Last month we discussed what trend monitoring is, why everyone should be doing it and how it can be done. You also had an assignment to go out to your plane and consider as many sources for aircraft data as possible. By now you should have a spreadsheet, or at least a list, to keep this data. This month we’ll see how that data might be converted into valuable information.
While we still have our dictionaries out from last month, let’s take a look at Data and Information. Merriam-Webster defines “data” as “factual information (as measurements or statistics) used as a basis for reasoning, discussion, or calculation”. Data is actually plural for datum. “Information” is defined as “knowledge obtained from investigation, study, or instruction”. My paraphrased version is that data is the raw numbers, while information is the knowledge gained by analyzing that data.
To understand a trend from our data, we must first have some sort of baseline or reference point. We should also establish what parameter exceedances create enough urgency to act. With good routine data sampling frequency, we might even see the data trending toward our preset limit and mitigate some catastrophe.
Everyone monitors trends on their tires, we probably just don’t think of it in those terms. We check the tire pressures routinely and note the rate of loss. At some point a decision is made to add air or nitrogen. To monitor this trend and react is an effort to keep tire wear to a minimum and tire performance at a maximum.
We also watch the tread loss rate. Short of a flat spot event, we have a pretty good idea how many landings remain before a change is needed. This is trend monitoring at its simplest and most obvious.
Oil Pressure vs Oil Temperature
Monitoring oil pressure can give us a good idea of the crankshaft bearings’ condition (main and rod). The concept is that at a given viscosity, the oil is pushed out of the gear driven oil pump into a relatively fixed outlet, i.e. the space between the crankshaft and its bearings. As the bearings wear, the gap increases, which presents less of a restriction to the oil flow, so oil pressure is reduced.
Lycoming engine oil systems are generally high pressure – low volume. One might expect oil pressure to run about 80 psi with oil temperature at 180 degrees C. The Continental engines are low pressure – high volume, so 50 psi at 180 degrees C is normal. For trend analysis, we’re more interested in changes than ultimate pressures. We’ll usually notice sudden changes, but documenting allows us to see trends over very long periods of time.
It’s important that pressure readings be taken at similar viscosities, meaning it should be the same oil type, weight and temperature. As the oil pressure trends down over the life of the engine, adjustments can be made to the pressure regulator. Any adjustments should be noted in the data records. At some point the regulator will have been adjusted to its maximum, and that will be the point at which the downward trend can give information on scheduling some engine repair action (basically means an overhaul is imminent).
Tracking oil consumption is one of the easiest trend monitors but so often ignored. Whether an engine consumes or expels one quart in twenty hours or one in four hours isn’t particularly important. However, noticing a change in that consumption is very interesting. A sudden increase in oil use could indicate the piston oil ring drain returns are restricted, or a piston ring is broken, or the cylinder walls suddenly glazed over. It could also indicate leaks in an oil cooler or a hose.
To get useable information, data point oil levels should always be checked during the same aircraft situation. By that I mean the engine should be at similar temps and with the aircraft level. The best scenario would be for the plane to have been sitting in the same place for at least twelve hours. This ensures oil from all the galleries in the engine have made its way to the sump, resulting in valid readings.
Cylinder Head Temperatures
Following cylinder head temperatures (CHT) at particular stages of flight with references to outside air temperatures, airspeed, and cowl flap positions can help determine effectiveness of baffling material and even magneto timing.
A change in normal CHT during a particular flight phase could indicate a piece of baffling is out of position. A sudden change in CHT after maintenance might be a warning the cowling wasn’t installed properly.
A sudden change in CHT after maintenance might also indicate improper magneto timing. Advancing magneto timing just a few degrees will increase the engine’s power output which in turn increases heat production. Conversely, lower temps might indicate the mag timing is retarded somewhat. A slow change in CHT could indicate wear of the magneto points which can effectively change the timing as well.
