What is a Finger Doubler?

"The finger doubler repair is, in the opinion of the author, the best compromise for a permanent repair to a basic fuselage structure."

T.Swift, Repairs to Damage Tolerant Aircraft", FAA-AIR-90-01

A finger doubler is a type of skin lap splice that has a number of advantages:

1. Does not hide cracks. Easier to inspect from the outside. The critical locations for future fatigue cracks are in the basic skin at the first attachment row in the doubler. A finger doubler does not degrade the inspectability of the basic structure because a crack propagating in the skin at this first row will be externally detectable.
2. The purpose of the fingers is to reduce the first fastener peak load stress and thereby increase fatigue life. The fingers at the end of the inner skin are flexible and softens the load transfer through the rivets from one skin to the other at the end of the splice.
3. Reduces skin bending stress by increasing the distance between rivets.
4. Increased distance between rivets permits any crack to grow longer and thereby easier to detect.
5. Avoids the need of drag-producing protruding-head rivets to reduce bearing stress. Allows the use of countersunk rivets because of lower peak load stress.

References:

Patent US5297760

Repairs to Damage Tolerant Aircraft by T.Swift, FAA-AIR-90-01

Improvements to the Slick Magneto

Since Champion purchased the Slick magneto line from GE, they have made some good improvements to the magneto. Expect more in the future!

Here is one example, the new ribbed distributor block (K3822 or K3823).

The ribs help capture conductive carbon dust that wears from the brush and provides for a longer dielectrical path that reduces flash-over potential.






A close-up of the K-3822 block showing ribs.










This is the older non-ribbed style:

This is what happens when with the old style distributor block if the carbon dust builds-up to the point that a conductive path exists from the carbon brush across the block.

If you repair or overhaul your Slick magneto you might want to replace the block with the newer style one:

K-3822 for 4 cylinder Slick 4300 series magnetos and K-3823 for 6 cylinder Slick 6300 series magnetos. You can purchase these from Sacramento Sky Ranch 800-433-3564 or www.sacskyranch.com

Do Aftermarket Oil Additives Reduce Wear Metals ?

John, I have to say that I'm really impressed!  You are right on both counts!  It was oil streaking in the bottom of the aircraft cylinder, and this was the first sample since I started using XYZ oil additive.  I've never been a believer in oil additives before, but all the reading I've done says that XYZ helps protect engines that don't fly a lot.  I don't think that the additive has anything to do with the metals being high, as they were high before I began using it.  I'm almost to an oil change, so will be interested in seeing what they do this time.


Customer picture below taken of a chrome cylinder barrel through a boroscope with oil streaking. Notice the wavy little lines (channels).

I wouldn't use metal ppm (oil analysis) to make a judgement on the effectiveness of an EP (extreme pressure) additives, such as phosphorus in aircraft engines. Their function is to prevent micro-welding between two metal surfaces during periods of metal-to-metal contact. They do not provide protection by keeping the surfaces separate, as oil does. (Of course, additives might have multiple functions, such as corrosion protection which would help to reduce iron levels.)

EP additives protect surfaces during those periods when the oil film is breached. They do this by reacting with the iron at high temperatures created by friction and oxidizing the surface. This oxide film prevents micro-welding which leads to spalling. The act of protection does cause a micro amount of iron oxide to form which eventually ends up in your oil analysis; whereas chunks of metal from spalling are too large and do not end up in the oil analysis. Thus erroneous conclusions about their effectiveness when only using oil analysis to judge their effectiveness or lack thereof. Who cares if a few ppm of iron is oxidized from the surface in the act of preventing a chunk from being torn from the surface!

Normally, high levels of EP additives are used in gear box oils and hydraulic oils, such as 5606, but not in combustion chamber oils as the act of bore polishing is detrimental to a honed surface (which is not a concern in your engine with channel chrome cylinders).

Should you pump the throttle on a carbureted engine?

I'm a CFI and have always instructed my students not to pump the throttle when starting a carbureted engine. A recent discussion between one of my students and another CFI has me wondering if I telling my students bad info concerning the accelerator pump on carburetors.

Can you confirm any reasons why it's either bad or OK to pump the throttle during start?

Sounds like one of those questions that generates lots of differing views, so here is mine: There is nothing worse than sitting in a burning airplane way out on the ramp without a fire extinguisher.

Our carburetors in flat Continental and Lycoming engines are up-side-down meaning that the fuel has to go up hill to get to the cylinders. Fuel squirted into the intake system just flows back down into the air box unless it's being sucked up into the cylinders. Too much fuel dripping out of the airbox can catch fire as my two personal experiences attests.

So just pumping the throttle with the propeller stopped is only useful for washing out the air box with fuel. In my C-182 it might be necessary to pump the throttle on a cold morning to keep the engine running - but only a little bit as too much pumping and the fuel starts draining back into the airbox - better to use the primer, if equipped.

So to sum up my recommendation:

• Don't pump the throttle on a stopped engine
• Avoid pumping or pump as little as possible

With that said, that is based on my limited experience. Some might have better recommendations that I would be interested in hearing.

