Until we figure out a way to power our airplanes by electricity, sunlight, or nuclear reactors, we will rely on fossil fuel. I will limit this discussion to airplanes that use either avgas or automotive gasoline. I will refer to these collectively as fuel.

When the clean fuel flows to the engine there is lots of noise and everyone is happy. When the fuel is contaminated there may be short interludes of quiet followed by prolonged silence, bar the screaming of terrified passengers. The silence is likely to soon be broken by the sound of bending aluminum. Not good.

So how does fuel become contaminated and with what? Water, sediments, or other foreign substances can enter the system and cause problems. There is also the possibility of adding jet fuel rather than gasoline. (That one can be easily avoided by insisting on personally supervising the fueling of the airplane and matching the label on the truck or fuel farm with the placards near the airplane's filler openings.) Update courtesy of Dan M.: "One can check for jet fuel by pouring a few drops from the sample on white paper (like the edge of a sectional). Very quickly it will vaporize and not leave any trace on the paper if there is no jet fuel mixed in. If there is even a tiny amount of jet fuel it will leave a stain that will not go away. I always do this when fueling at places that have jet fuel on the premises, no matter what the sign on the truck says." Thanks, Dan! Great idea.

Water is the most common contaminant. It can be introduced through the addition of fuel already contaminated with water. This is rare if purchasing fuel from a reputable FBO who complies with regulations regarding filter maintenance and system testing. But, failure of even the best operations is possible. Using a can to transport fuel to an airplane is a more common cause.

More likely the water entered the fuel tank through a leaky cap. The recessed fuel caps of the high performance airplanes are the most susceptible. They should be thoroughly inspected by a qualified technician on a regular basis and rebuilt or replaced promptly if a problem is noted. Water can also enter a fuel tank due to condensation in a partially filled tank. Keeping the tanks filled when the airplane is parked will help mitigate this problem but that isn't always possible due to takeoff weight restrictions.

Sediment and other foreign substances can enter the tank from the fueling source. Again, this is rare if fueling is done by a reputable operator and not so rare if fueling from a can. Most aircraft fuel caps cannot be locked and sabotage is not as uncommon as one may think. That happened at a flight school I was associated with some years ago. A flight instructor broke up with his girlfriend and she decided to teach him a lesson by pouring sand into the fuel tanks of our low-wing fleet. It was detected before anybody flew because she wasn't very precise and spilled sand on all the wings. This was before most airports were fenced and access was limited, but it's still pretty easy to get onto the ramp at many smaller GA airports.

Contamination of solids can come from within the tank. I have seen sealants decompose and contaminate the fuel. There is also the possibility of biological contamination in terms of microbes, bacteria, and fungi. This is more common in jet fuel, but I personally encountered it once in avgas. Any debris is bad. A few large pieces or many small pieces can clog up the fuel system.

So what to do? Sample, sample, and sample some more. The first flight of the day must be preceded by taking a sample from each fuel drain. Sometimes that can be quite a task depending on how many fuel drains an airplane has. (Do you know for sure how many fuel drains your airplane has?) As the day progresses, each tank to which fuel has been added must be sampled before flight, but long enough after fueling to allow any water or sediment to settle. Water is detected because it is heavier than fuel and will produce a defined line between fuel and water in the sampler. If the tint of the fuel isn't clearly visible, sniff the sampler to make sure it contains fuel and not all water. I have had many occasions when the first sample or two contained not fuel at all, just water. Solids can generally be seen in the bottom of the sampler or by a general discoloration of the sample. It's a good idea to wipe off the fuel drains before sampling to avoid false positives from debris kicked up by tires.

Is it possible to get a clean fuel sample and still have water in the system? You bet it is! I have seen that more times than I can count. Sometimes the water hides in the tanks. Single engine Cessnas have been particularly prone to this due to the small dihedral in the wings. Most light GA airplanes have "wet wings" using an integral aluminum tank. Some airplanes use flexible bladders. A pilot also recently told me that he had the recurring water problem once and they found that there was a wrinkle in a fuel bladder providing a place for the fuel to hide. Some airplane manuals instruct us to perform a "rock and roll" procedure in which the airplane is rocked from side to side and tail-down to loosen any trapped water and allow it to reach the drain points. Conventional procedure is to keep draining till the sample is entirely fuel. I would suggest caution after clearing water. I would do some more "rock and roll" and then wait at least 15 minutes to give time for the water to find the lowest point before taking another sample.

So in summary, pilots should take every possible precaution to prevent contamination from entering the fuel system. Once it is there, no matter how slight, we must make sure it is out before flight.

Click here to take a short review course on aircraft fuel systems. (Opens in a new window.)

"Caution, wake turbulence." How many times have we all heard that? Lots of times? OK, how many times have we heard that and ignored the warning because we were busy with other tasks in the terminal area? How many times have we ignored the warning because we have operated behind large airplanes before and not encountered a problem?

I have been looking at accidents and incidents that occurred because of an encounter with wake turbulence. The vast majority are not really caused by wake turbulence but rather by pilot complacency. Sure, the airplane was upset by the wake, but the airplane got into the wake because the pilot had become complacent about the danger. This is perhaps the flight risk that is most likely to generate complacency. We hear the warnings frequently in the terminal areas shared by large aircraft. We usually neither see nor feel any evidence of the wake turbulence. It's like when our mothers warned us of the dangers of not eating our vegetables. We didn't heed the warnings and we still grew and felt fine.

Recall that the wake turbulence is found behind, below, and downwind of the generating airplane. These horizontal tornadoes can be deadly but they usually are relatively small compared to the airspace within the terminal area. They are also relatively short-lived, especially if there is substantial wind. That means that the odds are pretty good that we won't fly into trouble. It also means that each time a pilot ignores the risk and gets away with it the likelihood of ignoring the risk again is increased. There is no way to know if disaster has been avoided by only a few feet.

