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Starters and Worm Gears in Aircraft Piston Engines

2009-11-07T08:59:00.000-08:00

Ever notice how Lycoming and Continental use completely different designs to interface the starter to the engine? We'll focus on the gearing differences between the two.

Continental uses a "starter adapter" for most of their engines that houses a worm gear and clutch mechanism.

Lycoming uses a starter ring gear" mounted to a large "Support Assembly" attached to the crankshaft flange. There are several engineering trade-off's of the two:

Lycoming keeps gearing out of the engine. If gear teeth break they don't contaminate the inside of the engine.
The heavy Support Assembly might provide some vibration dampening.
The starter adapter is compact and light weight.
The starter adapter can serve multiple purposes by mounting a pully or dampner to it.
The Continental starter adapter has been an expensive item to repair at overhaul (or in between) so Continental engines have one more expense.
The Support Assembly is like a large flat plate out in front of the engine that prevents tapering the cowling at the nose.
You can mount a pully or pully's to the Support Assembly to drive alternators and vacuum pumps.
The Support Assembly can have electrical contacts in the form of circumferal grooves to transfer electricity to the propeller deice system.
Both systems have had their own unique problems so it is a trade-off in my opinion as to which one is more reliable. But the Lycoming is less expensive to maintain.

The fast rotating starter needs to be geared down to the proper starting rpm for the engine. This is where the comparison becomes interesting: Lycoming uses a pinion and gear (small gear on the starter is called a pinion) to achieve the proper gear ratio. Continental uses a worm and gear "worm gear" to achieve the proper gear ratio. Lets compare the two:

The Lycoming starter ring gear is on the outside and the Contintal worm and gear is on the inside. Notice how compact the worm gear is? Worm gears are often used when large reductions in gearing are needed. In this example, a fast turning starter motor turns the worm. The engine is then turned by the gear - at a much slower rate. Compare this with the large Lycoming starter ring gear that does the same thing but is 2 feet across.

This is a close-up of the Continental worm and gear. Mounted on the back is the clutch spring and drum that disengages the starter when the engine starts. When it works it works well, but there are lots of parts that have the potential of failing and releasing bits of metal into the engine.

The typical worm gear has a brass gear mounted to a steel worm. There's a lot of rubbing motion across the gear teeth so lubricant needs to be continually applied. The combination of steel on brass prevents cold welding (galling) between the surfaces as lubrication is in the boundry zone where the lubricant isn't always between the surfaces.

Inspecting High Strength Materials

2009-10-26T08:46:00.000-07:00

The Aardvark Syndrom - built strong but easy to break

Drop a glass onto the floor and it shatters, drop a block of wood onto the floor and it doesn't - yet both have approximately equal tensile strengths. A high tensile strength steel bolt might be twice as strong as a mild steel bolt yet it fractures in two whereas the mild steel bolt bends but still holds the structure together.

We learn to handle glass objects differently than wooden or metal objects because "they break easily"and "are brittle." For the same reason if we use brittle materials in structures we need to 'be careful" and provide additional protection to avoid breakage. A high tensile strength steel bolt might be 3 times stronger than a mild steel bolt but it takes 10 to 100 times less energy to break!1.

A mild steel bolt can handle small nicks, or a little bit of corrosion pitting not because it is strong but because it is hard to fracture (crack). Even if it does crack, the crack growth is so slow that we use various NDT methods to detect cracks and replace the bolt before it breaks.
Not our high-tensile steel bolt. The energy required to break it is 10 to 100 times less. A bit of corrosion that creates a pit that concentrates stress might be all it takes to start a crack. Because it doesn't take much energy to grow the crack, the part may fracture as fast as a broken glass.
There are various methods of using "high-strength-low fracture energy materials, such as better envirnomental protection, non-critical applications, redundent load paths, crack arresting structures. But using a high tensile steel bolt ("Grade 8") as a single point of attachment on a trailer hitch that is bathed in road salt and submerged in lakes where corrosion occurs hidden under the head or shank is not one of them. A better idea would be to match strength with fracture energy!

