The Graphic Engine Monitor is ready to operate the moment electrical power is applied. Within seconds after starting the engine, orange bars stacked in 4 or 6 columns will begin to appear on the GEM display. Each column corresponds to the exhaust gas temperature of a cylinder. The lowest exhaust gas temperature that can be displayed by the GEM is 800°F. In some engines, the throttle will have to be opened to the fast idle range to get an EGT indication for all cylinders. As the cylinder heads begin to warm up, the display will indicate CHT for all cylinders as a dark (unilluminated) bar in each column. Until you shut down your engine at the end of the flight, the Graphic Engine Monitor will continue to indicate CHT and EGT and can be referred to at any time for leaning purposes, or to diagnose a possible engine malfunction.

First Flight with the GEM

During the first flight with the GEM the pilot should determine that the instrument is correctly calibrated. The GEM is calibrated at the factory for the average engine, but may require adjustment for some engines. Before your first flight familiarize yourself with the basic operating procedures outlined in this section. Establish a cruise altitude of five or six thousand feet and follow the steps outlined in Leaning Using Lean Mode to adjust your mixture to peak EGT. If your GEM is correctly calibrated and your engine is leaned to peak, the instrument(s) should resemble the photograph on the front or back cover. The highest bar(s) should be even with the asterisk reference mark and each column of bars should show one dark bar indicating CHT. If the highest bar is above or below the asterisk, the instrument requires calibration. After noting the position of the bars, complete the mixture adjustment procedure by enriching the mixture an appropriate amount. Should the instrument require calibration, this can be done in flight by a mechanic or on the ground. For GEM 602 or GEM 603, the mounting screws must be removed and the instrument backed out of the hole to gain access to the adjustment screw. Turn the screw clockwise to raise the bars and counter-clockwise to lower the bars.

For GEM 610 or GEMINI 1200 systems the palmtop computer is used to adjust the EGT column indication. Please see the chapter on 610 and 1200 for details.

Using GEM on the Ground

The temperature range of the GEM extends lower than most traditional EGT systems to include temperatures normally encountered at start-up. Under normal engine operation at 1,000 to 1,200 rpm, the GEM will produce a one or two bar EGT indication for each cylinder. The precise indication will vary from one installation to another, and it is not unusual to observe fairly large EGT differentials between cylinders at idle or taxi power settings.

One very useful feature of the Graphic Engine Monitor is its ability to detect abnormal combustion during the pretakeoff run-up. The primary purpose of the pretakeoff engine run-up is to verify the airworthiness of the engine's ignition system, plus carburetor heat and propeller control. Pilots without extensive engine instrumentation are accustomed to detecting engine and/or ignition problems by an rpm drop or roughness during the run-up. With the GEM, a much more accurate diagnosis of problems is possible.

As you run your engine up to 1,700 or 1,800 rpm (or as recommended in your aircraft's Pilot's Operating Handbook), you will observe a rise in EGT for all cylinders, to about one third of full scale. Normally, these indications will vary somewhat from cylinder to cylinder. The GEM should be carefully observed during the magneto check. Combustion is initiated by two spark plugs firing simultaneously in each cylinder. Under single mag operation, only one plug is firing, producing only one flame front in the combustion chamber, resulting in a slower, more prolonged combustion. This places the point of peak combustion pressure later in the power stroke and the tachometer will register a drop of 50 to 150 rpm. Since the exhaust gases have less time to cool before being expelled from the cylinder, the exhaust gas temperatures of all cylinders should rise two to four bars (50 to 100°F).

Various problems can be detected easily during run-up with the aid of the GEM. The absence of an rpm drop or EGT rise on single-mag operation operation indicates trouble in the form of a hot mag or defective ignition switch. A more common indication of trouble is the total disappearance of an EGT indication for one or more cylinders after switching to single-mag operation, indicating a faulty ignition wire or spark plug. If the affected cylinder returns to a normal EGT indication when operating on the other magneto, you have isolated the problem to a single spark plug (or lead) in a single cylinder.

In the absence of adequate engine instrumentation, the initial diagnosis of fouled spark plugs is usually made on the basis of a greater rpm drop for one mag than the other. Manufacturers' handbooks generally warn the pilot to regard any difference of more than 50 rpm between mags as suspicious. But it is important to note that an rpm drop will register only if more plugs are fouling on one mag than on the other. If each magneto harness harbors one bad plug or lead this would cause a uniform mag drop and the double fault would go completely undetected. On the other hand, an entirely different malfunction such as a partially plugged injector could create the same symptoms. Careful analysis of GEM data can help a pilot determine the precise cause of mag drop, or pinpoint problems hidden behind a uniform mag drop. In both cases cited above, the GEM would indicate higher EGTs for the affected cylinders.

