Airframe and Powerplant

What you can do to keep your airplane flying

Joe Pilot phoned Ace A&P to gripe about a dragging brake. Ace was waiting on the ramp as Joe taxied up. As he knelt down to look over the offending brake and wheel, Ace could feel heat from the brake almost as soon as he got within spitting distance.

Light-airplane brake assemblies, called spot or disc brakes, are very simple and dependable. As the pilot applies pressure to either brake — in many light airplanes by pressing the top of the rudder pedal — a piston moves in the master brake cylinder. As the piston moves, pressure is applied to hydraulic fluid below the piston. This pressure is transferred through aluminum tubing or flexible hose to a piston in the brake assembly that's larger in surface area than the piston in the master cylinder. As the fluid flows into the space behind the brake assembly piston, the piston moves out, pushing the brake linings against a brake disc, applying stopping pressure. When the pilot releases pressure on the brake pedal, fluid pressure from the master cylinder to the brake piston is relieved, releasing the pressure on the brake disc. There's no brake cylinder retract mechanism — the drop of fluid pressure is all that's necessary for brake release.

Light-airplane brakes depend on two things for proper operation. Two loose-fitting pins — called anchor pins — that are rigidly attached to the brake assembly float in two holes in the axle-mounted torque plate assembly. The pins must be kept clean and rust free to permit them to move freely. Second, bolts inserted through holes in the brake assembly and used for holding the back plate assembly in position must not be overtightened, or over- torqued.

Over-torquing distorts the housing and prevents the piston from retracting when brake-pedal pressure is released. When the piston can't retract freely the brake will always be partially applied. The result is constant partial brake application, which causes rapid brake wear, which results in high brake and wheel temperatures. If you're thinking that this might be the cause of Joe Pilot's brake problem, you're right. Joe serviced his own brakes and, in a misguided attempt to be thorough, he torqued the heck out of the back plate assembly.

Well intentioned but unaware

The reason Joe's brakes were incorrectly serviced was because the pilot performed his own maintenance. That's not exactly correct — the reason is because the pilot performed his own brake maintenance without the correct training and appropriate maintenance manuals.

Pilots who wish to cut costs and save time are permitted to perform certain preventive maintenance tasks. This is really good news. A pilot with a basic knowledge of the five types of stress — tension, compression, bending, torsion, and shear — and how these stresses dictate the performance of maintenance fasteners will be able to gain a big advantage in lowering costs and maintaining his airplane's airworthiness.

Preventive maintenance tasks aren't difficult but they must not be taken lightly — the same performance rules that apply to FAA certified airframe and powerplant technicians also apply to the pilot taking on preventive maintenance tasks. This may sound like a Catch-22 — the regulations give pilots the freedom to perform certain tasks but as soon as they decide to take advantage of this freedom they're told that they are bound by what can be interpreted as restrictive performance rules. No one's picking on pilots who want to take a more active part in their airplane maintenance — everyone involved in aircraft maintenance is bound by the same performance rules. So let's take a look at Federal Aviation Regulation Part 43 for the basic performance rules that govern aircraft maintenance for the pilot.

Part 43 — rules to wrench by

Part 43 contains a mere 13 paragraphs and six appendices — not much considering the importance of this part, which is titled Maintenance, Preventive Maintenance, Rebuilding, and Alteration. There's only one paragraph that speaks to the actual performance of maintenance.

The performance paragraph

Each of the two subparagraphs in the performance paragraph starts out the same way. Paragraph (a) says it like this: "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." There's more related to instruction for continued airworthiness, but you get the idea.

Paragraph (b) says: "Each person maintaining or altering, or performing preventive maintenance, shall do that work in such a manner and use material of such a quality, that the condition of the aircraft, airframe, aircraft engine, propeller, or appliance worked on will be at least equal to its original or properly altered condition." Again, there's more, but the gist of this paragraph is apparent.

In other words, a certificated pilot performing his first preventive maintenance task must work to the same performance standards as the veteran A&P. It may sound as if the whole point of this article is to create obstacles designed to scare even the most adventurous pilot away from trying to learn about preventive maintenance. That's not the point.

Earlier, a story was told about an airplane with a dragging brake. The pilot created that problem when he overtorqued what Cleveland's brake division calls the back plate tie bolts. On page A-4 of the appropriate Cleveland wheel and brake maintenance manual it says, "Overtorquing could cause depressions in the brake cylinder, which result in dragging or bound up brakes."

The proper torque value for these bolts is 80 to 90 inch-pounds of dry torque — meaning the bolts must not be lubricated before installation. This is a very small torque value because that's all that's required to hold the brake back plate in position. The reason it's so light is that the load on the bolt is a shear load, which means that the load is perpendicular to the bolt shank. Overtorquing the bolt would not increase its resistance to shear since the bolt's resistance to shear is a function of the diameter of the bolt shank.

The first step is to clean all crud and corrosion off the brake assembly. A soft brush will do the trick. Using either hand pressure or a C-clamp, push the piston back into the housing. Clean off the anchor pins and the back plate tie bolts. Scotch-Brite pads work well for this. Insert the anchor pins in the appropriate holes in the torque plate and move the brake assembly in and out to check for binding and alignment. If OK, insert the tie bolts through the brake assembly and screw them into the back plates. Torque the tie bolts to the correct torque and safety wire the bolts. Apply pressure to the brake pedal to seat the pads. Condition the brake pads before using. Directions for conditioning can be found online ( www.parker.com/ag/wbd/cleveland/pdf/wbtech.pdf).

