Sunday, March 7, 2010

Mag Check

If you fly a piston-powered aircraft, you undoubtedly were taught to perform a "mag check" during the pre-takeoff runup. But do you know how to do it correctly, what to look for, and how to interpret the results? Surprisingly, many pilots don't.

To begin with, most POHs instruct you to note the RPM drop when you switch from both mags to just one, and give some maximum acceptable drop. This archaic method makes little sense for aircraft that are equipped with a digital engine monitor (as most are these days), because EGT rise is a far better indicator of proper ignition performance than RPM drop. You should focus primarily on the engine monitor, not the tachometer, when performing the mag check. What you should be looking for is all EGT bars rising and none falling when you switch from both mags to one mag. The EGT rise will typically be 50 to 100 degrees F, but the exact amount of rise is not critical, and it's perfectly normal for the rise to be a bit different for odd- and even-numbered cylinders. You should also be looking for smooth engine operation and stable EGT values when operating on each magneto individually. A falling or erratic EGT bar or rough engine constitutes a "bad mag check" and warrants troubleshooting the ignition system before flying.

The "mag check" is poorly named, because because the vast majority of "bad mag checks" are caused by spark plug problems, not magneto problems. (It's really an "ignition system check.") How can you tell if the culprit is the plugs or the mags? Simple: A faulty spark plug affects only one cylinder (and one EGT bar on your engine monitor), while a faulty magneto affects all cylinders (and all EGT bars).

If you get an excessive RPM drop when you switch to one mag, but the EGTs all rise and the engine runs smooth, chances are that it's not a bad mag but rather retarded ignition timing. This is sometimes caused by mechanic error in timing the mags, but it can also be caused by excessive magneto cam follower wear (possibly due to inadequate cam lubrication) or some other internal mag problem. Retarded ignition timing also results in higher-than-usual EGT indications.

Conversely, advanced ignition timing results in lower-than-usual EGT indications, and also higher-than-usual CHT indications. Advanced timing is a much more serious condition because it can lead to detonation, pre-ignition, and serious engine damage. If you observe low EGTs and high CHTs after an aircraft comes out of maintenance, do not fly until you've had the ignition timing re-checked.

The usual pre-flight mag check is a relatively non-demanding test, and will only detect gross defects in the ignition system. To make sure your engine's ignition is in tip-top shape, we recommend performing an in-flight mag check at cruise power and a lean mixture (preferably a lean-of-peak mixture). Because a lean mixture is much harder to ignite than a rich one, an in-flight LOP mag check is the most demanding and discriminating way to test your ignition system. It's a good idea to perform one every flight or two.

The in-flight mag check is performed at normal cruise power and normal lean mixture (preferably LOP). Run the engine on each individual mag for at least 15 or 20 seconds. Ensure that all EGTs rise, that they are stable, and that the engine runs smoothly on each mag. If you see a falling or unstable EGT, write down which cylinder and which mag, so your mechanic or SAMM account manager will know which plug is the culprit. 

Changing the Oil

I normally recommend oil changes every 50 flight hours or 4 months, whichever comes first. (If you fly less than 150 hours a year or your flying tends to be seasonal, the 4 months will usually come first.) I often suggest shortening the oil-change interval after certain engine maintenance (e.g., cylinder change) or if oil analysis results reveal elevated wear metals.

It's important to understand why the oil needs to be changed so often. It'snot primarily because the oil breaks down in service and its lubricating properties degrade. Petroleum-based engine oils retain their lubricating properties for a long time, and synthetic oils retain them nearly forever. Consider that cars typically go 7,500 miles between oil changes, which is the equivalent of 150 to 200 hours. Studies have shown that synthetic automotive oil like Mobil 1 can go 18,000 miles without appreciable degradation, and that's equivalent to 400 to 500 hours.

The reason we change oil in our aircraft engines every 50 hours or less is not because it breaks down, but rather because it gets contaminated. In fact, it gets downright filthy and nasty. That's because compared to automotive engines, our piston aircraft engines permit a far greater quantity of combustion byproducts -- notably carbon, sulfur, oxides of nitrogen, raw fuel, partially-burned fuel, plus massive quantities of water -- to leak past the piston rings and contaminate the crankcase. This yucky stuff is collectively referred to as "blow-by" and it's quite corrosive and harmful when it builds up in the oil and comes in contact with expensive bottom-end engine parts like the crankshaft, camshaft, lifters and gears.
To make matters worse, 100LL avgas is heavily laced with the octane improver tetraethyl lead (TEL), which also does nasty things when it blows by the rings and gets into the crankcase. (Back in the days that cars ran on leaded gasoline, oil change intervals were typically 3,000 miles instead of 7,500.) So one of the most important reasons that we need to change the oil regularly in our Continentals and Lycomings is to get rid of these blow-by contaminants before they build up to levels that are harmful to the engine's health.