Run-up is also a good time to check carburetor heat (if present) and mixture control. Application of carburetor heat causes a reduction in the density (and therefore oxygen content by volume) of air coming into the engine, inducing an over-rich condition. This is indicated by a noticeable drop in engine rpm and exhaust gas temperature. If the application of the carburetor heat control fails to produce these effects, it is likely that the carb heat control is misrigged, causing the airbox flapper valve to hang open and allowing hot air to leak into the carburetor on a full-time basis. This should be remedied as soon as possible.

During the mixture check, a uniform rise of EGT indications for all cylinders will confirm that the mixture control is functioning correctly. The amount of temperature rise will depend on the degree of mixture control movement, but four bars or more would be typical before the onset of engine roughness from fuel starvation. Each cylinder should show a rise in EGT upon leaning. Failure of a cylinder to show a significant rise, or an abnormally large EGT differential between cylinders in fuel injected engines, may indicate a fuel injector nozzle constriction. In many engines, a large intercylinder EGT spread is normal at low power settings (even with fuel injection) so a diagnosis of this type is impractical until the pilot becomes thoroughly familiar with the normal indications for his or her engine. Even so, this type of diagnosis, easily made with the GEM, is virtually impossible with other EGT systems.

Using the GEM on Takeoff

The Graphic Engine Monitor can be used during takeoff to identify a very serious class of combustion problems that can result from poor fuel distribution at takeoff power settings.

The combustion phenomenon known as preignition can do extensive damage in a matter of a few seconds if left unattended. This combustion process produces abnormally high temperatures in the combustion chamber which result in immediate full-scale EGT indications followed by a rise in cylinder head temperatures. Should this type of indication occur during the takeoff roll, the takeoff should be aborted. If takeoff has proceeded beyond the point of no return, power should be reduced immediately (maintaining flight) and the mixture enriched if possible to make the temperature drop in the affected cylinder(s). A precautionary landing should be made as soon feasible. Preignition can be caused by red-hot cylinder deposits or overheated exhaust valves. Regardless of cause, preignition, once started, causes an extreme temperature rise in the combustion chamber and is self-sustaining until engine failure occurs (often in as little as 20 seconds). Broken connecting rods, melted pistons, and cylinder head separation are among the common preignition induced failures. A second type of preignition that does not fit the previous definition is magneto induced preignition. It results from extreme timing errors in magneto adjustment or failure of the magneto itself.

Detonation in automobiles is commonly referred to as ping or knock. It is an unusually rapid form of combustion that follows ignition induced combustion and is caused by high compression, high temperatures and a lean mixture. The rapid combustion of detonation is significantly advanced by the time the exhaust valve opens and the temperature encountered by the EGT probe is lower than normal. Detonation results in higher peak combustion temperatures and pressures which translate into higher CHTs and lower EGTs. More importantly, detonation imposes significantly greater stress on the engine components than normal operation. It may be caused by excessively lean operation at high power settings because of fuel system malfunctions, injector nozzle constrictions, improper mixture control settings, insufficient fuel octane or avgas contaminated by jet fuel.

Leaning for Takeoff

Leaning normally aspirated engines for takeoff is advisable for best performance under high density altitude conditions and this is something that can be done with confidence and accuracy with the GEM. Remember that the full-throttle, full rich-mixture setting is designed to provide an enriched fuel flow for proper engine cooling during takeoff at sea level on a standard day. This over-richness is a FAA-mandated minimum of 12% above the worst case detonation-onset fuel flow.

With increasing density altitude, this over-richness robs your engine of power. Leaning on a high altitude takeoff can restore a significant amount of power and add measurably to aircraft performance. Consult the Pilot's Operating Handbook for the airplane manufacturer's recommended high altitude takeoff procedures. On some aircraft equipped with fuel flow gauges, the full-power altitude reference marks indicate acceptable fuel flows for various altitudes (typical reference marks are S.L., 3000, 5000, 7000). Sometimes a specific temperature (150°F rich of peak EGT for example) is specified as the takeoff power mixture guideline.

After some experience with the Graphic Engine Monitor to determine the location of peak EGT, the GEM can be used to set the mixture using this guideline, or (with careful operator technique) to produce the EGT indications similar to a normal sea level takeoff (4 to 6 bars below the asterisk reference mark).

Leaning Normally Aspirated Engines in Climb

Most normally aspirated aircraft benefit from mixture leaning during climb with less plug fouling, better engine performance, smoother operation and increased economy. The full throttle, full rich mixture setting is designed to provide an enriched fuel flow for proper engine cooling during takeoff at sea level on a standard day. As the aircraft climbs, the air density decreases causing an effective enrichment of the mixture, eventually robbing the engine of power. Leaning in climb is advisable for best performance and will result in a cleaner engine and easier cruise leaning later on.

