Which Model?

Other than a few changes in engines and the addition of a rear window, the 172 hasn't changed much since the first prototype, a 170 converted from a tail wheel to a nose wheel.

Perhaps "evolution" best describes the gradual changes over the years. However, certain changes to the airframe and power plant are important enough to focus the shopping on certain years and models.

If you want to keep the purchase price as low as possible, look at the 172's built before 1968. The main reason for the lower price is the difference in the engine. It will be a Continental O-300 six cylinder instead of a Lycoming O-320 four cylinder. These Continental-powered 172's can offer good flying value if you're willing to accept a few compromises.

• With two additional cylinders the engine can be more expensive to maintain and rebuild. Parts can be harder to locate and more costly because the engine has been out of production for quite a while. The good news is that it shares components with the Continental O-200, which Cessna used in the 150.
• Although the Continental 300 is more smooth running than the Lycoming and plenty trustworthy, it doesn't share the same level of near-legendary status for reliability as the Lycoming 0-320 E series.
• The older airframes are "draggy" and noticeably slower. Expect cruise of around 100 knots. Not an issue unless you plan on traveling.

The major difference in the 172's built before 1963 is the missing back window. Not a big deal but a rear window is nice for looking around at the scenery, spotting traffic in the pattern and checking your tail control surfaces during run up. Another drawback is that the pre-1963 "fastback" airframe profile is immediately recognizable as an "old airplane" so it detracts a bit from market appeal. Another visual characteristic of the fastback airframe is the two-piece windshield.

1957 fastback with straight vertical stabilizer

1962 fastback, swept vertical stabilizer, pointy wheel pants

Airframes 1963 and newer (with the rear window) look enough like a new Skyhawk that most people can't tell one year from the next.

1963 model D - rear window, one piece windshield  introduced

1967 model H - the last Skyhawk built with the Continental engine

A nicely restored and repainted 1968 Skyhawk with '74-on wheel pants will look just like a Skyhawk built in the 70's, 80's, or 90's to the untrained eye.

In 1964 Cessna switched from manual flaps to electric flaps. Some will argue the virtues of manual flaps, which are worth knowing about:

• You can raise and lower them instantly, handy for touch-and-goes.
• If you overlook your airspeed indications on approach, the extra hard pull on the flap lever will alert you to the excessive wind load on the flaps
• Even though you don't need flaps to land your airplane, if you lose your electrical system, manual flaps still work
• Although the electric flap motors and jackscrews are reliable, the manual flaps run on stone-simple cables and pulleys, just like the other control surfaces.

Between 1968 and 1976 the differences in the Lycoming-powered Skyhawks are minor.

1968 model I ushered in the Lycoming. Wheel pants were still pointy

In 1971, beginning with the 172L, the main landing gear was changed from flat spring steel to tubular steel. The spring steel legs are flexible in the up-down direction but stiff in the fore-aft direction. This stiffness stresses the gear leg mounting structure, especially on rough strips. The tubular legs flex fore and aft, absorbing the energy of rough strip takeoffs and landings instead of transmitting the extra loads to the mounting assembly.

1972 model L - long dorsal fin introduced for added stability

In 1973, to further improve low speed stability, Cessna changed the contour of the wing leading edge. The difference is easy to see. It looks like a slight droop in the leading edge. It's called a camber cuff, often seen on STOL (short take off and landing) aircraft. However, on "serious" STOL aircraft the camber contour is more pronounced and the effectiveness is further enhanced by stall fences and other devices.

In response to superior cruise speeds from competing Piper and Grumman products, Cessna cleaned up the aerodynamics in 1974 and redesigned the cowl for less drag - all yielding a slight improvement in cruise speed. You can expect about a four knot speed advantage with the post-73 Skyhawks.

In 1974 Cessna also introduced the extended baggage compartment which opened up a significant amount of useful space in the tail cone. It's a great place to put things you want to keep out of sight and comes in handy when you need to transport something long with the back seat folded down.

Another minor improvement in 74 was the heater air inlet. I don't know if the pre-74 heaters needed improving but I can attest to the effectiveness of the '74 heater - it's excellent - even better than the heater in my '98 Toyota Camry.

1976 model M with the modern wheel pants and cowl

In 1977 Cessna made two blunders with the N model.

The first blunder was the paint. To speed up production, Cessna took some shortcuts with the paint materials and application process, contrary to the paint manufacturers advice. This caused corrosion to form underneath the paint but it took a few years for the problem to show up. The remedy is to strip the paint completely, remove and neutralize the corrosion and properly refinish the airplane. The paint problem affects all N models, so you need to watch for filiform corrosion on the N models, especially with aircraft stored near wet, coastal climates. This type of corrosion looks like irregular shallow bumps and clumpy blisters in the paint. You will know it's filiform corrosion when you puncture the bump and find white powder inside. The white powder is aluminum oxide, the equivalent of rust on steel. Corrosion can be serious enough to ground an airplane, even permanently.