Another reason we need to change the oil frequently is to replenish the oil's additive package, particularly its acid neutralizers. When sulfur and oxides of nitrogen mix with water, they form sulfuric and nitric acids. If you remember these dangerous corrosives from your high school chemistry class, you'll appreciate why you don't want them attacking expensive engine parts. To prevent such attack, aviation oils are blended with acid neutralizer additives. These are alkaline substances that neutralize these acids, much as we might use baking soda to neutralize battery acid. Because these acid neutralizers are consumed by the process of neutralizing acids, it's imperative that we replenish them before they get used up to an extent that might put our hardware in jeopardy.

At each oil change, it's essential to change the oil filter and cut open the old filter for inspection. We also strongly recommend sending an oil sample to Blackstone Laboratories in Ft. Wayne, Indiana, for oil analysis. Although there are a number of different labs that do such analysis, we think Blackstone is head and shoulders above the rest, and we urge our clients to use this lab exclusively.

Watch Your Language!

When an aircraft component becomes inoperative or unairworthy, we usually have a number of options. We can replace the bad component with a new one, a rebuilt one, an overhauled one, or perhaps send out our defective component to have it repaired. These four words -- new, rebuilt, overhauled and repaired -- have distinct, specific meanings in the context of aircraft maintenance, and it's important to understand precisely what they mean and how they differ:
  • New means never used. Dimensionally, a new component meets new fits and limits (obviously).
  • Rebuilt means a previously used component that has been overhauled to new fits and limits (possibly using approved oversize or undersize parts) by the original manufacturer. (For example, only the TCM factory can "rebuild" a TCM engine, although any A&P mechanic can "overhaul" one.)
  • Overhauled means disassembled, cleaned, inspected, repaired as necessary, reassembled and tested in accordance with the manufacturer's approved technical data (normally the overhaul manual). The word "overhaul" implies conformance to service limits, not necessarily new limits, so if you want new limits you have to specify "new-limits overhaul." (A new-limits overhaul is essentially the same as "rebuilt" except that it doesn't have to be performed by the original manufacturer.)
  • Repaired means inspected and repaired as necessary ("IRAN") to restore the inoperative compoment to proper working condition. This term implies nothing about fits and limits, because there is no requirement to measure anything when performing a repair. One could, for example, remove a cylinder, replace the exhaust valve and guide, and then put the cylinder back on the engine without measuring anything, and call it a "repair." A repair differs from an overhaul primarily in that there's no obligation to follow the fits, limits, mandatory replacements, and other procedures in the manufacturer's overhaul manual.
The words "overhauled" and "rebuilt" are defined in FAR 43.2, and have very specific regulatory meanings as described above. If a mechanic documents that something is "overhauled" and hasn't complied with every jot and tittle of the overhaul manual, he can lose his A&P certificate. However, if he documents it as being "repaired," he can do as much or little as he sees fit to do, so long as he is satisfied that his repair work is airworthy.

In short, if you ask for a "repair" you give the mechanic or technician considerable discretion to do only as much work as he believes needs to be done. If you ask for an "overhaul" you eliminate his discretion and require him to do everything precisely "by the book."

Repair is almost always the lowest-cost method to get a problem resolved. Often, the cost of having something overhauled will be much higher than the cost of having it repaired. For example, a propeller "overhaul" is typically twice as expensive as a "reseal repair." In some cases, having a malfunctioning gyro flight instrument "overhauled" can cost ten times as much as having it "repaired." The "O-word" is one of the most expensive and overused words in aviation maintenance. It is invariably a waste of money to have something overhauled if a simple repair will suffice -- often a lot of money!

Of course, the "R-word" is even more expensive than the "O-word" and the "N-word" is the most expensive of all. The object of the game is to use the least expensive word that will get the job done. So be careful to use these words carefully when approving maintenance work on your aircraft.

Fast, Good or Cheap: Pick Two

There's an old saw in product design that goes, "fast, good or cheap: pick two." It means that:
  • If you want both quality and fast turnaround, expect it to be expensive.
  • If you want both quality and low cost, expect it to take a long time.
  • If you want both low cost and fast turnaround, expect inferior quality

The same principle usually applies to aircraft maintenance. When my company manages maintenance on behalf of aircraft owners, we usually use our best efforts to obtain the highest quality maintenance at the lowest possible cost. In other words, "good" and "cheap." To the extent an owner applies time pressure to get the maintenance done quickly, there will have to be a sacrifice in either quality or cost. Since sacrificing quality is often not an option in aviation maintenance (especially if it could compromise safety), faster usually implies more expensive. But since rushed maintenance invites mistakes, faster sometimes results in errors that wind up causing additional downtime and expense to correct.

Occasionally, getting the airplane back in the air as quickly as possible is worth increased maintenance cost. But often, aircraft owners apply time pressure to maintenance providers without giving any thought to the adverse consequences of doing so. It's always a good idea to think carefully before you ask for quick turnaround. There's no such thing as a free lunch.