Q&A: Resealing Windows, “Flashing the Field” on a Generator

Q&A: Resealing Windows, “Flashing the Field” on a Generator


Is there a way to reseal the rear window of my Cessna 172 without removing the window? My airplane was recently in not one, but two, torrential downpours at my airport. I discovered leaks around the rear window and a mega leak around the baggage door.

I can easily repair the seal around the baggage door, but I’m wondering if there is a shortcut for the rear window repair. Being a desert rat, I rarely venture into rainy areas and I don’t want to spend a lot of money on the fix. Is there an easy and inexpensive way to accomplish this?



Hi Arlene,

Well, maybe. Part of the problem is that all Cessna windows “float” in between a couple of flanges. 

On your Cessna 172’s window, the skin of the aft fuselage forms the outer flange. A riveted piece inside the airplane forms the inner flange.

Initially, all windows had a strip of felt installed around the edges. Any space or gaps surrounding the window(s) were cushioned with putty. In the old days, Cessna used zinc chromate putty. 

Time takes a toll; the felt begins to compress, the putty migrates and the window becomes loose in the flange, allowing water to enter the fuselage. When it’s been years since a windshield installation, it’s not unusual for floating windshields to clunk from movement when taxiing over rough ground. 

Today, Cessna uses windshield sealant tape. You can look for Cessna Part No. U000927S. It’s oftentimes listed as “Sealant,” “Tape” or “Perma Gum.” It’s a non-hardening sealer and is used along with felt.

I’ve tried various methods for resealing rear windows such as yours.

The quickest and least expensive method is to use semi-flexible RTV sealant found in hardware stores. Try to find a clear, cold-weather, high-adhesion sealant. 

First, clean the window with isopropyl alcohol. Clean the metal around the edge of the window. Next, mask around the edge of the window and the edge of the metal. You want to lay the tape about one-quarter inch from the edge of the window and one-quarter inch from the edge of the metal. This will create a one-half inch gap between the tapes. This gap should be evenly spaced along the window/skin joint.

While wearing examination gloves, squirt a bead of the sealant where the metal and window meet, then smooth it out to the tape edges using your fingers. Have plenty of rags close by because you’re going to be removing some sealant as you smooth out the bead. You’re looking to create a smooth, not-too-thick coat of sealant across the metal/window gap. 

After it looks good, pull the tapes, being careful not to spread the sealant or drop any on the back window. If you do, use isopropyl alcohol to clean up.

Having said all that, it’s not likely that this fix will seal out moisture for more than a couple of years, simply because the window will continue to move in the flanges. 

A more expensive and longer-lasting trick uses PPG Aerospace’s P/S 890 Class B-1/2. This two-part sealant is more durable (and about five times more expensive) than RTV. It is a brown color. The same prep method is used.

Regardless of the method you elect to use, it’s very important for Cessna owners to understand that sealing the back window prevents corrosion. Here’s why. When water leaks into the aft fuselage of any Cessna single, it saturates the carpet. Later, when the sun comes out, that moisture in the carpet evaporates and rises. Some of it condenses on the inner skin of the upper fuselage, above the headliner. This moisture, in contact with bare aluminum, is one of the components needed for the formation of corrosion. 

Cessna didn’t paint the inner skins of its single-engine airplanes. One of the most likely places to find corrosion on a Cessna single such as yours is on the inner side of the upper fuselage skin.

Happy flying,




Hi Steve,

I fly a Cessna 140 that I have owned for six years. It’s has served me well and fits my needs perfectly. 

Recently, I joined with five other local Cessna 140 owners for a breakfast flight to an airport about 100 miles away. There are few things I love more than an early-morning flight; except maybe a late evening flight in my little 140.

My wife came with me because we are avid antiquers and had been planning to visit this little out-of-the-way town to look around and shop at the stores.

The others had left by the time we got back to the airport for our flight home. After starting my 140, I saw that the ammeter needle was deflected to the “not good” or discharge side. 

I taxied back to the tiedown area, without a plan. I didn’t know what was wrong or how we were going to get home.

I opened up the cowl to look around and before long, an older gentleman appeared and asked what was wrong. (I’m 72, so when I say older, I know what I’m talking about.) I told him the ammeter was going the wrong way.