More inspections are required for high-strength low fracture energy materials:

More frequent inspections for corrosion.
Protection from scratches and marks.
Protected tooling that won't mar the surface.
More frequent application of corrosion inhibitors.
More adequate and detailed inspection and rejection instructions.
Increased education of mechanics and their employeer on why this is so.

For procurement, better quality audits of the manufacturing process, as these parts require more precise materials , process, and heat-treat.

Note 1. Approximate work of fracture J/(m squared) for mild steel is 100,000 to 1,100,000. For high tensile strength steel it is approximately 10,000.

Fingerprint Corrosion on Aircraft Products

2009-10-24T08:40:00.000-07:00

Many years ago our aircraft hose manufacturing shop http://www.sacskyranch.com/ set up a program to identify and eliminate products that might cause chloride contamination. Every chemical introduced into the area is screened for chloride or other potential corrosive materials. This program is not a bad idea for any repair facility working on advanced aircraft products. What we couldn't eliminate was the human touch and the secretions deposited onto surfaces. Fingerprints cause corrosion, and police are using it to identify individuals who have touched brass cartridges years ago:

"We recently showed how fingerprints on brass cartridge cases that we left out for several days in open air at room temperature can still produce corrosion sufficient for visualization, even after they have been washed in warm water and detergent to remove the residue"

Our concern is corrosion pitting that might damage a critical aircraft part. So what can you touch and what should you be careful with? How do you clean fingerprints from parts? read more on my web site...

Slick magnetos and propeller strikes

2009-10-06T15:15:00.000-07:00

Hi John, a couple of us mechanics have been talking about what a Slick mag needs after a prop strike, I've been on the Unison website and can't find any info, where could I look for something in print?

I am not aware of any Slick recommendations for inspection after a propeller strike. Of course, Champion/Slick would be the people to ask.

In my experience of doing propeller strike inspections (approx. 12 per year for 10 years), magnetos were never part of the inspection. This was before Continental/Bendix added their magneto inspection requirement. My recollection (it is often faulty at this age), is that they blamed distributor gear tooth breakage in the magneto on sudden stoppage/propeller strike. Picture below is the distributor gear with broken teeth:

Our opinion is that this breakage is caused by worn bushings creating a conical oscillation about the center axis (whirl or shopping cart wheel flutter). This is a very rare event but we have witnessed it on a magneto test bench.

For more opinions on propeller strikes:

Should an engine be torn down after a prop strike?

What is a Finger Doubler?

2009-09-27T10:09:00.000-07:00

"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:

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.
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.
Reduces skin bending stress by increasing the distance between rivets.
Increased distance between rivets permits any crack to grow longer and thereby easier to detect.
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

2009-09-21T08:35:00.000-07:00

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 ?

2009-09-19T19:45:00.000-07:00

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?

2009-09-18T11:10:00.000-07:00

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

2009-09-18T05:34:00.000-07:00

"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!

2009-09-15T10:31:00.000-07:00

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

2009-09-05T07:35:00.000-07:00

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.

Cessna 152 Poor Idle - Carburetor Foam and Hollow Floats Sb-2

2009-08-12T16:37:00.000-07:00

A quick tip from one of our customers who services a small fleet of Cessna 152's. Rough and poor idle can often be corrected by:

Each 100 hours pressure testing the intake and exhaust system to find air leaks.
Make sure carburetor float is working properly.
Working on the carburetor won't make the engine run any better if the problem is an air leak in the induction system.

This is the newest Volare foam float. You can purchase these from www.sacskyranch.com or call 800-433-3564

Below is a link to the Volare Service Bulletin

volare Foam Float Service Bulletin SB-2

This is the old style float. Notice that the float is half filled with fuel!

Bad Valve Seat Job

2009-08-12T07:01:00.000-07:00

Bad valve seat workmanship stands out like a sore thumb if you know what to look for.

The first think to look for is the alignment between the seat and the guide. Are they concentric? In other words do they have a common center? In our picture below notice that the center of the circle formed by the seat is not centered to the circle formed by the guide. The circles are eccentric.

What I'm looking at is the thickness of the top of the seat at the black lines. Notice the width is thicker on the right side than on the left side. Of course a face must be ground into the seat before you can check concentricity. What happened here is that the seat grinder pilots from the guide so it wants to align itself to the guide center. This causes the grinding stone to grind more on to the left in this picture. If the seat and guide were concentric then the seat grinder would grind dead-center and there would be equal widths.