Stresses and how they apply

Fasteners can be classified by their type, and equally as important, by the application they're being used in. There are two common applications. Tension is defined as a stress that is applied parallel to the shank of a bolt and tends to pull apart or stretch the bolt-nut fastener. Shear is a stress that acts perpendicular to the shank of a bolt or fastener — when the force applied to one of the components overcomes the bolt's resistance to shear, the bolt will yield. Common scissors cut paper with a shearing movement between the two blades.

Torque is the amount of turning force applied to the bolt or nut when it's being tightened. Torque is measured in inch-pounds or foot-pounds. Like weight changes in preflight weight and balance computations, inch- pounds and foot-pounds are simply multiplication problems involving arms — measured in inches or pounds — and weights. A 24-inch-pound torque could be defined by 2 pounds applied through an arm that's 12 inches from the bolt or nut being torqued — or by a 3-pound force applied through an arm 8 inches long. Torque wrenches usually take care of the math, but you get the idea.

Torque values for tension and shear applications for common steel aircraft quality bolts and nuts are shown in tables in aircraft maintenance manuals and in FAA advisory circulars. Torque values for fasteners in tension loads are always higher than the values for fasteners used in shear loads for like-size fasteners. The point of this story is to introduce owners to an example of how critical it is to understand the importance of peripheral tasks when doing preventive maintenance. It's always better to shine a light on what you're getting into, and what the rules are for the task you're considering, than it is to blunder along in the dark. It's a lot safer, too.

The correct method to use

Since Cleveland brake maintenance has been introduced earlier, let's look at the steps that an owner would be required to complete in order to successfully reinstall the brakes after working on either or both main wheels.

The first step to learning about any preventive maintenance task is to watch an experienced A&P technician as he works through the stages. Everyone crawls before they walk.

Take note of the tools, lubrications, and handy tricks that the technician employs. Since the back plate tie bolts require the application of a certain torque, access to a calibrated torque wrench will be required. The service manuals and parts manuals for many light-airplane components are available on the Internet. Cleveland brake product manuals can be viewed and downloaded from the company Web site ( www.parker.com/ag/wbd).

The maintenance manual also contains instructions for conditioning both nonmetallic and metallic brake linings after installation. The procedure for nonmetallic linings consists of getting the linings hot by taxiing 1,500 feet with the engine at 1,700 rpm while applying enough brake-pedal force to reduce the taxi speed to 5 to 10 mph. Allow the brake to cool 5 to 10 minutes and then determine if the brake linings have been sufficiently glazed to hold the airplane when the engine is run up to high static power.

Doing the paperwork

With freedom comes responsibility — that means owners who perform preventive maintenance are required to make maintenance record entries of all work performed. The paragraph in Part 43 that pertains to content, form, and disposition of maintenance, preventive maintenance, rebuilding, and alteration records tells exactly what's required.

A description of the work performed, the date of completion of the work — it's customary and expected to include the tach or Hobbs meter reading — and the signature, certificate number, and type of certificate held by the person returning the airplane to service are required. Owners are often concerned that entering their name in the maintenance records exposes them to liability for all previous maintenance. This is untrue; Paragraph 43.9 (a)(4) says, "The signature constitutes the approval for return to service only for the work performed."

Here's an example of an entry for replacing the left tire on a typical Cessna 172: "November 8, 2004. Lubricated both main landing gear wheel bearings in accordance with the Cessna 172 service manual, paragraphs 5-20 through 5-24. Torqued brake back plate tie bolts in accordance with procedures and limits in Cleveland Component Maintenance Manual AWBCMM0001-6/USA dated 1997. Joe Pilot, Private Pilot #12345678."

E-mail the author at [email protected].

Visit the Web site ( www.aopa.org/members/files/guides/prevmain.html) to see what tasks pilots can perform under preventive-maintenance regulations.

Owner maintenance

The following is a list of tasks an owner can perform as preventive maintenance under FAR 43.? The inclusion on this list must not be construed as an approval for an owner to disassemble any primary structure or any primary operating system nor do these tasks permit any action that requires complex assembly operations. — SWE

Landing gear

1. Repair, replace, and service wheels, tires, and tubes.
2. Clean and grease wheel bearings.
3. Service shock struts with air (or compressed gas) or oil or both, and replace elastic shock-absorber cords (bungees) on landing gear.
4. Add fluid to hydraulic reservoir.
5. Lubricate the landing-gear mechanism.
6. Replace wheels and skis where no weight and balance computation is required.

Engine

1. Change engine oil and filters. Clean engine oil screens.
2. Clean and gap, or replace, spark plugs.
3. Clean or replace fuel filters.
4. Replace prefabricated fuel lines.
5. Replace any hose connection except hydraulic hoses.
6. Remove and check magnetic chip detectors.

Electrical

1. Replace and service the battery.
2. Replace reflectors, bulbs, and lenses in landing and position lights.
3. Troubleshoot and repair broken circuits in landing-light wiring.

Cosmetics and interior

1. Apply paint, except on balanced control surfaces (rudders, elevators, and ailerons).
2. Apply corrosion proofing if it can be applied without disassembly of a primary structure or any operating system.
3. Repair upholstery.
4. Replace seat belts, seats, and seat parts with approved parts.
5. Make simple repairs to fairings, cowlings, and cover plates.
6. Replace nonstructural fasteners.
7. Make simple fabric patches that don't require rib stitching or removal of structural parts or control surfaces.
8. Replace side windows.

General

1. Lubricate anything that's accessible without disassembly.
2. Replace defective safety wire and cotter keys.
3. Install restricted-size fuel filler ports if installation can be accomplished without disassembly of existing filler port openings.
4. Replace any cowling not requiring the removal of the propeller or disconnection of flight controls.