After safely clearing the field, observe the location of the tops of the bars on the GEM. As you ascend, the effective mixture enrichment that results from the decreasing air density causes the EGT reading to fall. Observe one column as a reference. When the reading drops one bar, lean the mixture until the reading goes up, restoring the dropped bar. Repeat this procedure each time the EGT reading drops a bar due to ascent into less dense air to ensure that highest EGT is 4 to 6 bars below the asterisk reference mark. Aircraft equipped with fuel flow gauges may have altitude reference marks to guide leaning during climb.

This procedure for leaning in climb does not apply to turbocharged engines which do not experience the same air density variations due to altitude.

Leaning without Lean Mode

There are occasions when the pilot may wish to lean manually. It is informative on the first GEM training flight to lean the engine without Lean Mode to get a feel for the instrument. As you lean, the bars will rise, reach a maximum, and then fall at the onset of engine roughness. The average of the bars should reach the asterisk reference mark before falling. If they do not, consult the calibration procedure in the GEM Installation Instructions. If you lean too far the engine will stop. Short flights in high traffic density Terminal Control Airspace (Class B Airspace) demand maximum pilot attention to traffic avoidance. When busy, the pilot may lean quickly by watching the bars rise and stopping when they are a couple of bars below the normal average indication. This procedure will be within a gallon or two per hour of the optimum mixture setting, and can be used as a temporary measure until time permits using the complete leaning procedure described below.

Leaning using Lean Mode

The basic GEM cruise-leaning procedure is as follows:

Establish cruise altitude and cruise power. Make initial trim adjustments, etc. as needed to establish cruise.

Perform a coarse leaning or preliminary leaning of the engine until the EGT bars rise to a bar or two below the normal cruise indication, or until experience tells you the fuel flow is within a couple of gallons per hour of the anticipated final fuel flow.

Pause for two minutes to allow the engine to stabilize and cylinder head temperature to return to normal. It is advisable to allow up to five minutes for the turbocharger (if so equipped) to stabilize in output before attempting final leaning. During this time you can make final trim adjustments to the airplane, reset cowl flaps, etc.

Push and hold the GEM Reset Button for a second or two to enter Lean Mode. When you have entered Lean Mode, the EGT annunciator will begin blinking.

Now slowly lean the mixture until one of the EGT columns blinks. This final leaning should take about five seconds. The blinking column of bars identifies the leanest cylinder (the first to reach peak EGT). The mixture may be slightly too lean depending upon how quickly the pilot has reacted to annunciation of peak EGT. Push the Reset Button briefly to stop the blinking.

Enrich the mixture as desired. There are several ways of enriching the mixture. If the aircraft has a fuel flow indicator the pilot may elect to operate the engine at a fixed margin (e.g. 1/2 gph) on the rich side of peak. Alternately, the pilot may choose to operate the engine at a fixed temperature drop on the rich side of peak. Enriching the mixture until EGT drops one bar will ensure that you are not on the lean side of peak and will establish a best economy mixture setting (see Figure 1). To select the best power setting the mixture should be enriched further to drop the EGT three to four bars from peak EGT (75-100°F). If the engine and airframe manufacturer approve continuous operation at peak EGT for the current power setting and operating conditions the pilot may elect to not enrich at all.

Note: Engine manufacturers differ in their approval of operation at peak. Lycoming recommends operation at peak for power settings of 75% and less while Continental recommends operation at peak for power settings of 65% and less.

Do not lean to peak EGT power settings greater than those recommended by the manufacturer.

If you have enriched the mixture after establishing peak EGT, push the Reset Button again to store this new exhaust gas temperature for Monitor Mode.

This procedure may not be applicable to all engines. In some aircraft the mixture may be dictated by other parameters: see Leaning Restrictions, Leaning by Turbine Inlet Temperature, and Special Considerations for Turbos.

Leaning by Turbine Inlet Temperature

Some turbocharged engines are designed to be leaned by reference to turbine inlet temperature. This may imply that the TIT is the first temperature to reach redline and is the overall limiting factor in the leaning procedure. Some manufacturers may put a limit on the TIT to increase detonation margins. In general, turbochargers are very much alike and most manufacturers specify a redline of 1650°F. Some operate as high 1750°F. Because indicated temperature is largely dependent on probe placement and exhaust flow, it may not be the same as that experienced by the turbo. Aircraft manufacturers have very likely taken this into account when deciding on the official TIT redline.

Leaning Restrictions

Some aircraft have restrictions on leaning that must be observed. The recommendations of this manual are not intended to supersede any specific requirements for engine operation as stated by the aircraft or engine manufacturer. The pilot should consult the Pilot's Operating Handbook and follow the manufacturer's recommendations. These restrictions typically, (but not exclusively) apply to aircraft with marginal cooling airflow at high altitude or high angles of attack or turbocharged engines where concern over turbine inlet temperature, compressor discharge temperature, detonation margin, or cylinder head temperature must dictate mixture settings.

