Article: Safety Margins

Budd Davisson, Flight Training, November, 1999

Margins: How much is Enough?

...And How Much is Too Much?

We heard a story once in which a pilot spoke of being at 13,000 feet in his Cessna and watching the fuel gauges 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 "...just one more airport." Then the stupidity of what he was doing suddenly 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 margins of safety is central to the larger concept of aviation itself. Nothing is pushed or designed right to the edge. Airframes are designed to take far more than the normal amount of abuse. Each bolt, for instance, is selected with a safety factor to cover possible material flaws, it is inserted into a fitting which was itself designed with a huge margin to cover possible fit problems and the fitting is part of an airframe design which was based on assumed flight loads which are likely to never happen. An airplane is safety margin layered upon safety margin. Still, this layering of safety margins is done with an intelligent plan which always 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 exact same concepts of safety margins should apply to the way we fly. We should design safety margins into our flight operations in such a way that they greatly increase safety but don't unnecessarily limit us.

Margins can apply to dozens of areas, some obvious, some not so obvious. The obvious ones, like fuel and weather, are talked about a lot. More subtle areas calling for personal limitations and margins include factors like the number of hours flown in a day and fatigue levels.

Margins are extremely personal and subjective. One pilot's acceptable margin may be cutting it razor thin for another person. That's to be understood because pilots vary so widely in their personalities and skill levels. Although it is tempting to list "skill" as one of the top decision factors in setting margins, skill doesn't always allow narrower margins. For instance, even though a person may be the best pilot on the planet, does it make sense to cut fuel margins below legal limits? No, because too many things which the pilot can't control could happen. Skill will only compensate for those factors over which the pilot has control. Fuel-burning headwinds, a mechanical problem or a herd of geese on the destination runway forcing him to use an alternate airport are just a few of many things the pilot's skill can't address.

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 in a variety of situations which, in turn, help him decide what margin is too narrow and what's too fat. Experience actually does two 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. Experience allows him to evaluate a situation for possible pitfalls and ways to take care or them. Experience also means that because of the aforementioned situations, he or she is unlikely to cut margins as thin as would a less experienced pilot. Experience gives the wisdom to accurately decide what can be given up and what must be held on to.

Familiarity with the airplane is another of those factors that goes into setting margins. If a pilot knows the machine intimately, he knows, for instance, exactly how many gallons it is burning and exactly how much gas is actually usable. He has also developed a feeling for how fast it's actually covering ground and can fudge his ETA's accordingly. Is all of this familiarity a good basis for cutting the fuel margin down to next to nothing? Absolutely not. Commonsense dictates that a pilot leave himself plenty and his definition of "plenty" probably exceeds legal minimums.

Familiarity with the route being flown and it's topography gives the pilot knowledge which will also allow him to cut things a little closer, but only a little closer. 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 situations like that, familiarity can work against you because the less familiar pilot would have turned back much earlier.

Certainly the biggest factor in setting margins is commonsense. Whether it's fuel, crosswinds, runway length or whatever, a person's commonsense and ability to understand not only his own abilities but 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.

So how much margin is enough? The obvious question is "...enough of what?" Let's talk about weather, first. How much ceiling is enough, for instance? Although the regs may let you come down to 500 feet in a given area, is that smart? A lot of factors come into play here. Let's say you have good visibility when you encounter these ceilings, a solid 5 miles. So, visibility says go ahead and do it. But, where are we? Oklahoma, here the tallest thing in the county is an oil derrick? Or the west slope Colorado where everything is high and getting higher? Is the weather Oklahoma type weather where a 500 foot ceiling is likely to stay 500 feet or is it Pennsylvania type weather where the ceiling is more likely to be ragged and unpredictable and the topography is the same way? And how well do you know the route? A lot of factors enter the equation and commonsense has to reign.

Probably the best way to look at setting weather minimums is what happens if things get worse? Will your margin give you enough room to find a safe haven. 500 feet in almost any situation isn't enough to guarantee you time to find an airport, if things get worse. 1,000 feet, on the other hand, lets you see the problem as it develops. If you raise your limits even higher, to 1,500 feet, however, you may find it's overkill and will keep you out of the air entirely too much.

The type of flight also has to be considered. If you're just going to go bounce around the pattern or the local area, where you're never more than a few minutes away from a local airport, then you can work an 800 foot ceiling with a degree of safety. Taking off on a 500 mile cross country, however, where it's predicted to be 800 feet most of the way is an entirely different level of risk and has to be approached cautiously.

The correct approach is to set an absolute limit in your mind. Pick an altitude, say 800 feet, and the instant you find yourself forced down to that level, you push the "nearest airport" button in your brain and draw a straight line to it. At times, when the destination isn't far ahead, that may be difficult to do. But, if you don't adhere to self imposed limits 100% of the time, you might as well not even set them.