He looked under the cowling for no more than two seconds, told me he would be right back with a tool to get me going, and within a couple of minutes showed up with a piece of wire with alligator clips on each end. 

He told me to get in and turn on the master switch. I didn’t see what he was doing, but after a few seconds, he secured the cowling and told me to try it. 

I started the engine and lo and behold, the ammeter was showing a charge. 

My question is: What did he do? 

140 Flyer


Hi Flyer,

Your mystery magician and airplane fixer performed a task called polarizing the generator, or “flashing the field.” 

The Cessna 100 series service manual states, “A generator of the type used on aircraft must maintain a residual magnetism in the pole shoes in order to produce a charge. To polarize a generator, connect a jumper (wire) momentarily between the armature and the battery terminals of the (voltage) regulator before starting the engine. A momentary surge through the generator is enough to correctly polarize it.” 

The next sentence in the manual warns that if the generator is not correctly polarized, the regulator and generator may be damaged. NEVER polarize an alternator-type system.

You’re fortunate; I would place a bet that the topic of flashing the field is not often discussed or known these days.

Happy flying,




Know your FAR/AIM and check with your mechanic before starting any work.


PPG Industries, Inc.

Those ‘70s Skyhawks

Those ‘70s Skyhawks

Over 16,000 Cessna 172K, L, M and N models left the factory in the 1970s. Most of these Skyhawks are still flying, and they’re gaining value, too.  

The Cessna 172 is the most successful General Aviation aircraft model of all time. It has weathered the storms of inflation, recession, crushing product liability lawsuits, and dwindling demand. 

The 172 was certified Nov. 4, 1955. A premium version called the Skyhawk was first sold alongside the standard 172 in 1961. The Skyhawk came with upgraded avionics and appearance packages. 


Four variants of the 172, the K, L, M and N models, and several modifications would take the 172 from 1970 through 1979. 

The 172K, L and M variants all came from the factory with the 150 hp, four-cylinder Lycoming O-320-E2D “Blue Streak” engine. The N model was powered by the 160 hp Lycoming O-320-H2AD.


Certified May 9, 1968, for the 1969 model year; 2,062 produced.

In 1969, the 172 had been upgraded with larger rear side window. Additionally, the rudder was fitted with a ground-adjustable trim tab at its base. In 1970, “drooping” conical-cambered fiberglass wingtips were added to the K model. 


Certified May 13, 1970; 1,535 produced.

In 1972, tubular-strut landing gear replaced the Wittman sprung-steel type gear. This change increased the footprint of the landing gear by 12 inches. The dorsal fin was lengthened to run the length of the fuselage to a point just behind the rear windows. 


Certified May 12, 1972; 6,825 produced.

With the M model, additional leading edge camber/droop was applied to the wings. This new wing was called the Camber-Lift wing and promised improved low-speed handling characteristics. Tinted skylights in the ceiling were offered as an option.

In 1974, the Skyhawk II trim package was added to the lineup. According to a Cessna marketing brochure from 1975, the Skyhawk II combined “…the businesslike blend of performance, economy and comfort [of the Skyhawk] with the nine most-wanted Skyhawk options.” 

The Skyhawk II trim package and Nav-Pac bundle grouped popular equipment options together to make ordering a new Cessna easier.


These two photos illustrate the differences between a 1975 172 sporting the Camber-Lift wing and extended dorsal fin and a 1963 model with straight wings and a smaller dorsal fin.

The brochure lists those nine options as: “Cessna Nav/Com with 360 channels for communications, and 160 for navigation with VOR indicator, Dual Controls, Emergency Locator Transmitter, Pitot Heating System, Alternate Static Source, Omni-Flash Beacon, True Airspeed Indicator, Navigation Light Detectors, Courtesy Lights.”

This cutaway drawing from a 1977 marketing brochure calls out the features of the Cessna 172.
Specifications sheet for the 1977 Cessna Skyhawk.
A fully-equipped Skyhawk panel, including the Nav-Pac option.

Certified May 17, 1976; 6,427 produced.

In 1976, Cessna stopped marketing the aircraft as the 172 and began exclusively using the Skyhawk designation. The “Skyhawk/100,” as Cessna called it, was introduced for the 1977 model year. 

The “100” moniker indicated that the aircraft was powered by a 160 hp Lycoming O-320-H2AD engine designed to run on 100LL. The previous O-320-E2D could run on 80/87 Avgas.