The practical effect here is that the stone pressure is mostly on the left where it is doing most of the grinding. Now drop a valve onto this seat. Just like the grinder, most of the seat contact pressure will also be on the left side. In analyzing eccentric seats in aircraft engines we found that they develop exhaust valve leaks at the narrow side. This makes sense as the narrow side is where the least amount of contact pressure occurs. Also, the valve is slightly tilted to one side. I would also presume that guide wear would be accelerated as the valve stem pushes into the side of the guide as the valve attempts to center itself onto the seat. No fancy 3 angle valve job is going to correct this seat.

What do you do about it? The only think you can do is move the holes back into alignment. This requires fixtures and cutters and cannot be done with hand tools. Also, to move a hole one must make it larger. You come up against maximum oversize limits quickly when you start moving holes.

AD2009-16-03 SAP Cylinder Cracking

2009-08-06T08:36:00.001-07:00

AD 2009-16-03 results from reports of cracks in the area of the exhaust valve and separation of cylinder heads from the barrels of SAP cylinder assemblies with certain part numbers.

So now 8,000 of you (8,000 engines effected) need to inspect your SAP cylinders for head cracks per AD2009-16-03. Two inspection methods are presented in the AD:

Visual inspection for combustion staining, and
Pressure test.

After spending 30 years running a cylinder overhaul shop I learned one thing is certain: cylinders crack. No surprise here. Cracks in aircraft cylinders are not limited to any one manufacturer - all have had problems with cracks and will continue too given the hard work we ask of them. So even if your airplane is not directly affected by this AD, checking cylinders for cracks is important and can be incorporated into your normal inspections without much additional time or expense.

Lets discuss both of these inspection methods and introduce a third method of my own making:

Both inspection methods presented in the AD only detect cracks that extend completely through the wall and large enough to pass gas. That is one big crack and on the verge of outright fly-apart fracture.

Lets look at a cylinder for what it is: a pressure chamber that gets pressurized and de-pressurized with very hot corrosive gasses 20 times a second. If a flight is 1 hour long you have one "thermal cycle" but 72,000 pressure cycles!

Pressure test:

There is nothing unusual in the requirement that if during a compression test, gas is leaking from the pressure chamber (and it's not going past the valves, and, its not going past the rings) then you need to start looking for cracks in the cylinder head. Well that's something you should do anytime you suspect your cylinder has a hole in the head and won't hold air. Only inconvenience is you have to do a compression check each 50 hours per the AD.

Staining Inspection ("black combustion leakage")

This inspection is much more interesting and anyone can do it with just a mirror and small flashlight.

The picture above shows staining from gas leakage out a crack in a Lycoming cylinder. At each 50 hour oil change you can look at the fins for staining. It just takes a few minutes of eye-ball time. This crack didn't get detected so lets see the end result:

So there you have it - spontaneous cylinder head separation. Notice the gas staining in the red circles. There was plenty of advanced warning. Inspection for exhaust staining on the outside fins of a cylinder should be a normal inspection item.

Lets take a more hidden example: See anything wrong with this O-200 cylinder below?

Well, you're not going to at this angle. You're not looking at the right angle nor in the right places. There's gas leakage and a crack right there between the yellow lines. Here is how you should look at the cylinder - look between the fins! That's where the crack penetrates - not at the end of a fin but at its base. Same cylinder but different view:

See the dark staining between the fins. Do you see the crack?

Back in the 1980's we did some experiments with gas or oil leakage, especially leakage between the barrel and head. We'd take cylinders that had oil and gas stains and pressurize them with 80 pounds of pressure to see if they leaked. Our experience was that they didn't leak in our test. Certainly if the crack is large enough and open it will leak but cracks like the one above would not leak air when pressurized. Our theory is that our test is at room temperature without the strains induced during the actual combustion cycle. Only cracks at their last stage - when they're big, large, and about to come apart, leak gas. So the compression test and soap and water is the final frontier - gas and oil staining come first.

