Everyman's Airplane

Anatomy of a Success

May 1, 1992


Deep within the breast of every pilot beats the conviction that flying is glamorous and inspirational — a high-tech indulgence capable of making us transcend the ordinary. So what accounts for the immense popularity of the Cessna 172/Skyhawk series of airplanes?

After all, the Skyhawk comes off as the most prosaic of light aircraft. It's plain Jane at its very plainest. The essence of ordinary. But take a look at the numbers — more than 36,000 sold, over an extraordinarily long, 31-year production run — and the Skyhawk comes out way ahead of all its competitors. In fact, the 172/Skyhawk is the world's most popular single- engine airplane.

It's been the step-up airplane for generations of pilots who have bided their time in two-place trainers. The airplane of choice for the family man — or woman — as an inexpensive cross-country machine capable of cruising at about 115 knots and carrying four people 600 nautical miles.

It's also shown great versatility. Skyhawks have served in roles ranging from bushplane to military service, wearing everything from skis to floats to munitions hardpoints. Pound for pound, dollar for dollar, the Cessna 172 series has perhaps the greatest utility of any single-engine airplane.

Clearly, Cessna must have done something right.

That something began in 1954, with Cessna realizing that its mainstay four-place single, the 170, was beginning to lose market share to the Piper Tri-Pacer. Suddenly, the Tri-Pacer began to look like the wave of the future. The C-170 had been in production since 1948 and had conventional (taildragger) landing gear. The Tri-Pacer, introduced in 1951, had a pilot-friendly, tricycle-gear arrangement.

Cessna engineers were doctrinaire in those days and at first scoffed at the Tri-Pacer's landing gear design. Conventional landing gear could handle soft or rough fields better than tricycle designs, they said. The gear created less drag and cruised faster, to boot.

The gospel at Cessna was that the 170 would stay. The plan was to bring out a new Model 170 (the 170C) for 1956, one with a larger horizontal stabilizer and more up-elevator authority. It seems that the 170 needed more elevator power in order to make three-point, full-flap landings easier to perform at forward-CG loadings. The C-170C was also to have a new engine — a derated, six-cylinder, 145 horsepower Continental 0-300-A of the type used in the 170B. In the 170B, this engine was rated at 155 hp.

But change was in the air. On their own, some Cessna engineers came up with a nosewheel installation for the C-170 and even went so far as to make a mock-up. But a manager's weekend stroll through the shop put a swift, if temporary, end to the nosewheel experimentation. A Monday-morning memo from the vice president of engineering ordered that the mock-up be destroyed. It wasn't. Instead, the conspirators hid it.

By spring of 1955, the Tri-Pacer once again became the subject of debate. Its brisk sales persisted. At one point, Cessna even rented a Tri- Pacer to evaluate the airplane firsthand. Finally, with some regret, Cessna management gave a secret authorization to develop a tricycle-gear version of the C-170C. Out came the hidden, once-forbidden mock-up.

Two months later, on June 12, 1955, the first trigear 170C had its maiden flight out of an abandoned sod strip in Kingman, Kansas — 50 miles away from Wichita and prying eyes.

You might think that something as simple as installing a nosewheel would pose no great challenge. That's what Cessna's engineers thought, too. But surprisingly, this was not the case. First, the airplane's center of gravity had to be lowered so that strong or gusty surface winds would not throw the airplane up on its nose and wing. Adding a nosewheel to the C-170's main gear resulted in an empennage that stood tall — tall enough for gusty air to lift the airplane's tail. Solution: Shorten the main gear and lower the entire airplane.

Other associated problems quickly became evident. A lowered airframe meant less propeller clearance, so an air/oil oleo strut was developed to dampen the airplane's bobbing motions as it taxied over rough surfaces. And what if the nosewheel bogged down during taxi? The engine's high thrust line would help dig the nosewheel in even deeper if a pilot tried to power himself out of trouble. Solution: Enlarge the horizontal stabilizer and elevator so that up-elevator forces would be enough to help lift the nose.

Finally, what about the nosewheel's steering linkage? Then-chief of Cessna flight testing and aerodynamics, William D. Thompson, took a dim view of the Tri-Pacer's arrangement. In the Tri-Pacer, the nosewheel is tied directly and continuously to the rudder control system. Whether on the ground or in flight, a push on a Tri-Pacer's rudder pedal brought both a rudder and a nosewheel deflection. In flight, this meant degraded directional stability due to the drag of the deflected nosewheel.