Weather is the most obvious area needing margins, but what about some operational items like runway length, for instance. Some of the most interesting airports and locations in the world have shorter than normal runways. Here "short" has a personal definition. It has to be defined by factors which pertain first, to your airplane, then to your skill and comfort level in it. Your home airport is 3,500 feet and you want to drop into this cute little 1,800 foot sod strip with good approaches in your C-172. What do you do? How close will 1,800 feet be cutting it? The POH says no sweat. The theoretical margin is there. 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 POH says it can still be done, but the margin is getting tighter and the burden of maintaining that margin is shifted over to you and your skill. You'll have to put it right on the numbers to make it work. Ignoring major factors like wind on the nose (which will have the effect of making the runway longer) or runway surface (which could make it shorter because of lack of braking action) or slope (which can be good news or bad news), the question becomes whether you can pull it off. Maybe the question becomes whether its even worth trying. Margins. How much margin do you want to have? One way to set that limit is through practice. Work at putting it down on your home field. Hit the numbers and use only moderate braking in a no wind condition. Count the runway lights (generally 200 feet each). Do that a number of times, then look down the runway after stopped and see how much runway you'd like to have in front of you when stopped. This again is a personal choice. 30% more? 50% more? Or maybe you're a daredevil and would be happy with only 200 feet in front of you. Don't forget, you still have to takeoff and the wind you landed in may not be there when you go to take off. 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.

Here's a daily concern for most pilots: How much crosswind are you willing to tackle? This is one area where more than a few pilots ground themselves needlessly because they are worried about their own capabilities. At the same time, a few don't have enough respect for the limitations of the airplane. In 90% of the cases, the airplane is capable of handling more wind than the pilot and the pilot knows he is the weak link. 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 either case, the airplane sets the absolute limit and the pilot still has to set his own limits.

In setting crosswind limitations, it's important to realize the gust spread is of more concern then the steady state level of the wind itself. A howling, 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 put in your personal limitations? 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 C-182, but it can be a real challenge to a Cub. Then, as that wind moves off the nose, it becomes more problematical. The gust factor becomes more critical as at the peak they can exceed the crosswind capability of the airplane. Make your margins bigger as the wind comes off the nose and the gust spread gets bigger. You may accept a 10 knot spread on the nose, but only 5 knots at 30°.

Yes, you'll add airspeed on approach to compensate for the gusts, but how much? Figure about half the gust spread and don't over do it. If you come whistling over the thresh hold at a huge margin over normal approach speed, you're going to spend a lot of time floating down the runway. That puts you at the mercy of the wind for a longer than necessary period of time and also eats up runway. Don't over do it. Too much speed isn't always a good thing.

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

Here's a subtlety we seldom talk about: pilot fatigue. It's also something that's really hard to get a handle on. In flying, there are actually two types of fatigue and you have to recognize both and deal with them. The first type of fatigue is that which you bring with you when you get in the cockpit: You've had a hard day at the office or didn't sleep well because the baby kept you up, so you're a little strung out when you get in the airplane. The second flavor of fatigue is that generated by being in the airplane. Airplanes cause their own brand of fatigue. Maybe its the intensity of flying, or the background noise and vibration. Something makes a few hours in an airplane the equivalent of twice the same number of hours in a car. Unfortunately, we seldom, if ever, take fatigue into account when setting personal margins.

It's never part of the accident report, but fatigue of one kind or another often plays a significant factor in airplane accidents. Both judgment and skill are seriously eroded by fatigue and we have to recognize that and set personal limitations to avoid that kind of situation. For instance, we should recognize that outside factors have affected our mental and physical ability to fly. We can't put a number on this kind of thing, but we should develop a subliminal understanding of "self" and how that part of us is functioning. We know when we aren't as sharp as we should be and we should use that feeling in keeping 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 on a given day? The conditions under which the flight is conducted (hot, bouncy, low ceilings, etc.) have a bearing, but in general, we should set limits with which we are comfortable. For those who don't do a lot of cross country flying, three hours is a long time to sit there droning along without a break. Brain fade for almost all pilots (and passengers) generally starts at around two to two and a half hours. Sure, lots of folks can set there for five hours straight, but that's not the norm, and even the professional x-country guy may be approaching a flat-line mental state at the end of a long, long flight. Six hours in a single day, broken into two legs, seems to be a fairly normal limit for most folks. Eight hours is often pushing it, but again depends on the pilots involved.

How much time are you willing to allow between flights before you take some dual time? Have you ever really thought about that? The regs don't address it except in the case of carrying passengers. Are you willing to go three months without flying and then go strap an airplane on? Six months? How long? Like everything else this depends on both the pilot and the airplane involved, but it is a limit which the pilot has to set for himself. A high time pilot with a thousand hours in a given, simple airplane may be willing to go six months, while a low time pilot new to the airplane 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 the wiser heads will generally get some proficiency flights at the point.

The concept of margins can be applied to just about everything in aviation, tangible and intangible. The important part of the concept, however, is that a pilot recognize that no matter what his time or experience, there are parts of his life as well as flight situations which are outside of his ability to conquer. No one is Superman. Everyone needs a personally-set limit which 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. Then he has no one to blame but himself for finding himself having to cope with uncharted territory. BD