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

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

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

Read more...
Keep Those Manuals Handy: Rules for Owner-Performed Maintenance

Keep Those Manuals Handy: Rules for Owner-Performed Maintenance

As an aircraft owner and pilot, you can legally perform some maintenance tasks, but you must adhere to strict guidelines when doing so. Steve Ells walks us through packing wheel bearings, while highlighting what’s important to stay legal.

As most readers of Cessna Flyer know by now, all aircraft maintenance tasks must be overseen or performed by an appropriately-rated person. For maintenance tasks, this means an A&P mechanic—or a technician, as some like to be called these days—is frequently both performing and signing off on the work. This mechanic must (by regulation) have up-to-date versions of the appropriate manuals, bulletins, tools and equipment necessary to complete the tasks. 

However, there are also a number of maintenance tasks that owners may legally perform. These are termed preventive maintenance (PM) tasks. There’s a long list of them in Appendix A of FAR 43. 

What is considered preventive maintenance?

Appendix A is titled, “Major Alterations, Major Repairs and Preventive Maintenance.” Paragraph (c) lists preventive maintenance tasks. Type “Appendix A of Part 43” into your favorite search engine (or find the link in Resources on Page 35. —Ed.).

There is a surprisingly long list of tasks allowed. For instance, owners are permitted to remove and replace batteries, replace bulbs, reflectors and lenses of position and landing lights, and replace prefabricated fuel lines. 

They can also remove and replace panel-mounted communications and navigations receivers and update databases in panel-mounted avionics such as GPS navigators. 

Great news, right? It is, especially if a pilot has the time and a place to do these tasks. The potential for saving money exists, but much more important is the satisfaction to be gleaned from learning how to take care of your own airplane. (For further reading, see the sidebar on Page 34. —Ed.)

Are you permitted to perform preventive maintenance tasks?

FAR 43.3 paragraph (g) says that “…the holder of a pilot certificate issued under Part 61 may perform PM on any aircraft owned and operated by that pilot which is not used under Part 121, 129 or 135 of this chapter.” 

So, according to this section, if the owner and pilot is not using his airplane for hire, whether on a scheduled service, an on-demand service or as a foreign carrier operating for hire in the U.S., he/she can perform PM.

But there’s a catch. It’s in 43.13. It’s titled “Performance Rules (General).”

 43.13 Performance Rules (General)

The following three points—from paragraphs (a) and (b) of the performance rules—have been abbreviated to simplify the important points the maintenance performance rules for owners. 

1. Each person performing maintenance, alteration, or preventive maintenance on an aircraft, engine, propeller, or appliance shall use the methods, techniques, and practices prescribed in the current manufacturer’s maintenance manual or Instructions for Continued Airworthiness prepared by its manufacturer, or other methods, techniques, and practices acceptable to the Administrator.

2. He shall use the tools, equipment, and test apparatus necessary to assure completion of the work in accordance with accepted industry practices. 

3. Each person maintaining or altering, or performing preventive maintenance, shall do that work in such a manner and use materials of such a quality, that the condition of the aircraft, airframe, engine, propeller, or appliance worked on will be at least equal to its original or properly altered condition (with regard to aerodynamic function, structural strength, resistance to vibration and deterioration, and other qualities affecting airworthiness).

In other words, if you’re going to do PM, you must follow the procedures in the manuals. It’s as simple as that. 

It’s important at the outset to understand that airplane maintenance, while seeming to be like automobile or other gas engine maintenance in that it must be done right, is different in a very important way. In airplane maintenance, there is a published protocol for every operation, even the tightening of a nut or bolt. 

Another peculiar-to-aircraft trait is this: the strength versus weight equation must always be kept at the forefront of every operation and decision. In other words, if you believe that more is better, whether it be the size of a bolt or the amount of torque, you’re going to do more harm than good. 

Gathering the manuals and bulletins to meet the requirements of the FARs is much easier and less expensive than it used to be. The secret is the internet. Manufacturers have come to realize that making their manuals and bulletins available at no cost or consolidating a double-shelf full of manuals onto a CD is a sound idea, simply because access to manuals makes it much easier for maintenance shops (especially smaller shops) to access the precise methods and techniques the manufacturer has developed for maintaining its product. 