There is another test that we used and that is the ping test. Here is how it originated: I got upset spending shop time cleaning cylinders and then checking them for cracks only to find out after 2 hours of labor that the cylinder was no good. Better to find the obvious crack before investing any shop time. Once a cylinder passes the ping test then we would further clean and inspect for less obvious cracks.

Take your finger nail or the plastic end of a pen and ping a cylinder fin. It should ring. If it goes thud then there could be a crack at the base of the fin. You can even take your pen and just stroke it down the cylinder head. The link below is a sound file so you can listen to the sound a crack makes! This doesn't work for fins that are in contact with baffling, they have to be free to ring like a bell.

Here is the sound

Additional Information:

I have a free e-book available (windows only) at my web site that you can download called "Crack Detection Using the Unaided Eye"

Visit my photo albums for more maintenance pictures

Visit www.mechanicsupport.com for more articles and software for mechanics

Hose Problems

2009-07-31T07:22:00.000-07:00

Hose Problems

Yesterday a friend brought by some hoses off of his racing car for pressure testing. Normally, I only do aircraft hoses, but for a friend --OK

So the hoses fail. Look carefully at the hose to the left and you can see that the hose

pulled out of the socket. The hose was only engaged into the socket 1/4 inch or so. This is why he had a hose leak - it wasn't assembled properly to begin with.

Of all the different types of aircraft hoses I build, this style is the most tricky to assemble correctly. It is also the style of hose that race-car and custom car builders love to use. We use very little of this stuff on aircraft and in general don't like it - except it is "pretty" so I guess that is all that matters.

There are multiple ways you can get into trouble screwing the fittings onto this style hose. Here is just one tip:

After you screw the hose into the socket to the correct depth, place a "back-out" mark on the hose. Then when you screw the fitting together go back and make sure the hose hasn't pushed out of the socket as you screwed the nut-nipple Assembly into the socket.

Also, don't flood the cavity with oil as you can create a hydraulic lock that prevents the rubber from sliding into the recess.

Here is a link to another article on hose assembly at my web site:

http://www.mechanicsupport.com/aircraft_hose_assembly.html

AN Thread Size and AN Fitting Size Chart

2009-07-27T08:12:00.000-07:00

Threadform: UNJ -3A or 3B
Note: There is an entire industry supplying so-called "AN" fittings that do not meet aerospace standards. See my article at http://www.mechanicsupport.com/articleStronger.htm anicsupport.com/articleStronger.htm

Dash Size ThreadForm
-2 : 5/16-24
-3 : 3/8-24
-4 : 7/16-20
-5 : 1/2-20
-6 : 9/16-18
-8 : 3/4-16
-10 : 7/8-14
-12 : 1-1/16-12
-16 : 1-5/16-12

Use of crush washer on AN Fitting Connections

2009-07-27T07:18:00.000-07:00

An "aluminum crush washer" or conical seal is sometimes placed between the flare surfaces. These are mechanic band-aids to be used when you have a leaking connection and no replacement fittings. The usual problem is using a new hose fitting against an old male nipple whose sealing surface has been damaged. It's easier to drop a conical seal into the hole than to replace the damaged fitting. Going back 10-15 years no one had ever heard of them.

Conical seals getting more popular now, in my opinion, because of the increased usage of steel on steel mating surfaces. Traditionally, AN plumbing mating surfaces were aluminum to aluminum or steel to aluminum. To seal any surfaces, one or both surfaces must conform (yield) slightly under pressure to seal any microscopic gaps between the surfaces. It helps if one or both surfaces are relatively soft. The more recent switch to steel on steel requires increased nut torque to get the two hard surfaces to yield sufficiently to seal. This should not be a problem with good surfaces and proper torqued.

Bastardized AN fittings - Which AN fitting goes into the hole?

2009-07-03T09:46:00.000-07:00

Automotive racing has adopted the aircraft AN fitting technology, bastardized it, and now sent it back into the aircraft industry. I've talked about the differences before in my article "What is the difference between aircraft AN and JIC fittings". The result of this bastardization is confusion and extra expense for the aircraft industry. Let me try to clear up some confusion when it comes to screwing fittings into straight-thread ports.