According to Thompson, project engineer Obed Wells found a solution based in part on the self-centering mechanism used on the Cessna 310's nosewheel, which at the time was one of Cessna's only two tricycle-gear designs (the T-37 military jet trainer was the other). Wells devised a centering cam that rode against the strut's collar, which was located above the torque scissors assembly. On the ground, rudder pressure would activate both the rudder and the nosewheel steering. But once airborne, the strut extended, the collar dropped down to meet the cam, the nosewheel steering disengaged, and the nosewheel automatically centered. Meanwhile, the rudder remained engaged. It was a brilliant solution; steering authority on the ground was not compromised, and directional stability in the air was enhanced.

By the end of 1955, the landing gear had been thoroughly tested. Cessna insisted on using pilots with no previous experience flying tricycle- gear airplanes and told them to experiment at will. The orders were to try as many takeoff and landing techniques as possible and not to pamper the airplane in the process. With a few exceptions (nosewheel vibration, strut noise, and tire wear after hard braking), the airplane came through with flying colors. The only major problem had to do with the tall tail and occurred when taxiing downwind, then using hard differential braking to turn sharply. Many times, pilots of early 172s would taxi to the run-up pad, pivot into the wind by locking the brake on one wheel, and be promptly blown over. In spite of the chopped main gear and the lowered airframe, many airplanes were nosed over on a wing tip after this kind of maneuver. Thus the origin of the taxiing diagram — the one showing the correct control positions for all wind directions — in the pilot's operating handbook of every Cessna single-engine airplane.

In a fit of sloganry, Cessna's marketing department dubbed the tricycle gear "Land-O-Matic," then touted the advantages of the mechanically actuated "Para-Lift" flaps. The windshield offered "picture window" visibility.

The 172 was a hit from the start. Pilots loved the airplane for its easy landings and improved ground handling. The airplane's light, well- balanced control feel made it one of the most maneuverable of the emerging flock of competing four-place, tricycle-gear singles.

There was — and still is — one handling quirk: In a forward slip with flaps extended, a 172 could pitch nose down, violently enough to push a pilot against his seat belt. This is caused by an upturned aileron's reducing the normally strong downwash of air over the horizontal tail. Though puzzling in its infrequency, Cessna saw the potential for trouble. Consequently, a placard is installed near the flap control: Avoid slips with flaps extended.

Apart from this rather rare, mysterious anomaly, the 172 proved virtually viceless, practical, and economical. Pilots accepted the nosewheel's 4-knot cruise penalty (the 1956 172's maximum cruise speed was 117 knots, compared to the proposed 170C's 122 knots) and bought 1,178 172s in the first year of production — at $8,750 a copy.

The first design change came with the 1960 C-172A, when the original square tail was replaced with a swept one. Cessna called it the "Flight Sweep" tail. While the swept tail might have looked modern, this change carried a price. The swept tail weighed more, and rudder effectiveness was reduced slightly, as was crosswind landing capability and overall directional stability. Other 1960 developments were float certification and the first use of the "Skyhawk" name for deluxe versions (which featured IFR instrument packages).

Still bearing the ground-upset problem in mind, Cessna shortened the 172's landing gear by 3 inches with the 1961 C-172B model. This also made it easier to board the airplane. The engine mounts were lengthened by 3 inches (to increase propeller ground clearance), and a new cowling (for improved engine cooling) was also introduced. One effect of all these changes was to alter the view from the pilot's seat. Prior to this time, visibility over the nose was exceptional, and the pilot rode tall in the saddle. But from 1961 on, forward visibility continued to be incrementally degraded, however slightly. A baggage door and a pointed prop spinner were other 1961 introductions.

The tinkering continued in 1962, with the addition of fiberglass wing tips and redesigned wheel fairings. In 1963, a rear window — a.k.a. "Omni- Vision" — and a one-piece windshield were the most apparent changes. An 8-inch increase in horizontal tail span was less noticeable but greatly aided pitch authority in the landing flare. This was necessitated, in part, by the airplane's 100-pound increase in gross weight — to 2,300 pounds.

The "Powermatic" versions of the 172 and Skyhawk also came out in 1963. These rare birds (only 68 were built) came with the same GO-300-E, 175- hp, geared Continental engines and constant-speed propellers that were used in the C-175 Skylark. The cruise speed of these P172D Powermatics was only slightly better, by 7 knots, than the standard 172s. The Powermatic never found its niche between the standard 172 and the faster 182 and was in production for just one year.