So, step one for owners that want to start working on their airplanes is to have or have access to manuals, and either have the tools or be able to manufacture the tools required to properly perform each maintenance task. 

Let’s look at an example of why manuals are important.

Greasing wheel bearings: a “simple” preventive maintenance task

Greasing the wheel bearings on an airplane may seem simple. At its most basic, it can be described in the following steps: First, jack up the airplane or axle enough to get the tire off the ground, then remove the axle nut and pull the wheel/tire assembly off the axle. Next, remove each bearing, clean it and the bearing race, inspect for damage or corrosion, replace if necessary, pack with grease and reinstall. Finally, reinstall the tire/wheel assembly, tighten the axle nut and lower the tire to the ground. 

Not so fast. There’s more to it. In fact, there’s quite a bit more.

To remove the tire/wheel assembly (TWA), the brake assembly must be partially disassembled. This disassembly requires the removal of two or four bolts to release what’s called the brake back plate(s). The TWA can be removed only after the back plate(s) have been removed.

Assuming the airplane has been jacked up far enough to lift the TWA, a large cotter pin must be removed prior to removing the axle nut. Then, the TWA can be pulled from the axle.

There is an inner and an outer bearing. From the earliest parts manuals, Cessna breaks out the parts in wheel assemblies. The bearings are always referred to as cups—often called the “race”—and cones—which are often called the bearings. 

Here’s another thing to know that is hard to find in any manual: Bearings and races are matched pairs. Don’t take the bearing assembly you removed from the race on the valve stem side of the TWA and install it in the race in the non-valve stem side of the TWA.

What grease to use?

Pre-1962 Cessna single-engine service manuals suggested that wheel bearings be cleaned and greased at the first 100 hours time-in-service, and thereafter at 500-hour intervals, unless operated in extremely dusty or sandy conditions. 

The manual specifies MIL-G-25760 grease. However, there’s also a sentence that says that the military specifications are not mandatory, but are listed to serve as a guide. It goes on to say that most products produced by reputable manufacturers meet or exceed these specifications. 

Later Cessna manuals give a different MIL-SPEC number, but cite “general purpose grease.” 

I personally like the 500-hour greasing interval. Unless I’m performing maintenance on an airplane for the first time, that’s the interval I use. 

Cleveland, the manufacturer of brakes and wheels used on Cessna singles, suggests the use of Mobil SHC™ 100 grease. 

Bearing removal, cleaning and greasing

After the TWA has been removed, the bearings are removed. This usually requires the removal of a snap ring, a washer, a felt grease seal and another washer. 

Bearings are then cleaned with Stoddard solvent, applied by either an air-powered solvent sprayer or a brush. Air can be used to blow the grease out, but never spin a bearing by directing compressed air perpendicular to the rollers. 

Directing a stream of air across—not between—the rollers in roller bearings is dangerous because the bearing cage is designed only to maintain the spacing between the rollers. It’s not strong enough to contain the rollers when they rotate at a high rate of speed; in other words, directing air across when bearings can result in fast-moving projectiles.

After the bearing is clean and dry, look for corrosion and/or pitting. If found, replace the bearing and matching race.

Bearings are repacked by putting a gob of clean grease in the palm of either hand and forcing the grease up into the bearing. Press the bearing down into the grease until the bearing comes into contact with your palm. Repeat this procedure until grease appears at the top of the bearing cage.

You can also buy a bearing packer and use it to pack the bearing. Look up “wheel bearing packing tool” on your favorite search engine. YouTube also has wheel bearing packing videos. (See Resources for an additional article that discusses wheel bearing service. —Ed.)

The last step is to look at the grease seals. For decades, Cleveland, the manufacturer of most GA wheels and brakes, has used felt pads to seal against sand and fine dirt. These seals are inexpensive and work well. 

Recently, Cleveland has replaced the felt pads with molded rubber grease seals. These may be used in place of the felt seals.