In non-aircraft applications ports are mostly straight thread "ORB" O-ring Boss. The automtove industry (heck I don't know what to call you guys - "race industry", is that better) has plenty of adapters to adapt AN to the port - pictured below.

There is no AN number for this adapter. It does not exist in the AN series - for good reason - it is not needed in aircraft. In aircraft, the traditional straight threaded boss is called a "AND" port (AND10050 or MS33656) and doesn't require an adapter fitting. This should be the end of the story for us aircraft people but it isn't.

Some automove style components with ORB ports are being used in aircraft. These require the adapter shown above. So now us aircraft people must be able to look at the port and tell what kind it is.

Is it a tapered pipe thread port (NPT), an ORB port or a AND style port? Curse you automotive people for bringing us your ORB crap! NPT is bad enough!

Here is a aircraft brake caliper with a male AN nipple sticking out of the port. It looks like out adapter above. But it's not.

Here is a picture of the entire fitting.

Notice that this is a standard AN fitting. Below is a picture showing how it installs with a boss o'ring.

Simple, just install a a boss O'ring onto the end of a standard AN nipple fitting and screw into the port. You can also use a bulkhead fitting and special nut for high-pressure hydraulic applications.

Here is a picture below.

This port (AND10050 - MS33656) has stronger (greater shear strength) threads (class 3 versus class 2). and can accomidate the extra length of the nipple. It is adaptable to a wide range of AN fittings and pressures.

AN fittings that can be screwed into AND ports are what the drawing calls a Type E style and can be used to seal on the flare OR seal on the nut with an O'ring. Not all AN nipple fittings are Type E. Below is a non-type E fitting.

Notice that there is no nut hex and no circumferal groove above the last thread. If you wish to use a Type E angle fitting then use the bulkhead series with a nut. You can now point the fitting any direction you wish.

What does a AND10050 port look like?

A AND port has a countersink around the top edge forming a 120 degree included angle. This port is also sometimes called a "Military Straight Tread Port". The latest drawing for this port is AS5202.

Sealing Aircraft Engine Cases

2009-06-18T12:48:00.000-07:00

John:
The sealing of engine cases seems to come up on aircraft groups often. Lycoming says to use POB #4 and some other materials. I have tried to ask question about where to purchase these materials and cannot get an answer. Some people say it is made by Perfect seal, and some say Permatex. Could you clearify this and show where and how to use these materials in a section? You could have pictures showing the thread and showing how much of the sealant is put on these areas.

Don,

I am reluctant to discuss crankcase sealing as my method (the traditional method) conflicts with both Lycoming and Continental. The way almost all overhaul shops did it 50 years ago is time proven - Titeseal and silk thread.

The Titeseal (usually medium weight) is used only to provide a tack surface for the thread; it does no sealing in itself. Thus the line of Titeseal need not extend completely across the surface, just wide enough to lay your thread. It should be absolutely thin.

The silk thread that Continental sells is the correct diameter (gage?) as too large a diameter will make a small divot into the crankcase parting surface.

The one problem with this method is that it's slow and large shops or factories think it takes too much time. They would rather slap some goop on the surfaces and torque it up.

Why Titeseal? It remains tacky so you have unlimited work time to lay down the thread. It doesn't cure into small balls or particles that can plug an oil passage (usually the oil passage through the rod bearing - only a few thousandths clearance). Also, Titeseal is a great NPT thread sealing compound so it has another purpose in the shop.

A possible shortcoming of silk thread is that silk thread is not tolerant to damaged or scored parting surfaces. I might consider a more elastic product when working with damaged faying surfaces.

Whatever method is used you have several objectives:

Thin is better as it retains the clamping force produced by torque. Thick gaskets or fluids that compress will loosen the joint and cause all kinds of problems.
Anti-creep. Product should not "run away from stress" i.e. it should be anti-extrusion and anti-oozing for the same reason as above. This eliminates many of the RTV style sealants unless applied extremely thin.
Product should not be capable of contaminating system. This eliminates RTV style sealants.
Product should be easy to remove at next repair.
Product should not create corrosion.

"The mark of an expert is decidedly not a big wad of hardened silicone out of every joint, but proper preparation of sealing surfaces." Greg McConiga, Motor Service, Feb. 2002.