Tracking take off power exhaust gas temperatures (EGT) is an excellent indicator of full power mixture settings. A common EGT at take off is in the 1300 degree range. Your EGT probe installation may give different numbers, but you can find the normal for your engine installation for continued reference. Changes in take off EGT could indicate fuel/induction system issues. Higher EGT indicates a leaner mixture, and lower temps show richer mixture. Normally aspirated engine mixtures at the full rich position will vary with density altitude, so remember to consider that.
Knowing the normal wide open throttle full rich mixture take off EGT is also very useful for leaning in the climb on normally aspirated engines. Take off with throttle, prop and mixture all forward and note the EGTs. Leave everything forward during the climb, except lean the mixture as needed to maintain the same EGT. This keeps the same relative mixture setting as at take off for the duration of the climb and should provide sufficient fuel for cylinder cooling as well.
Amps and Volts
Most legacy aircraft have an ammeter but no voltmeter. A voltmeter is infinitely more useful for determining the condition of the charging system. The combination of the two provides enough data to create some information for analysis. At a given buss voltage, any electrical system on the plane should use the same amperage. A lower current draw on electric prop heat,for example, would be a first indication of an open on one of the heating elements. A higher current would indicate a short in one of the elements. In both cases, you’d know flying into a potential icing situation would be a poor decision. The same can be said of almost any electrical system in the plane.
Electrical landing gear motors can be monitored for trends in current draw, operation cycle time, and pressure reset cycling for those with electrically driven hydraulic pumps. This bit of information can be used to plan for future maintenance or to prevent an unexpected hydraulic system failure.
The annual FAA required compression check has been the most commonly used measure of engine condition and trend monitoring. Unfortunately, compression checks don’t actually tell us much of anything about the overall condition of the engine. At best, it can tell us if there’s good reason to perform a borescope inspection of a cylinder.
I realize the compression check is somewhat of a sacred cow, but the truth is that it can’t be relied on as a linear indication of cylinder wear. If you have the same mechanic perform the compression checks every year for many years using the exact same technique, then you do have a chance of getting usable data. However, there are very few of us that will own our airplanes long enough and use the same mechanic long enough to see any real benefit from tracking compressions. From my view, compression checks are mostly a current situation test and not a trending indicator.
Cylinder Lean Check
The cylinder lean check, also known as the GAMI lean check, is a test to determine the relative performance of the cylinders. As the mixture is leaned, the fuel flow at which each cylinder’s EGT peaks is noted. A fuel flow difference between the first cylinder to peak and the last is called the GAMI spread. A GAMI spread of less than 0.5 gallons per hour is pretty good.
For trend monitoring, we’re not all that interested in the ultimate fuel flow spread, but rather any change that occurs from one check to another. When the test is performed under nearly identical conditions, the total fuel flow spread and the order in which the cylinders peak is usually very similar.
A restriction to free fuel flow is the most likely failure of an injector nozzle. This most commonly occurs after maintenance is performed on the fuel system. A restricted nozzle will cause one cylinder to run leaner than the others and will show on the lean check as peaking prematurely.
A lean check can also indicate leaks beginning in the induction system. There are several variables in this case based on whether you fly an engine that’s normally aspirated, turbo normalized, or turbocharged. If it’s a Continental engine, the under cylinder induction piping will respond differently than the overhead tuned induction systems. Detail discussion on the possibilities is a bit longer conversation than I have time to type, so we might talk about this another day.
A magneto check at 1800 RPM during preflight is mostly only going to reveal something that’s totally failed. It’ll show a spark plug that has a speck of debris shorting across the electrodes or a magneto that’s falling apart. It won’t give you an idea about the performance of the ignition system. An in-flight check at cruise power and altitudes is very informative and can expose components that are beginning to perform at less than desired levels.
In-flight mag checks should be just like those on the ground; when switched to just one mag, the EGTs all rise and the engine should run smoothly. Switch back to both, and EGTs go back down to normal and all still runs smoothly.
