is done with an intelligent plan that keeps the goal of structural simplicity and strength-to-weight ratios in mind. There is such a thing as building in so much strength that performance suffers because of unnecessary weight.

The same concept of always having a safety margin should apply to the way we fly. We should design margins into our flight operations in such a way that they increase safety without unnecessarily limiting us. Margins can apply to dozens of areas, some obvious, some not so obvious. The obvious ones include fuel and weather; the more subtle ones include the number of hours flown in a day and stress levels.

Margins are extremely personal and subjective. One pilot's acceptable margin may be too thin for another person. That's because pilots vary in their personalities and skills. Although it's tempting to list skill as one of the top factors in setting margins, skill will only compensate for those factors over which the pilot has control. For instance, does it make sense to cut fuel margins below legal limits? No, because too many things could happen that even the best pilot can't control.

Experience

How about experience? Where does that intangible fit in the margin-setting equation? This one is near the top because experience gives the pilot reference points that help him decide what margin is too narrow and what's too fat. Experience actually does several things when it comes to setting margins. First, experience means the pilot has been in a given situation many times and knows what to expect. Second, experience allows him to evaluate a situation for possible pitfalls and ways to avoid or at least fix them. Third, experience gives the wisdom to accurately decide what can be given up and what must be guarded.

Familiarity

Familiarity with the airplane is another factor that goes into setting margins. If a pilot knows the machine intimately, he knows exactly how many gallons it is burning and exactly how much gas is actually usable. Is all of this familiarity a good basis for cutting the fuel margin to next to nothing? Absolutely not. Common sense dictates that a pilot have plenty of fuel and plenty should exceed legal minimums.

Familiarity with the route being flown and its topography gives the pilot knowledge that may also allow him to cut things a little closer, but only a little. A huge number of very experienced pilots have died thinking their familiarity with the local terrain combined with their experience would let them push on in questionable weather. In these situations, familiarity can work against you because the pilot who was less familiar with the area would have turned back much earlier.

Certainly the biggest factor in setting margins is common sense. Whether it's fuel, crosswinds, runway length, or fatigue, a person's common sense and ability to understand both his own limitations and the level of risk involved sets the final number. If it doesn't make good sense to do something, don't do it. End of conversation.

Ceiling And Visibility

So how much margin is enough? How much ceiling is enough, for instance? Although the regulations may let you fly as low as 500 feet in a given area, is that smart? A lot of factors come into play here. Let's say you have five miles visibility when you encounter low ceilings. Visibility says go ahead and do it. But, where are we? Oklahoma, where the tallest thing in the county is an oil derrick? Or the western edge of Colorado where everything is high and getting higher? Is the weather in a place where a 1,000-foot ceiling is likely to stay 1,000 feet or is the ceiling is more likely to be ragged and unpredictable? And how well do you know the route? A lot of factors enter the equation and common sense has to reign.

Probably the best way to look at setting weather minimums is to ask yourself what happens if things get worse. Will your margin give you enough room to find a safe haven? Five hundred feet isn't enough to guarantee you time to find an airport if things get worse. Two thousand feet, on the other hand, gives you more time if a problem develops. If you raise your limits to 3,000 feet, however, you may find it is overkill and will keep you out of the air entirely too much.

The type of flight also has to be considered. If you're going around the pattern, then you might be able to work a 1,500-foot ceiling with a degree of safety. Taking off on a 500-mile cross-country, however, is an entirely different level of risk. This is where an instrument rating becomes invaluable.

The correct approach is to set an absolute limit in your mind. If you don't adhere to self-imposed limits 100 percent of the time, you might as well not even set them.

Runways

What about operational issues such as runway length? Some of the most interesting airports have relatively short runways. Here short has to be defined by factors that pertain to your airplane and your skill. Your home airport runway is 3,500 feet and you want to drop into this cute little 1,800-foot sod strip in your Cessna 172. How close will 1,800 feet be cutting it? The pilot's operating handbook says no sweat. The airplane can do it. The question is, can you?