The second blunder was the engine, ending up as a painful embarrassment to Cessna and Lycoming. For reasons related to simpler engine manufacturing and maintenance, Lycoming encouraged Cessna to replace the excellent, time-proven O-320-E2D with a new design, the O-320-H2AD. This engine change began with the N model. It didn't take long before N model owners discovered lifter and camshaft damage were common problems with the H engine, especially in cold climates. In a nutshell, the lifter face that rubs against the camshaft lobe was not broad enough to prevent chipping and flaking during the first few moments of engine warm up before the oil begins flowing. This event not only destroys the critical dimensions and contours of the lifter and cam, it also contaminates the oil with steel particles, causing other problems inside the engine. Cessna did a recall and enhanced the warranty. Lawsuits, airworthiness directives, the "T" modification to the crankcase, aftermarket oil spray nozzles, and a special lubricant (LW16702) emerged as remedies. To this day nothing has solved the problem entirely. The Cessna Pilots association recommends 100 hour lifter inspections for the H engines. Quoting the Cessna Pilot's Association technical document: "The purpose is to try and detect a failing lifter before it tears up the crankshaft."

How bad is the H engine? Depending on several factors, it seems the engine can be your sweetheart, your worst nightmare or somewhere in between. There are plenty of reports of H engines doing fine in temperate climates with regular flying and proper maintenance. Flight schools have typically run H engines to the 2000 hour TBO without any lifter or camshaft problems to speak of. However, I prefer keeping life simple so when shopping for my Skyhawk, I made an early decision to avoid the H engine and hold out for an E or D engine Skyhawk or an O-360 "SuperHawk" conversion. More on that later.

In 1981 with the model P, Cessna gave up on the troublesome H engine and replaced it with the D engine. The D engine is pretty good. It doesn't have the lifter-camshaft design flaws of the H engine but the valves can be less reliable than the E engine. During my airplane shopping I included D engine Skyhawks in my search but as you can see from the production numbers, there aren't as many of them around. Additionally, they tend to be priced higher but the airframe (other than cosmetics) is identical to the M and N models so the extra cost might be difficult to justify.

With those major differences out of the way, this table might be helpful to straighten out the confusing model letters. To keep the table easy to read I didn't list all the changes, just the distinguishing ones.

As just about everyone knows, Cessna interrupted production of all light planes in 1986. The Skyhawk production of that decade ended with the P model. There was a similar Q model, called the Cutlass. It was equipped with a Lycoming O-360 engine and Cessna produced them in small numbers between 1983 and 1986. The current Skyhawk is the R model, with a Lycoming O-360 engine. Other noteworthy 172 airframes beyond the scope of this website are the P172 Powermatic and the R172 Hawk XP. For further information on Cessna model differences, I highly recommend The Standard Catalog of Cessna Single Engine Aircraft, Jones Publishing, Inc., Jim Cavanagh, Kim Shields, and The Cessna 172, Tab/McGraw Hill, Bill Clarke.

Recommendations

If keeping acqusition cost to the minimum is the single most important issue, the Continental-powered fastbacks offer a lot of airplane for the money. Just be sure to take into account the effects of age on polymer and rubber-based parts. The countless rubber parts, including sections of fuel line, are typically hard or even brittle. They are difficult to get at and usually don't get replaced unless there's a leak. Wiring glitches and corrosion damage are often issues with the older airframes. A "best buy" in a fastback would be one that's had all the age-related problems fixed but the price doesn't reflect the considerable labor-hours involved. The slight step in cost to the rear window Continental-powered 172's isn't that much more and I believe the market appeal associated with the "modern" Skyhawk look will help retain the investment value.

Most Cessna 172 experts consider the M model Skyhawks the most desirable. Their reasons convinced me. I bought one myself and would enthusiastically rate the 172M as a "best buy."

• It's equipped with the Lycoming O-320-E2D engine, widely regarded as the best aircraft piston engine ever made.
• Modern aerodynamics, including the cleaner post-73 airframe (they cruise a wee bit faster) and the camber cuff leading edge for improved low speed handling
• 14 volt electrical systems, which opens the door to more electrical and avionics options than the 28 volt systems
• The handy extended baggage compartment introduced in '74
• The M's are the oldest model with a rear window, new style wheel pants and the long dorsal fin. Translation: A new-looking Skyhawk airframe at the lowest cost
• Prices are competitive. There is a fair supply of M's in the marketplace competing for buyers because Cessna produced over 4,800 of them

The N model Skyhawk with a high time engine can be a great value because it is a splendid candidate for a 180 HP "SuperHawk" engine upgrade. The N has the modern aerodynamics but it's value is depressed by the H engine stigma. By replacing the H engine with a Lycoming O-360 you eliminate the H engine problems (and stigma) and gain considerable climb rate, cruise speed and legal useful load. 

Two good companies offering well proven 180 HP engine conversion services, kits and STC's are Penn Yan Aero in Penn Yan, New York, and Airplains Services in Wellington Kansas.

Of course there's much more to learn about the history and models of the Cessna Skyhawk than I've covered here. The goal of this effort is to share the key facts I wish I would have known about when I decided to buy a Cessna 172 and discovered all the bewildering model letters to figure out.

I hope this information helps others in the same situation.