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Then And Now: Spiral Instability

Technology and training have changed in the last 50 years, thank goodness

Like many of you, I took my initial flight training in extremely basic airplanes—read, “cheap.” The first was a re-engined, 85 hp Piper J-3 Cub on skis, but the second was even more basic, if that’s possible. It was, in fact, several steps below a Cessna 150.

That airplane was a Champion Tri-Traveler, one of the first planes Champion Aircraft built after it acquired Aeronca in 1956. The Tri-Traveler was essentially an Aeronca Champ fitted with a nosegear and a 90 hp engine.

That was quite a few years ago, when nosewheels were still something of a novelty for general aviation. The Tri-Traveler proved that Champion Aircraft didn’t know quite what to do with them.

I wasn’t wild about the Tri-Traveler, but it was the cheapest thing on the flight line at the time, and money was a major constraint. At the same time, I have to admit that the Tri-Traveler was one of the most docile and forgiving airplanes I’ve flown. (I later got my license in a Piper Colt, another minimum machine.)

I hope this doesn’t generate hate mail in this age of $120/hour Cessna 152s, but I seem to recall that the Tri-Champ (as it came to be known) rented for $6/hour (wet) and the instructor charged $3/hour. If memory serves, the Private Pilot flight test cost $25.

Even if you were a slow learner (me) and demanded 58 hours to earn the private (me, again), you could spend about $600 and walk away with a license to fly, or, at least, learn to fly. Even given inflation, that’s still quite a deal.

I bring it up because the Tri-Champ was, above all, ridiculously stable, and that helped to reinforce one of the great private pilot myths of all time.

Tell me you’ve never heard this advice or something resembling it from the instructor’s seat. “OK, now you’re over-controlling. This is an easy airplane to fly. It doesn’t require heavy control inputs in any axis. In fact, it has positive stability. It will fly better than you will. If you trim it properly in a bank or dive and let go of the stick, it will eventually recover to straight-and-level flight all by itself.”

The derivation of this particular brand of nonsense was Civil Aviation Authority Bulletin 32, and instructors took that advice as gospel. After all, it came from the Federal government, so how could it possibly be wrong?

Apparently, not many instructors took the time to test the premise, or they would have discovered in one test that most aircraft of the time were not that stable.

In fact, hardly any aircraft (if any at all) have the kind of positive stability necessary to fly themselves out of a date with disaster. Most of the time, the CG is constantly changing in flight as the aircraft burns off fuel, and lateral stability is especially susceptible to even a moderate wind gust.

PHOTO: “N79849” by wiltshirespotter – CC BY-SA 2.0/Flickr

The myth was exploded early on when the prestigious Cornell Aeronautical Laboratory of Ithaca, New York, suggested that lack of spiral stability in “the majority of general aviation airplanes…means that the tendency for the airplane to maintain a constant wings-level attitude, if left unattended, is at best marginal.” The Cornell study suggested “the aircraft will enter a turn in which the angle of attack slowly increases and the nose slowly drops with a resultant increase in airspeed…Once the spiral develops (unattended), the airplane will eventually fly into the ground in a spiral dive….”

The military discovered the truth of the stability argument in the 1920s, determining that not only were airplanes inherently unstable but also that pilots were similarly challenged. The Army experimented with both pilots and pigeons to see if either had any inherent stability sense when flying blind.

The pigeons, fitted with tiny blindfolds, one assumes, did stalls and spiral dives trying to gain control. “Finally, holding their wings in a high dihedral (attitude), they descended to the ground in much the same manner as a parachute.”

Predictably, pilots did even worse. They were blindfolded and tested in cars and boats on the ground and in water, and the inevitable result was a circuitous course and then loss of control.

Today, the inherent inability of light planes to recover from a loss of control remains the biggest risk factor. The FAA has been fighting this battle for years, investigating accidents in which pilots either cheat and attempt to operate IFR without proper training or do so without bothering to earn the rating in the first place. The result of such attempts is nearly always predictable and usually earns a place in the local newspaper the following day.