Another high-powered variant of the 172 was the R172, more popularly called the "Reims Rocket." Built by Cessna's French partner, Reims Aviation, the Reims Rocket has a 210-hp Continental IO-360-D engine and a constant-speed propeller. It was first tested in 1963; it later enjoyed a 590 unit production run from 1968 through 1981. The Reims Rocket also served as the basis for the 195-hp Hawk XP, designated the R172K, which was in production from 1976 to 1981. The Rocket was also the foundation behind the military version of the Cessna 172, the T-41. The T-41s, called "Mescaleros" by the U.S. Army, served as trainers and utility/ support vehicles during the Vietnam era. A few still survive as primary military trainers, some serving at the U.S. Air Force Academy. In all, 783 T-41s were built between 1965 and 1975.

The 1960s saw a continued, steady progression of improvements to the 172, improvements that cinched the 172's dominant market share for all time. By 1968, the airplane had a panel in the now-standard "T" configuration, center-mounted avionics, a shock-mounted cowling, a 60-ampere-hour alternator in place of the earlier generator, and electric flaps. The most radical change, however, was the decision to switch to a different engine.

In the mid-1960s, Cessna began to contemplate a snazzier replacement for the 172. This was to become the Cessna 177, or Cardinal, series. (Although the original plan was to give the first Cardinals a 172J designation.) To power the Cardinal, Cessna chose the four-cylinder, 150-hp Lycoming 0320-E2D engine. The company was so convinced of the Cardinal's success that it ordered 4,000 of the 0-320s. Needless to say, it came as quite a shock when, in 1967, the prototype Cardinal missed several performance targets using the 0-320.

The 172 was the big beneficiary of this miscalculation. With thousands of 0-320s on hand, Cessna decided to try using them with the C-172I, beginning in 1968. It turned out to be a stroke of genius. The 0-320s ran smoother and cooler than the old 0-300s, were lighter, and, with four cylinders, less expensive to maintain and repair. To accommodate the new engine, 1968 172s had redesigned cowlings and new engine mounts.

Cessna went ahead and introduced the Cardinal that same year, but the 0-320-powered model lasted just one season. Subsequent Cardinals went on to have 180-hp Lycomings and retractable gear. Meanwhile, the 172 soldiered on.

In 1971, steel-tube main landing gear replaced the flat-spring gear used in previous models. The idea here was to eliminate the kind of landing gear bulkhead damage that hard landings and side loads could impart with the spring gear. This model also saw the landing light moved from the left-wing leading edge to the nose cowling. Overhead skylights were also offered for the first time.

The following year, the 172's dorsal fin was extended all the way forward to the rear window. This helped virtually eliminate the nose-down pitch problem in sideslips and made the airplane much more spin resistant. According to Thompson, in his book Cessna Wings for the World: The Single- Engine Development Story (published by Maverick Publications, Incorporated; Post Office Box 5007; Bend, Oregon 97708), Cessna received complaints from some flight instructors because of this characteristic. Demonstrations of fully developed (three-turn) spins were difficult to perform; the result was usually a spiral.

With the 1973 Model 172M, yet another aerodynamic change was introduced. In a modification that borrowed heavily from a design developed by the Robertson Aircraft Company, the wing leading edges were drooped. This produced an increased stalling angle of attack. While most Cessna singles stall at about a 12-degree angle of attack, 172s with the drooped leading edges stalled at 15 to 18 degrees. According to Thompson, the drooped leading edges produced no noticeable improvements in performance. In fact, the drooped leading edges could cause a problem: a sudden drop of a wing at the stall. Other changes for 1973 were a 1-inch reduction in propeller diameter, a locking baggage door, and the use of the Skyhawk name for all 172s. The deluxe versions were now called "Skyhawk IIs." To those in the know, the 172M is often considered one of the most desirable of the entire series.

As 80-octane fuel was phased out in the late 1970s, Cessna chose the 160-hp Lycoming 0-320-H2AD engine for the 1977 172N models. While these engines accomplished one goal in that they were designed for the more common 100LL fuel, they fell woefully short in another. Yes, the extra 10 hp gave the Skyhawk four more knots in cruise and an 1,100-foot boost in service ceiling, but the engine's camshaft and cam followers suffered from inadequate lubrication.

An expensive airworthiness directive was issued — AD 77-20-7 — that required an inspection of the camshaft and replacement of all valve lifters. Another AD required oil pump impellers to be replaced. Yet another called for the replacement of the crankshaft. At one point, Cessna recalled all affected Skyhawks in Operation Blue Streak, a program that prorated the engine warranty up to TBO and paid for certain major repairs. But the damage was done. All these problems made Skyhawks with the -H engines the least desirable.