Now I must say that I enjoy gambling. But I only gamble with money and only with money that I can afford to lose. I will not gamble with my life or the lives of my passengers. A pilot is taking a life gamble when he or she does not take adequate precautions against the dangers of wake turbulence.

For a very brief (5 minutes) review course on the basics of wake turbulence avoidance, click here. (Opens in a new window.)

Accidents discussed in this section are presented in the hope that pilots can learn from the mistakes of others and perhaps avoid repeating those mistakes. It is easy to read an accident report and dismiss the cause as carelessness or a dumb mistake. But let's remember that the accident pilot did not get up in the morning and say, "Gee, I think I'll go have an accident today." Nearly all pilots believe that they are safe. Honest introspection frequently reveals that on some occasion, we might have traveled down that same accident path.

On September 8, 2008, at approximately 11:47 AM local time, a Diamond DA-20 collided with terrain near Pueblo, Colorado. The flight instructor and student pilot were seriously injured.

As they entered the traffic pattern for Pueblo Memorial Airport the tower controller instructed the crew that they were to follow a C-130 and informed them to be cautious of wake turbulence. With direction from the flight instructor, the student pilot turned and entered the downwind leg. The flight instructor recalled seeing the C-130 on a wide pattern and elected to lengthen the downwind leg and began to turn to final approximately 1 nautical mile past the runway threshold. Once the airplane was level on final, the student pilot was attempting to "keep it high," but noted that the glide slope indicators displayed "2 red/2 white."

The reference to "keep it high" refers to the need to stay above the flight path of the airplane generating the wake turbulence. The wake is always found below, behind, and downwind of the generating airplane. The reference to "2 red/2 white" refers to the on glide path indication of the Visual Approach Slope Indicator (VASI). That would put the training aircraft dangerously close to the C-130's wake since the large airplane probably flew the approach on the glide path.

At approximately 1/2 mile from the threshold, the airplane encountered wake turbulence from the preceding C-130. The airplane began an uncommanded pitch up, rolled left and then began descending. Full throttle and opposite flight controls were applied. Despite attempts to regain level flight by both the student pilot and the flight instructor, the airplane descended until it collided with terrain.

A review of tower radar returns revealed that the accident airplane flew 56 to 62 seconds behind and within 1/4 to 1/2 nautical miles downwind of the preceding C-130's flight path.

This is a classic case of either not understanding how to determine the location of the dangerous wake or being complacent about the perils of a wake turbulence encounter.

The National Transportation Safety Board determines the probable cause(s) of this accident as follows:

The student pilot's inability to maintain aircraft control due to an encounter with wake turbulence from a preceding larger aircraft. Contributing to the accident was the instructor pilot's failure to intervene.

Click here to read the full NTSB accident report.

Even though we live in an area not populated by poisonous snakes, my wife will absolutely come unglued at the site of any snake regardless of its size or pedigree. You can imagine her reaction last summer when our six year old grandson proudly displayed the live, seven-inch long garter snake that he had just discovered. I quickly analyzed the situation and realized that laughing would probably result in my sleeping in the hangar that night so I decided to calmly study the situation.

It was truly a demonstration of contrast. My wife was about to have a stroke from believing that somehow this reptile which, at about the size of a pencil, was going to devour or otherwise do away with this child. The grandson was just talking to the snake and announcing that he had just found a new friend.

In this case the snake was dark green and harmless. Had we been in another part of the country, a snake that size could have been the colorful but deadly coral snake. I am confident that the reaction of both grandmother and grandson would not have been different had the snake been deadly. Both grandmother and grandson were reacting not to the actual danger posed by the snake, but to their respective perceptions of the danger.

Yes, this does relate to aviation safety. The point of all this is that as human beings we do not react to the dangers in our world. We react to our perception of those dangers. The pilot who is trying to squeeze the last bit of camping gear into the airplane probably has the perception that flying a little outside the loading envelope is OK. The pilot with the bathroom scale weighing the passengers and the cargo probably has the perception that operation outside the envelope can be catastrophic. The pilot who engages in "scud running" probably doesn't have the perception of the dangers of CFIT or spatial disorientation.

Our perceptions of the possible problems we may face as pilots are based on our knowledge and experience. Think of our perceptions as being a filter. We see and hear lots of things before and during a flight. Our five senses pick up sights, sounds, sensations of feel, odors, and tastes. Perhaps taste is the only sense not needed for flight. Our perception filter takes each and every one in and processes it. Most sensations are quickly dismissed as normal. But the alarms go off when we see the traffic approaching, hear the miss in the engine, feel the buffet of the approaching stall, or smell something burning. The filter lets that information through for action to be taken. These of course are obvious problem areas. We need to educate our perception filter to recognize which situations are hazardous.

I know of a case in which a pilot ignored an indication of zero oil pressure while flying over a reasonably large airport at night. He continued another thirty miles to his destination and landed without incident. The engine seized while taxiing to the ramp. He saw the indication of zero oil pressure but he did not perceive the situation as critical.

Contrast that with the pilot who wrecked his airplane during a precautionary landing when he noticed that the fuel gage for one of two tanks was indicating empty. The fuel gage for the other tank was indicating three-fourths full. It turned out to be an indicator problem but even if one tank had been empty, he had plenty of fuel in the other tank to execute a safe landing at a suitable airport. He perceived a minor problem as being critical.

We can become safer pilots by educating our perception filters. We can do that by learning more about the systems in our airplanes, thinking through possible abnormal and emergency procedures before they are needed, reading aviation publications, attending safety seminars or webinars, and participating in the FAA Wings Pilot Proficiency Program.