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

Written by
Rate this item
(1 Vote)

Steve,

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?

Arlene

 

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,

Steve

 

 

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,

Steve

 

 

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

Resources

P/S 890 CLASS B-1/2 SEALANT
PPG Industries, Inc.
ppgaerospace.com/Products/Sealants/Fuel-Tank-Proven-Standards/P-S-890-Class-B-Fuel-Tank-Sealant.aspx