Silk Thread part number from Continental: 641543
Titeseal is available from most aircraft parts houses. Lightweight titeseal works well on gaskets to keep them from leaking. Doesn't harden so the gasket removes easily during later repairs.

Inspecting the magneto coil

2009-06-16T07:04:00.000-07:00

I received this email inquiry:

John, Recently we have found, during a 500hr Slick Mag Inspections, defective coils. The engines in both of these cases were running OK when brought in for the inspection. When coils were tested found primary coil to be within tolerances but the secondary coil showed open resistance. In both cases we replaced coils. We just want to understand why the mags were still operating so we can give an "intelligent" answer to the customer as to why we had to replace coils when all seemed to be operating fine.

Cross-section of Slick coil. Large primary windings next to core. Smaller secondary windings on outside.

Interesting question as sometimes inspection standards seem arbitrary without an explanation of what their intended purpose. by "open resistance' I assume that you had infinite resistance or a open secondary circuit.

Your ohm meter does its check by flowing a tiny bit of current through the secondary wire at a fraction of a volt. Any broken wire will stop the current flow and indicate an open circuit. However, when the engine is operating the voltage in the secondary wires is high enough to jump a spark plug gap so it will easily jump across a small break in the secondary wire inside the coil and the magneto operates fine - for awhile.

The arching inside the coil causes the coil to get hot. On a high-voltage coil tester when you pick the coil up it's like a hot potato! The burning inside the coil starts to melt insulation and burn the secondary wires. Gradually the number of secondary windings reduces as the current shorts across adjacent wires. Now the ratio of primary windings to secondary windings is reduced so the transformer effect of stepping-up of voltage is reduced.

The first indication for the pilot is hard starting; at the slow rotation speed during start there is not enough voltage to spark the plug. Get it started and it runs fine for awhile as the high rpm creates enough current in the primary to off-set the degradation in the secondary. Eventually, the burning inside the coil reaches the outside by burning a hole out through one end of the coil.

It is amazing when you see this because you know that the aircraft was operated with all that internal coil damage. Eventually, enough secondary wires are damaged that the magneto won't produce enough voltage to fire the plugs. At that point the aircraft is grounded in some hole-in-the-wall airport with an angry wife and screaming kids.

Checking secondary resistance on a Bendix S-1200 series magneto

Editorial on inspections: The I-35W bridge that collapsed into the Mississippi river and killed 13 people was operating fine the moment before it collapsed. It had failed previous inspections and was rated as "structurally deficient". These inspections were ignored because it was operating fine. Operating fine - don't fix it or "if its not broke don't fix it - kills innocents

If it's not broke - don't fix it

2009-06-16T07:01:00.000-07:00

God I hate that saying. It's right up there with the pilot telling me "it will be OK. I'm in a hurry and gotta go."

The I-35W bridge that collapsed into the Missippie river and killed 13 people was operating fine the moment before it collapsed. It had failed previous inspections and was rated as "structurally deficient". These inspections were ignored because it was operating fine. Operating fine - don't fix it or "if its not broke don't fix it - kills innocents.

I have this problem with my aircraft engine...

2009-03-12T12:22:00.000-07:00

Thankyou for your inquiry regarding your aircraft engine. It has been over 15 years since I sold my engine overhaul business and I find my knowledge is dated and sometimes fading from memory. Even though I cannot answer your specific question, I would like to express my thoughts on airworthiness that might be of some help.

In the aviation industry airworthiness is based on objective evidence. This standard exists in most countries and is exemplified by the use of "inspectors" and "documents". Your question about the "airworthiness" or safety of your engine is a valid question and you do not need a reason to ask. In fact, inspector's will pull job sheets at random and ask for proof of airworthiness. Inspectors and inspection based on documents are an integral part of aircraft maintenance at every level.

How does your mechanic, maintenance shop, manufacturer, etc. show airworthiness? All of the following are typically required:

Evidence of approval from governing body,
Evidence of training,
Necessary tools and equipment at hand,
Appropriate maintenance manuals,
Calibration reports to show measurement integrity,
Inspection reports and check sheets,
Functional tests.