A Cessna dealer sales notebook lists increases in performance nearly across the board for the 1977 172 over the 1976 model.

In 1977, in-flight-adjustable rudder trim was available as an option, and pre-selectable flap control came standard. In 1978, a 28-volt electrical system was installed. In 1979, the flap extension speed was increased to 110 kias. 

This page from a Cessna dealer’s 1977 sales binder lists the performance improvements of that year’s new models.
Color choices and combinations for the 172 have tended to follow the trends of the times, as shown in this color chart for the 1974/75 model year.

The 172 continued to roll off the assembly line in the 1980s, until production was halted in 1986 due to unprofitability driven by onerous product liability lawsuits. The passage in 1994 of the General Aviation Revitalization Act (GARA) did indeed revitalize the General Aviation industry. Cessna began producing Skyhawks (and other aircraft) again in 1996. The Skyhawk continues to be produced today. 

In fact, as of this writing, the Cessna 172 may be more popular than ever. In an August 30, 2018 report from AOPA about the current hot market for used aircraft, the 172 is mentioned for its recent rapid appreciation (see link below). 

“Prices for 40-year-old airplanes do not typically jump 20 percent in the space of a few months, but that’s exactly what’s happened to Cessna Skyhawks produced between 1968 and 1976.” 

The report continues, “Rodney Martz, a senior aviation technical specialist in AOPA’s Pilot Information Center, said the recent jump in sale prices for Skyhawks produced in the 1960s and 1970s stands out as the largest percentage increase he has seen in many years, if ever.”

That, as we would say in the 1970s, is a really “far out” achievement. 

Jennifer Dellenbusch is president of the Cessna Flyer Association. Send questions or comments to .



Aircraft Owners and Pilots Association

McFarlane FAA-PMA Cowl Flap Hinges for Cessna Aircraft & Steering Rod Boots for Cessna 182

McFarlane FAA-PMA Cowl Flap Hinges for Cessna Aircraft & Steering Rod Boots for Cessna 182

Baldwin City, KS, September 5, 2018: McFarlane Aviation Products is excited to announce new approvals for Cessna Aircraft! 

McFarlane now has FAA-PMA approval for improved cowl flap hinges for Cessna 172, 180, 182, and 185 aircraft. “Cowl flap hinges wear out due to aerodynamic vibration and vibration of the engine cowling. A lot of high frequency engine vibration is transmitted to the cowling through the baffle seal material” stated Andy Pritchard, McFarlane IA technician. “This vibration and related hinge wear would be reduced if McFarlane’s low friction Cowl Saver® baffle seal material was installed on the engine.” McFarlane FAA-PMA cowl flap hinges utilize selective fit pins to eliminate looseness that allows the start of vibration wear. The improved design provides a superior hinge at a fraction of the replacement cost of the original hinges.

McFarlane now has FAA-PMA approval for the steering rod boot and attaching parts on the Cessna 182. McFarlane has improved the design of both the boot itself and the retaining flange. The Cessna boot is single ply fiberglass with a stiff silicone rubber coating that is prone to ripping and tears caused by fatigue and premature infrared heat related material break down. McFarlane has utilized a unique deeper convolute design, a three ply material design incorporating both Kevlar and fiberglass, and a supple high temperature rubber coating. “The Kevlar and fiberglass work together to prevent wear, fatigue, and heat failures while providing extended fire protection at 2,000 degrees F.” said Dave McFarlane, “We have worked for over twenty years to perfect this boot. It has a tough job to do.”  McFarlane has upgraded the original aluminum flange to a stainless steel firewall material. The boot is located right behind the muffler and holes and tears in the steering rod boot could allow for carbon monoxide, smoke, and even fire to penetrate the firewall and enter the cabin. “You would not want a standard boot on your aircraft if you watched it crumble and fail during our fire testing” said Teeyana Wullenschneider, McFarlane Project Engineer. Routine inspection and replacement of the boot and attaching parts are so very important to maintain safety. 

McFarlane Aviation Products has provided general aviation with high quality replacement aircraft parts at an affordable price for over 30 years and were recently awarded the 2018 Kansas Governor’s Award of Business Excellence! For more information, go to www.mcfarlaneaviation.com or call 888-352-0076 or 785-594-2741.