Putting it all back together

The newly-greased bearings are reinstalled in the side of the wheel which they came from. Slide the TWA onto the axle. If it doesn’t slide all the way on, you’ve got the large steel washers on each side of the felt seal in wrong. Swap the washers around until the TWA slides all the way onto the axle.

Thread the axle nut onto the axle. 

How tight should it be? I couldn’t find definitive information on how tight the axle nut should be. Field experience suggests to tighten the nut up well to seat the bearings, then loosen it until you can feel a slight movement of the wheel in and out on the axle, then snug it back down until the TWA spins without resistance and no in-out movement is felt.

Now, to reassemble the brake. Two or four bolts were removed so the back plate could be removed to free the brake disc from the inner and outer brake pads. 

Whenever I have a TWA off the axle, I clean up the brake guide pins with a Scotchbrite pad. I also clean the guide pin holes in the torque plate. These guide pins must slide in and out to allow the brake to self-adjust as the brake pads wear. 

The devil is in the details

The last step is often missed as it’s not in the Cessna manual. It’s found in the Cleveland Wheels and Brakes Component Maintenance Manual, Appendix A titled, “Wear Limits and Torque Values.” This manual, and all of the Cleveland wheel and brake manuals, are available for free on the Cleveland website. Start by downloading the Technician’s Service Guide. (See link in Resources. —Ed.)

           

Oftentimes, similar-looking parts call for different torque values. It is crucial to use the correct value for your part.

 

This critical step in reassembly is applying the proper torque to the two or four back plate tie bolts. Overtorqueing the bolts can deform the brake housing. 

The proper torque on almost every Cessna single engine brake is 75 to 90 inch-pounds (6.25 to 8.5 foot-pounds). That ain’t much. It doesn’t need to be much since these bolts aren’t in a compression application. They are loaded in shear, and as long as these bolts are snugged down to the proper torque, that’s sufficient. 

Sign off your work

The good news is that owners can legally do a lot of work on their airplanes. However, as mentioned, there are catches. Catch No. 1 is that you must own or have access to the manuals. Catch No. 2 is that you must enter the work you performed in the aircraft records in a manner that’s acceptable to the Administrator. That’s FAA talk for the head of the agency. 

The requirements for these entries are listed in FAR 43.9. It says if you perform PM, you shall make an entry in the maintenance records containing the following information:

1. A description of the work performed.

2. The date the work was completed.

3. The name of the person performing the work.

4. If the work was performed satisfactorily, the name certificate, certificate number and signature of the person performing the work. The signature constitutes an approval for return to service only for the work performed. 

(This is a summary of FAR 43.9. Please refer to Resources for a link to the complete text. —Ed.)

Notice that the regulations do not require the entry to include the aircraft total time or tach time, but it’s extremely helpful to include that information. 

An example1 of an entry for the work described above would read:

Month/day/year. “Greased left and right main landing gear wheels in accordance with information in the Cessna (model number) service manual and the Cleveland Wheel and Brake Component Maintenance Manual, Appendix A, paragraph A3.”

Signed: Joe Pilot Cert # 1245654

The point of this article is to make sure owners understand the freedom and the limitations that are part of owner-performed PM. Go ahead and do it, but make sure you do it right; by the book. 

1For more about complete and detailed logbook entries, see “Deciphering Logbooks” by Kristin Winter in the November 2017 issue. 

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

Steve Ells has been an A&P/IA for 44 years and is a commercial pilot with instrument and multi-engine ratings. Ells also loves utility and bush-style airplanes and operations. He’s a former tech rep and editor for Cessna Pilots Association and served as associate editor for AOPA Pilot until 2008. Ells is the owner of Ells Aviation (EllsAviation.com) and lives in Templeton, California, with his wife Audrey. Send questions and comments to

Resources

FURTHER READING

Part 43.3, Part 43.9, Part 43.13, Appendix A to Part 43
Electronic Code of Federal Regulations

Technician’s Service Guide AWBTSG0001-1

Cleveland Wheels & Brakes – CFA supporter
 
Component Maintenance Manual AWBCMM0001-12
Cleveland Wheels & Brakes – CFA supporter
 
“DIY Wheel Bearing Service” by Jacqueline Shipe
Cessna Flyer, July 2016 
Read more...