A spark plug may fail under flight loads due to the more extreme vibrations, heat, or pressures. A spark plug will also give about the same failure at any altitude. You’ll know exactly which plug it is because the EGT on that cylinder will drop immediately. A spark plug wire failure will present very similarly to a bad spark plug, but is often affected by altitude.
Plug wires have a center conductor, an insulator, and then a shield around the outside. For various reasons the insulator can deteriorate, allowing the high voltage in the center conductor to arc over to the shield. Since air is a good insulator, lack of air due to high altitudes will create an easier path for the arcing.
One may find that at relatively low altitudes, say below 5,000’, the in-flight mag checks are good. However, on the same flight a check at 12,000’ feels very rough on one mag, but the EGT display appears normal. Another check at 18,000’ might feel terrible with one EGT display being a bit erratic. This would be a classic indication of a plug wire failing.
Your mechanic can try to confirm the in-flight discovery using a high voltage tester, but it’s very unlikely that he/she will be able to duplicate the problem. The test box only puts out about 1/10th the power as a magneto, and the plane is at very low altitude (on the ground). For mag wire monitoring, an in-flight mag check at low and high altitude is the most reliable method.
Finding and addressing spark plug or plug wire problems early on will greatly prolong the life of the magneto points. Performing these in-flight routine trend monitoring checks is not only the the best way to find problems but just about the only way.
Oil analysis is pretty straight forward, and most of us have figured out the advantages. The important part is to take samples regularly. As with any trend monitoring, one bad analysis isn’t very meaningful if not supported by previous trends.
Flight rigging checks are a slightly special case in terms of trends. Short of some structural damage, airplanes don’t get out of rig on their own or due to wear. Changes in rigging are mostly maintenance induced.
It is true that control cables get loose, but the change in rigging actually occurs when a mechanic tightens the cables. If not done properly, the control will be slightly out of rig. The first cable adjustment won’t be noticed, but the same unbalanced adjustment repeated over many years eventually gets things out of whack.
A post maintenance flight check should expose any significant flight control problems. With even fuel in each wing and even weight in the left and right seats, a plane should fly straight and level with the ball centered. It should also hold a 15 degree bank all the way around a 360 turn, hands off in left and right turns. Deviations from that indicate incorrect adjustments have been made.
Lycoming O-235 Extra
The 152 owner with the O-235-L2C engine might want to track valve tappet adjustments. At each 100 hours in service, the hard lifter gaps should be measured. Tracking the results of these measurements and adjustments provides very good data. This could give valuable insight into cam lobe wear, cam follower wear, rocker arm wear, and even push rod condition.
Logging and parsing the data
There are many ways to record data, depending on what’s available to you. Recording electronic engine monitors make the task very easy. All can be uploaded to www.savvyanalysis.com for simplified viewing in graph form.
Most of us have cameras in our mobile phones which make a very handy tool for documenting what we see in the plane. That video camera in the phone comes in pretty handy too. It’s amazing what you can pick up on when a picture or video is reviewed in the quiet of sitting at your computer. Of course, remembering you have that phone camera/video with you in the plane is the trick.
As a last resort, there’s always pencil and paper for jotting down notes or keeping a log of oil temp vs oil pressure, oil consumption, max RPM, max FF, etc. for future review. It’s tedious, but transcribing all that into a spreadsheet for display on a graph really brings the data to life.
Collecting data is a continuous effort. The more data you have, the more information can be gleaned from it. Taking data points at each preflight, flight, and post flight is ideal. As a bare minimum, data should be collected before and after every maintenance event, especially annual inspections.
What’s the Pay Off?
Knowing what the norm is helps to know when and what a problem is in flight. Having already researched the cause and effect, one has a much better insight as to the urgency to react.
Some of the monitoring possibilities I used as examples may seem more like troubleshooting tools. In reality, that’s exactly what trend monitoring is-troubleshooting for future problems. Whether you own a 1962 150 or a 2014 TTX, monitoring any data that’s available is helpful. Truly, knowledge is power.