Objective standards of airworthiness are not based on a "personal" relationship between inspector and technician. In fact, this often hinders the objectivity of the inspection.

Your question involves a measurement issue. This can only be answered objectively by either:

obtaining the inspection reports showing dimensional recordings, with calibration sheets to back up measurement quality.
Re-taking the measurements.

The burden of proving "airworthiness" rests on the shoulders of the maintenance facility. Generally, there is a record retention time limit for such questions to be asked, often 3 years or longer.

Leaning a Carbureted Lycoming or Contintal past Peak

2008-12-21T07:42:00.000-08:00

If your objective is to lean of peak (LOP) then lean until the engine gets rough. However, I see no purpose for this. First outline the problem and the solution:

1. operate at best power to climb over that mountain, or maximize speed, or,

2. operate at the lowest specific fuel consumption to maximize range and minimize $, or,

3. operate the engine in a manner that minimizes lead deposit build-up.

Objective 2 just happens to be somewhere on the backside of an egt curve.

Objective 3 is indeterminate as it depends on the engine and power setting but as a general rule leaner is generally better up to a point.

If your objective is 2 then very SLOWLY lean until you feel a slight roughness and then enrichen slightly to remove the roughness. You are probably very close to lowest specific fuel consumption. So how does this relate to LOP? At least one cylinder is LOP. EGT readings show some cylinders somewhat hotter egt (closer to peak) or somewhat lower egt (before peak, or after peak). All very confusing display for the pilot and somewhat worthless data.

To add the the EGT confusion, there is no standard temperature drop past Peak EGT that produces the leanest operation without engine roughness. And it is engine roughness that limits lean operation, not some exhaust temperature reading. So lean to engine roughness and then enrichen slightly.

engine roughness defined for this purpose: A very slight and non-regular vibration pulse that your passenger probably won't notice. It is NOT a "rough engine"

All leaning suggested above is at power settings below 75% power.

Oil Filter Examination - continued

2008-08-09T09:17:00.000-07:00

John,
So, being a reformed lab-rat myself, I set up a little scope and had a look at my filter washings. Here's a little show if you have a minute to view and comment. I realize there are services that will do this and I will likely send the next filter to them to see what they say but I thought you might like to see what I came up with...Incidentally, under magnification, most of the tiny reflectivity's I was seeing on the filter in the sun represented light reflecting off of oil dampened flat surfaces of a variety of tiny non-metallic looking debris. My stereo scope:

I washed one half of the total available oil filter, after cutting it out, with solvent into a cup.For reference, the cup's base diameter is about 1.5 inches.Here's all that I got in washing:

Under magnification it looks like this (20-40X)

I threw in a hair (ouch!) for reference:

The vast majority of the material appears to be non-metallic junk (silicates, carbon, or whatever). I was able to see only a couple of very tiny magnetic bits using a magnet. You would never have seen them move with the naked eye. I next did a trial of drano with some aluminum filings so I'd have something as a reference. Bubble and fizz as advertised.I then applied the drano solution to the washings. I was able to locate only 2 or 3 microscopic fizzies out of the whole lot. Not much aluminum in there. What is notable, and you can see it in the photo immediately above, is that there is a scattering of metallic looking particles (red circle).These are mostly the width of a hair and smaller. They are not aluminum and are not magnetic. They have the appearance of microscopic bits of aluminum foil as you can see.Plating of some kind? Any thoughts? Know anyone crazy enough to look at their filters this closely?

I hope your not tiring of this! A little more... Re-reading your metals analysis post, I proceeded to look at the effect of HCL on the debri.I tried 25% HCL first with very little observable effect. I then went to 100% HCL. At this concentration, the tiny shiny bits were clearly seen to bubble and begin to dissolve. No green tinge was seen anywhere so I doubt chrome. There is no blackening seen either. Based on your post, I'm guessing tin. So, I would characterize this as scattered, microscopic platelets consisting of tin. Question is, is this a matter of concern or a normal wear finding? Any opinion? As I said, I will send the next filter out and see what the professional lab has to say.

-----------------

I thought this was a professional lab! Your microscope looks familiar - just like mine! A good bench microscope is a handy shop tool. Now that I'm older I use it to read tiny part numbers off of parts.