In fairness, accident statistics suggest that many pilots stumble into IFR weather accidentally, get trapped on top, fail to make the critical 180-degree turn when the weather goes down, become disoriented at night or make a combination of other mistakes that expose them to unnecessary risk.

The FAA studied all IFR accidents between 1983 and 2013 and discovered that, in 533 cases, non-instrument-rated pilots were at the controls. These accidents resulted in 955 fatalities.

For their part, aircraft manufacturers have been trying for years to develop automatic systems to guard against neutral or negative stability and spatial disorientation.

Back in the mid-1960s, Mooney was the first manufacturer to offer a safety margin for pilots operating in accidental IFR. The company began installing Brittain wing levelers as standard equipment in all of its airplanes.

The Mooney/Brittain system, known as Positive Control (inevitably, PC), was different from most autopilots in that it worked in reverse. It was on full-time unless you disabled it. It had only one function—level the wings.

There was a pressure-sensitive switch on top of the yoke’s left ram’s horn that disabled the wing leveler; otherwise, the system would stubbornly maintain level flight continuously. (For better or worse, PC didn’t offer any automatic pitch control.)

My current Mooney had the PC system installed when I bought it, and like so many other IFR-rated Mooney owners, I disliked it. In order to maneuver, you needed to press the button and hold it, not exactly my idea of fun flying. You could overpower the servos, but it required considerable yoke pressure.

Accordingly, I taped the button down to disable the system completely on the premise that if I needed half an autopilot, I could simply remove the tape. At one Mooney fly-in with 32 airplanes in attendance, I counted 17 older Mooneys (pre-1976) with PC installed on the ramp, and 15 of them had the override button taped down.

More recently, I flew a new Cessna TTx in Wichita, and Cessna had its own, less-obtrusive and considerably more sophisticated stability augmentation system installed. Part of the Garmin G2000 system in the TTx, the cleverly named Electronic Stability Protection (ESP) keeps the airplane out of trouble whether the autopilot is on or not. ESP is far more friendly and permissive than Mooney’s PC system—Mooney today has ESP in its airplanes, too.

With ESP monitoring your every move, roll and pitch limits are more restricted than the airplane’s certification parameters but don’t constrain normal maneuvering. If you exceed 17 degrees of pitch up or 19 degrees of pitch down, ESP will gently counter by recovering to within its prescribed tolerances.

Similarly, try to roll past 45 degrees of bank, and ESP will correct the bank angle back to 30 degrees, though, as with the Mooney, you can overpower the autopilot, though it takes some force to do so.

Before you object and insist this full-time protection is something George Orwell might have envisioned, consider how many pilots this could save each year who accidentally stray into IFR conditions.

The Cirrus SR-20 and SR-22 (which today boast ESP as well) are the world’s most popular singles. They employ an innovation that’s the simplest of all the current stability augmentation devices. Aeronautical engineer Jim Griswold, perhaps most famous for leading the team that designed the world-beater Piper Malibu in 1983 and, later, the Questair Venture, designed the Cirrus control system with a simple feature to improve stability and recovery.

Everybody loves the Cirrus, and it’s not hard to understand why. Like the Cessna TTx, Cirrus mounts a side stick on the left arm rest, but apparently the company recognized that left hands may not be as strong or adept as right hands.

Perhaps to that end, Griswold fitted the Cirrus side stick control with a break-out force, a simple mechanism that’s spring-loaded to return to neutral if the pilot merely releases pressure. This operates in both pitch and roll, and it’s, once again, a passive system.

By themselves, clever design features such as those on the older Mooney, Cessna TTx and Cirrus models won’t eliminate loss of control when pilots stumble from VFR into IMC. That won’t happen until pilots fully appreciate that instrument flight is a skill totally separate from VFR operation.


As of January 1, 2016, Senior Editor Bill Cox has logged 15,100 flight hours in 321 types of aircraft. He also holds 28 world city-to-city speed records, has made 211 international delivery flights, and owns and flies a LoPresti Mooney. You can email Bill at flybillcox@aol.com.


Check out more Cross-Country Log flying stories from ferry pilot and Senior Editor Bill Cox.

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