Stuck Valve Check - Quick, Easy, Inexpensive

"Anyway, about an hour and 15 minutes in to the flight, I felt a barely discernible roughness, and then a significant cough, and the RPM dropped about 200. I quickly pulled the carb heat out, but then it got even worse. So I pushed the carb heat back in, waited for a minute, and pulled it back out. My engine was still running, but my RPM I am inclined to think that I was experiencing a sticky valve problem. What do you think John? "

Here is one of my favorite checks as it is fast, easy, inexpensive, and works quite well as I tested it for years in the cylinder shop. I would suggest as part of your compression test you remove the rocker covers and with your two thumbs pushing on top of the valve - pop the valve open and shut. You will feel any dragging if the valve is sticking. If the guide is really worn you will feel two thumps as the valve face closes onto the seat first on one side and then another

There are so many possibilities that only a systematic troubleshooting process will reveal the answer. The O-200 normally is rare to stick the exhaust valve since the original guide material was relatively soft aluminum-bronze. As deposits build-up in the guide, the valve would just wear away the guide. Then Superior had a great idea, use the harder ni-resist guide that the bigger engines have to give better wear - well yes, except now the valve sticks. I'd rather have it wear. It's been so long since I've been in the cylinder repair business that I don't know who is using what guide. 

Do not fly if you suspect a stuck valve. If the valve sticks closed and doesn't open in 1/2 propeller revolution the rocker arm supports will blow-off leading to total loss of engine power. Even if that doesn't happen, with each revolution the high opening forces are smashing your camshaft lobe flat. This is why I do not recommend Marvel Mystery Oil and other solvents to fix a sticky valve - yes they might unstick it in time but while it's doing it's work you risk expensive and dangerous engine damage. Also, the root cause of valve sticking is not corrected. A O-200 engine should not stick valves - if it does then something is wrong with the engine (could be the shape of the exhaust port). 

Camshaft lobe damage. Trying to push open a sticky valve smashes the camshaft lobe ($$$). 

Rough Engine on left mag - nothing you do fixes the problem!

O-320 Lycoming won't run on left magneto

Nothing you do will fix it. Mag check -- engine coughs and sputters on left magneto.

• Replace magneto - still rough
• Check ignition harness - checks fine - still rough
• Replace "P" lead - still rough

So lets replace the harness "just in case" --Success engine runs fine. 10 hours later problem returns!
One other item interest; can't lean engine, as soon as the mixture is pulled back the engine sputters.

So what's the solution?

The big hint here is the mixture control. It points to the carburetor and only the carburetor. But why just on the left magneto? Lets say you have crappy fuel atomization and fuel distribution. Not real bad, but just bad enough. With two spark plugs firing it ignites a leaner mixture and the engine runs fine. But lets impar the ignition system slightly by weakening it. Turn off 1 magneto and now try to ignite the mixture - it can't do it as well and the engine coughs and shudders. Lean out the mixture just slightly and now even with sparks flying out of two spark plugs it can't ignite the mixture every time. It misses a few strokes then enough fuel has gathered that it ignites and burns and then the cycle repeats. The carburetor nozzle was replaced and now the engine runs fine.

More Magneto Ignition Troubleshooting Tips

Start with the easy and inexpensive and work your way from there. The spark plug should be one of the first items you check. A spark plug resistance check is a fast and easy method. You will need an inexpensive ohm or multi-meter.

Spark plug terminal well showing arching lines. Plug lead is arching to ground by way of the plug metal shell. Replace plug. At the bottom of the well there is a contact. Check spark plug resistance by placing 1 end of the ohm meter lead to this contact and the other end of the lead to the center electrode on the firing end of the spark plug. Resistance should be 800 to 1200 OHMS. Replace any plug above 5000 OHMS.

 This is what the resistor looks like inside a Champion REM40E spark plug.

One reason why not too drop spark plugs and a good reason why to carefully inspect the terminal well (shown here) for cracks.

More causes of rough engine

Fuel Hose Installation and Electrical Wiring in Aircraft AD2009-15-01

I was blown away by this Airworthiness Directive 2009-15-01 on HAWKER BEECHCRAFT CORPORATION G36 "the next generation Bonanza". To quote:

This AD results from reports of chafing between the wire harness/connector(s) and fuel line. We are issuing this AD to detect and correct chafing between the wire harness/connector(s) and fuel line. This chafing could lead to fuel leaking into the cockpit and fire in the cockpit if wiring arcs through the fuel line. One report indicated arcing from a chafing wire harness burned a hole through the fuel tube.




A fuel line in the cockpit resting against an electrical wire bundle? Makes me shudder in fear!

For all of you building homebuilt aircraft, here is a very safe, simple and sensible standard for installing aircraft hose and electrical wiring:

MIL-W-5088L Wiring, Aerospace Vehicles

Wiring shall be supported independent of and with the maximum practicable separation from all fluid-carrying lines, tubes and equipment.

Where this routing is not practicable, the wiring shall pass below the lines at an angle rather than parallel to the lines.

Wiring shall not be attached to fluld carrying lines, tubes and equipment unless they require electrical connections or their separation is less than two inches, In areas where separation is less than two inches, the wiring shall be installed to maintain positive separation of at least .500 inch. 


There is more in the mil-spec but this about sums it up; don't place a fuel line against an electrical wire bundle. Chafe protection is just a delaying method and does not provide positive and long term protection.