Your sample looks very clean. If you have blow-by of combustion gas past the rings, the sample will be darker with more flakes of black carbon. On turbocharged engines the filter paper will be almost black and brittle.

What I used to do was to squish all of the debris between two microscope slides. If it squished with no scraping sound - good - no sand or rocks. If it scraped, scratched, and made noise then I had some sand particles - time to check the induction system for leaks; or stop pulling out the alternate air door on run-up if the runway was dirty!

If your tiny bit of shiny metal is brittle then suspect chrome, otherwise tin if it passes the drano test. Either way one microscopic bit is not important.

I would use a stick magnet to collect all of the bits of iron. Usually they were not shiny and looked like iron filings that you collect from dragging a magnet in sand. If they encircled the tip of a stick magnet then there was cam or cam follower problems. If less, then everything was OK.

Bendix D3000 magneto redundancy

2008-08-09T08:34:00.000-07:00

John, I purchased and read your magneto book, which I found informative and interesting. After reading the book, I do have two questions I'm wondering if you could answer given your experience with magnetos.

1. Our Cardinal has the dual magneto, although I am considering a conversion to a non-D at the upcoming overhaul. My question regards redundancy: what internal elements are common to the two magnetos within the D housing?

Do they only share a common drive shaft (and gear), or are any other electromechanical components shared - e.g., cam, magnets, etc.? Do you know of any source where I could see internal photos or mechanical or cutaway drawings of the D3000 magneto so I could better understand its design?

-------------------------------
About the only parts that are truly redundant is the coil, points, and capacitor.

The D3000 magneto is a good magneto, easy to work on and reliable. Like any device it has its limitations. The following three areas should be well understood, respected and maintained, as all have caused fatal aircraft accidents:

Impulse coupling spring (part number 10-51324)
Hold-down clamps
Cam retaining screw

Impulse Coupling Spring:

Breakage retards timing causing complete loss of engine power. Cessna 172N N738BC ditched at sea with 2 fatalities. On any magneto the impulse area sometimes gets rusty from condensation. The impulse spring gets tiny rust pits that create stress corrosion cracking. There is no warning - it just breaks. With two magnetos you also loose timing but you can turn the bad magneto off. With the D3000 magneto the broken spring retards the timing on both magnetos and you lose power.

Follow TCM's instructions and recommendations to the letter in regards to inspecting and replacing the impulse spring. Personally, if I lived in a corrosive area I would replace it every year.

Hold-down Clamps:
See my article at http://www.sacskyranch.com/eng410.htm

Cam Retaining Screw: "Everything hangs on this connection"

Here is a copy of an email I received some time ago from Germany:

A pilot came to an aircraft workshop with the problem,that the engine did not start well and did not reach more the 2200 RPM. (Cessna 170 N engine Lyc.O-320-H2AD SNR L-8408-76T)

The workshop made -a test run on ground:

the engine rpm drop was 120-130 rpm.-
The different pressure in the cylinders was 1.)80/78; 2.)80/79; 3.)80/78; 4.)80/77.-
The intake tube of the Cylinder No 3 was leaky -it was renewed.-
The timing of the ignitions examined and adjusted.-
The air intake filter was dirty and cleaned -
The following ground check did show no problems: rpm drop 70-80 rpm max rpm 2320 rpm.

After this the aircraft made a take off and crashed immediately in the ground, as the engine lost power. The investigation did show, that the screw (Fig 1-14 D-3000 Magneto manual ) was loose and so the cam breaker could turn on the cone.

The problem here is that the mechanic did not order a new cam screw and re-used the old one. You loosen this screw when you adjust adjust internal timing. Continental says to replace it with a new one.

Continental (Bendix) has addressed each one of these areas in their maintenance manual. Personally, if the mechanic did not have the maintnenance manual for the D3000 in hand I would not let him touch the magneto.

There is an interesting discussion of this screw and the problems associated with installing it that I highly recommend be read by anyone working on this magneto. http://www.aaib.dft.gov.uk/cms_resources/Mooney%20Aircraft%20Corporation%20M20J,%20G-EKMW%2011-06.pdf