Operating Superstitions-Part Two

Last month's feature on the frustrating persistence of engine operating superstitions generated a number of comments from readers, most appreciating the ammunition to fire back at the Old Wives' Tale-spouting "experts" at their local airport. Several referenced other operating superstitions—engine and airplane—regularly being passed off as time-honored aviation truths and asked if they could be addressed.

In the spirit of reader service, we pulled up a handful of the most common. Here goes:

This one just won't seem to die. It may be because instructors didn't get good training and thus don't teach student pilots to routinely lean the mixture anytime the airplane is flying level. Also, on student cross countries, some instructors teach just going high enough to avoid hitting anything and tell their students not to lean the mixture below 3000 feet (the most common altitude quoted in my unofficial survey, with 5000 feet a distant second).

The other possible reason for the myth of full rich in cruise below an arbitrary altitude is that most POHs call for leaning the mixture during climb beginning at a given altitude and pilots have deduced, incorrectly, that such guidance also extends to cruising flight.

Busting the minimum altitude for leaning myth only requires a quick look at the POH sections that discuss cruising flight. The checklists direct the pilot to lean the mixture in cruise—without stating a minimum altitude. The cruise performance charts refer to fuel consumption with the mixture leaned according to the checklist—at all altitudes.

Cruising at full rich means burning expensive fuel at an excessive rate and leaving lead deposits on valves, valve seats and spark plugs at an accelerated rate. I've heard pilots who run full rich in cruise say they do so to make sure they don't improperly lean the mixture and damage the engine. That's an issue of education—the POH or engine manufacturer's operating manual gives guidance on leaning, although many call for operating at 50 degrees F rich of peak, which can lead to high enough cylinder head temperatures to reduce cylinder life. For guidance on leaning, we recommend John Deakin's excellent series of articles on engine operation right here on AVweb.

Cruising at full rich can have adverse consequences. As a 17-year-old private pilot, I was flying a Cessna Cardinal while a friend of mine followed me toward home plate in an ag airplane that the company I worked for had just purchased. He ran out of fuel after two and a half hours of flying because, as he put it later, we were only at 2500 feet and he couldn't lean the mixture that low. It took several months to repair the damage from the forced landing that resulted. Had he simply leaned the mixture, that airplane had about four hours of endurance and the accident would have been avoided as we were less than a half hour from our planned fuel stop.

I've often wondered where this one comes from, as there is no data to support it. I first looked for data correlating engine failures with power reductions following takeoff more than 25 years ago—and found there wasn't any. I've done the search from time to time since then. In addition, as part of researching the accident history for each type of airplane featured in the Used Aircraft Guide in our sister publication, Aviation Consumer, I've looked for a correlation between engine failures and the first power reduction and have found none.

NTSB accident data indicates that engine stoppages due to a mechanical malady are, happily, rare and that there is no phase of flight where they are more likely to happen. In terms of personal risk management, the data also show that if you have an engine stoppage, the most likely reason is that you've mismanaged your fuel.

When I hear that one, I wonder about the pilot's understanding of the forces acting on an airplane in flight. It's pretty basic, if the turn in coordinated, the resulting forces don't cause the fuel to flow one direction or the other in the tanks. If the turn is not coordinated, the fuel will slop toward the same side as the ball that is telling you about the quality of the turn.

Oxygen is oxygen. It's an element on the periodic table. There is no aviation-grade. If you're servicing your airplane's oxygen system and need to buy the gas in bulk, go to a welding supply store where the price tends to be the lowest. One of John Deakin's AVweb columns addresses the subject in detail.

For some pilots, TBO seems to be a magic number for engine operation. They are adamant that running an aircraft engine past TBO is (your choice) a violation of the FARs, will void your insurance, increase the risk of engine failure, make the subsequent overhaul more expensive and/or void the manufacturer's warranty.

For Part 91 operators (and most Part 135 operators), none of that is true. TBO is a recommendation, not requirement, made by the engine manufacturers. The FARs do not require overhaul at TBO for Part 91 operators. While it is required for Part 135 operators, the vast majority take advantage of procedures set out by the manufacturers and the FAA to extend that time, so they are not overhauling at TBO.

Your insurer doesn't care about your engine's TBO—from a mechanical standpoint, its only real interest is that your airplane is in annual. Operating your engine past TBO won't void your insurance—flying your airplane after the annual has expired probably will (unless you're on a ferry permit and have gotten specific agreement from your insurance company).

There is no data that indicates that operating a properly maintained aircraft engine past TBO increases the risk of it failing. The parts of the engine most likely to go south on you are the parts you ordinarily repair or replace on condition such as cylinders, mags, ignition harnesses, spark plugs and so forth. An engine overhaul targets the "bottom end" of the engine, the crankshaft, camshaft, crankcase, gears and bearings. When they start to go bad, it's usually not in a catastrophic fashion and an increase in wear will show up during oil analysis before the risk of failure starts to ramp up.

The data do show that there is an increased risk of engine failure in the first 50 hours following an overhaul. This infant mortality is due to the very real risk that, as with surgery, when the engine is opened up for major work, something will be done wrong that leads to a failure soon after the overhaul. Because of the infant mortality risk, overhauling your engine at TBO rather than on condition probably increases you risk of catastrophic engine failure.

Running the engine until its condition indicates it is time for overhaul rather than doing so at TBO does not increase the cost of the overhaul. The things that impact the overhaul cost are an unserviceable crankshaft or a cracked crankcase. Neither of those items are any more probable for an engine operated beyond TBO.

Void the warranty? What warranty? By the time your engine has reached TBO, any warranty on it has probably long since expired.

This one may finally be dying out. Rightfully so. It held that if you climbed a few hundred feet above your cruising altitude and then dove down to the desired altitude, the airplane would reach some magical equilibrium allowing it to cruise a few knots faster than if you just leveled off at altitude and then set cruise power.

While it sounds attractive, it just ain't so. You can prove it for yourself any time you wish. Do whatever you want to set up cruise operation at a given power setting. Wait 10 minutes. No matter what you have done, the airplane will be at the same speed.

This one states that if you lose an engine on a twin, you should never turn toward the dead one or you will lose control. It just isn't true. Every turn has two halves: half will be a roll toward the dead engine and half will be a roll away from the dead engine. The only difference is which comes first. So long as you keep the speed above Vmc when operating single-engine, you can turn toward or away from the dead engine.

Again, not so. Fewer than 15 percent of ditchings, fixed or retractable gear, result in fatalities. At least 85 percent of the time, everyone gets out of the airplane.

The fact is that ditching near shore may be the best option depending on terrain if you're faced with a forced landing and water is available. Ditching should not be rejected out of hand because of a myth about its safety. AVweb's editorial director, Paul Bertorelli did extensive research into ditching safety and wrote what is widely regarded as the definitive article on the subject.

Operating superstitions can be more than just silliness repeated as aviators swap stories over coffee—they can be dangerous. Running out of fuel because you flight planned for the fuel consumption in the POH and than cruised at full rich isn't any fun. Nor is a catastrophic engine failure following an overhaul performed just because the engine hit TBO. It's best to operate relying on data, not superstition.

Rick Durden is a CFI and ATP with type rating in the Douglas DC-3 and Cessna Citation. He is the author of the just-released Volume 2 of The Thinking Pilot's Flight Manual or, How to Survive Flying Little Airplanes and Have a Ball Doing It, Volumes 1 and 2.