The reality is that 1,800 feet isn't very short and a little practice on your home field will prove that to you. But what about 1,500 feet? Or 1,300 feet? Now the pilot's operating handbook says it can still be done, but the margin is getting tighter. You'll have to put it right on the numbers to make it work. The question becomes whether you can pull it off. Maybe the question becomes whether it's even worth trying.

How much margin do you want to have? One way to set that limit is through practice. Work at landing on your home field. Hit the numbers and use only moderate braking in a no-wind condition. Count the runway lights (generally 200 feet apart). Do that a number of times, then look down the runway after you've stopped and see how much runway you'd like to have in front of you at the end of the rollout. This again is a personal choice. Maybe you're a daredevil and would be happy with only 200 feet in front of you, but maybe you'd like to finish your rollout on the first 50 percent of the runway. Don't forget, you still have to take off, and the wind you landed in may not be there when you go to leave. Or it may be hotter. Also, remember that load has more effect than anything else, so go out light. Consider it all when setting your margins, but set them and don't attempt something you're not sure you can do.

Crosswinds

Here's a concern for most pilots: How much crosswind can you tackle? In most cases, the airplane is capable of handling more wind than the pilot. There are two ways of handling safety margins in this case: The first is to recognize your limitations and set the margins accordingly. The second is to get some instruction and increase your capabilities. In setting crosswind limitations, it's important to realize that the gust spread is of more concern than the sustained wind. A steady wind, unless it's well off the nose, isn't generally a major problem. A nasty, gusty crosswind, however, can be at a fairly low level and pose a significant threat.

So how much crosswind margin do you allow? Have two limitations, the first being the gust spread you're willing to accept based on the stall speed of your airplane. A 10-knot gust spread is easily handled by a Cessna 182, but it can be a real challenge to a Piper Cub. Make your margins bigger as the wind comes around the side and the gust spread gets bigger. You may be willing to accept a 10-knot spread on the nose, but only five knots at 30 degrees.

Yes, you'll add airspeed on approach to compensate for the gusts, but how much? Add about half the gust spread to your normal approach airspeed and don't overdo it.

Here, too, common sense rules, but don't let crosswinds keep you grounded. If you don't learn to conquer them, sometime one is going to creep up on you and ruin your day.

Fatigue

Here's a subtlety we seldom talk about: pilot fatigue. In flying, there are two types of fatigue. The first type is the fatigue that you bring with you when you get in the cockpit. You've had a hard day at the office or didn't sleep well, so you're a little strung out when you get in the airplane. The second type of fatigue is generated by being in the airplane. Maybe it's the intensity of flying or the noise and vibration. Something makes a few hours in an airplane the equivalent of twice that number in a car. Unfortunately, we seldom take fatigue into account when setting margins.

Fatigue of one kind or another often plays a role in airplane accidents. Judgment and skill are eroded by fatigue. We know when we aren't as sharp as we should be, and we should use that feeling to keep ourselves out of the cockpit.

The airplane part of the fatigue mix we can control. How many hours are we willing to fly on a given leg or a given day? The conditions under which the flight is conducted (hot, bouncy, low ceilings, etc.) have a bearing. For those who don't do a lot of cross-country flying, three hours is a long time. Brain fade generally starts at around two to two-and-a-half hours. Six hours in a day, broken into two legs, seems to be a normal limit. Eight hours is often pushing it, but it depends on the pilot involved.

Staying Current

How much time are you willing to allow between flights before you take some dual time? Are you willing to go three months without flying and then strap on an airplane? Like everything else this is a limit that the pilot has to set for himself. A high-time pilot with 1,000 hours in a given airplane may be willing to go six months, while a low-time pilot shouldn't go nearly that long. What's a good limit? For a low-time pilot, two months between hops is pushing it. For a medium-time pilot, three months is probably a fair limit, although wiser heads will get some proficiency flights at that point.

The concept of margins can be applied to just about everything in aviation. The most important part of the concept, however, is that a pilot recognize that there are situations that he can't conquer. No one is Superman. Everyone needs a limit that they will not exceed no matter what. Without that limit, it's only a matter of time before a pilot finds himself dealing with something for which he is neither trained nor equipped.