In 1981, the -H engine was replaced by the Lycoming 0-320-D2J, which also produced 160 hp. Gone were the lubrication problems, and gross weight was upped to 2,400 pounds in the bargain.

Another Skyhawk offshoot was the 172RG, or Cutlass. This was a retractable-gear version with a 180-hp Lycoming 0-360 engine and a constant- speed propeller. It was in production from 1980 to 1984. More than 1,100 Cutlasses were sold.

From 1981 to 1986 (the last year of production), the Skyhawk was designated the 172P, and improvements were basically creature comforts. Air conditioning became an option in 1981, and thicker windows, side panels, and insulation provided more soundproofing.

Flap travel was reduced from 40 degrees to 30 with the 1981 Skyhawk, in response to a spate of handling problems during go-arounds. If a Skyhawk's flaps were retracted during a nose-high, full-flap, go-around configuration, the airplane showed an untoward tendency to stall or sink rapidly. Reducing the flap travel kept pilots out of trouble by minimizing drag during such critical configuration changes. Another early 1980s aerodynamic improvement was a slight change in the horizontal tail's angle of incidence, an alteration that was advertised as improving the airplane's pitch authority. As for speed, the 172P's maximum cruise reached 123 knots, the fastest of all the 172s.

According to Cessna, unreasonable product liability awards brought down the Skyhawk — and all other Cessna piston aircraft — in 1986. The corporate view was — and still is — that the success of airplanes like the Skyhawk is now a curse. Just the thought of more than 36,000 Skyhawks must be enough to churn the ulcers of Cessna's legal department. So many airplanes, so many pilots, so many passengers, and so much flight time, all of it conceptualized as potential disaster. How times have changed.

It's been a long time since the burgeoning, innocent days when Land-O- Matic gear drew excited praise, and a self-centering nosewheel was big news. It would be nice to see those days return.

If and when they do, we'll be sure to see more new Skyhawks. Officially, Cessna has said that production of the Skyhawk and other singles will resume as soon as the laws governing product liability are reformed.

Let's hope they are. The huge fleet of Skyhawks is aging rapidly, and reasonably priced new airplanes are needed to replenish it. Apart from continuing to fulfill the need for practical four-place airplanes, the production of new Skyhawks could also stimulate a growth in the pilot population — the way it did back in the good old days.

That's something that Cessna should never forget. The venerable Skyhawk's phenomenal sales achievement formed, in large part, the financial base for Cessna's growth and later diversity. Without Skyhawks, today's fleets of Citations might never have been.

Where the Trouble Is

How Skyhawks crash


With just over 24,000 Skyhawks still flying, it should come as no surprise that the airplane shows up so frequently in accident reports. Any vehicle exposed to a good deal of use is bound to experience mishaps, and the Skyhawk — forgiving though it may be — is no exception.

Questions arise. What kinds of accidents are Skyhawk pilots most likely to have? And how does the Skyhawk compare with other popular fixed-gear singles?

The AOPA Air Safety Foundation's accident database was able to provide some insight on these issues. For the years 1982 through 1988, a computer sort was ordered from the database. We asked for the total number of Skyhawk accidents, plus all those experienced by the Piper Tri-Pacer (the impetus for the 172 in the first place), the Cessna 170 (the 172's predecessor), and the 150- and 160-hp versions of the Piper PA-28 Cherokee (the 172's strongest competitor). In addition, we asked for the top five leading accident causes for each of these airplanes.

As expected — because there are more of them — the Skyhawk had the most accidents: 1,610 over the seven-year period. Coming in second were the Cherokees, with 1,134 accidents, followed by the Tri-Pacer with 253 and the 170 with 180.

How did Skyhawk pilots go wrong? Again, no surprises. The biggest cause of Skyhawk accidents was the pilot's loss of control while landing in crosswinds, gusty conditions, or tailwinds. This was also the biggest cause of C-170 accidents.

In second place was low-level flying, terminating with collisions into terrain or obstacles. Third was loss of directional control during landing. Fourth was collision with obstacles after a late or delayed go-around. Fifth was fuel exhaustion.

The Tri-Pacer and 170 had remarkably similar profiles. The few exceptions include the Tri-Pacer's third-place cause (power loss for undetermined reasons) and the Cherokee's first-place accident cause — fuel exhaustion. Fourth and fifth place for the Cherokees was landing short and landing long, respectively.

This all proves that some things never change. Whether the wheel's on the nose or the tail, most accidents occur during the landing phase with a loss of directional control.

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