There are certain times when you should not lean to peak or even attempt to find peak. In full power climb or any time the engine is operating at power settings in excess of 75%, leaning to peak could result in detonation and serious engine damage. This is especially true for high performance engines and turbocharged aircraft. In lieu of specific manufacturer's recommendations, lean manually to obtain EGTs no higher than 6 bars below the asterisk reference mark.

The Importance of Measuring Turbine Inlet Temperature

The measurement of turbine inlet temperature has become popular in recent years with some aircraft coming so equipped right from the factory. Although turbine inlet temperature is an invaluable operating parameter, a great deal of confusion still surrounds TIT indications and their meaning.

Turbine inlet temperature is measured by a single probe mounted in the exhaust inlet to the turbocharger. The TIT display shows the temperature of the exhaust gases that drive the turbo. In many cases this probe is just a foot or so downstream of all the EGT probes. At first glance this measurement appears redundant. Why read the temperature again when it is just the collection of all the EGTs? TIT is not a simple function of the collective exhaust gas temperatures. It may be hotter than the hottest EGT that feeds it or cooler than the coolest EGT. The temperature measured by the EGT probe is the average of the pulse of high temperature gases that exit the cylinder when the exhaust valve opens. The TIT probe sees the collection of pulses from all cylinders that feed it and will indicate a higher temperature.

Turbo action is throttled by the wastegate valve that forces a portion of the exhaust gases to bypass the turbo. At low altitude, with little demand for turbocharging, the wastegate will direct a large part of the exhaust past the turbo and the TIT probe will read a lower temperature. At higher altitudes the wastegate will close to direct more energy to the turbo and a higher TIT will be indicated.

TIT is not just a simple function of EGT and this is very important to consider when operating a turbocharged engine. A power setting and fuel flow that may be well below peak EGT and well below the TIT redline temperature at 9000 ft may easily exceed the TIT redline at 16000 ft. The higher temperature results from more exhaust gas driving the turbo to restore the manifold pressure at the higher altitude.

The TIT reading is a key factor in leaning the turbocharged engine. It also provides diagnostic information that is unavailable from other sources. A wastegate system malfunction will affect TIT readings under conditions where other indications are normal. Should the wastegate stick closed at high altitude, all indications would appear normal. Subsequent throttle power reductions for descent would show a deceptively normal decrease in manifold pressure but abnormally high TIT readings for that situation. Other factors such as ignition, fuel distribution, induction, or compression that affect EGT will also affect TIT; sometimes with detrimental results. For example, ignition failures that cause the EGT to rise may increase the TIT past redline.

Special Considerations for Turbos

Turbocharged engines exhibit some special characteristics that result from the interaction of the turbocharger, throttle, wastegate controller, and other engine components. These interactions will vary in degree depending on the engine type and installation. In the normally aspirated engine, the components of combustion are essentially fixed for a given throttle and mixture setting. Any mixture control change results in a direct mixture change. The turbo has one additional complication that results from mixture changes. A change in mixture changes the exhaust gas energy that drives the turbo. This change in turbo drive energy changes the induction or manifold pressure and temperature and may or may not be compensated for by the turbo wastegate controller.

The turbo also has significant inertia which causes a lag in response to changes in drive energy. The result of this turbo bootstrapping is a change in the EGT/Fuel Flow Curve depending on the direction of mixture movement. This lag must be understood and taken into consideration to properly lean the engine. This change in the curve (see Figure 4) becomes evident if the pilot tries to enrich the mixture to drop the temperature one bar. In most turbocharged engines it will take considerably more fuel flow to drop the temperature one bar than it did to achieve that temperature on the way up. For example, in a normally aspirated engine, enriching for a 25 degree drop may take a 1/2 gph increase in fuel flow. The same model engine when turbocharged may require a 2-4 gph increase in fuel flow to get the same 25 degree drop. Paradoxically, the pilot may even see EGT rise when he starts enriching before it begins to fall.

Another observable characteristic is that the required fuel flow is dependent on altitude under conditions of constant rpm and manifold pressure. It may seem reasonable that the optimum mixture for a given power setting should remain constant. However, when the turbo compresses the induction air it also increases its temperature and reduces its density. Although the manifold pressure is restored, the oxygen content of the induction air is reduced because it is a function of air density. It should be remembered that the exact nature of this complex and confusing issue is dependent on the engine and installation. For this reason it is difficult to make generalizations about the leaning characteristics of turbocharged engines, but one thing can be said with certainty: a generous enrichment of the mixture from peak will prolong the life of exhaust valves, the wastegate and the turbocharger itself.

Special Considerations for Twins

Some twin engine aircraft exhibit an unusual mixture control reversal characteristic. We speculatively attribute this to the long flexible cable used to link the cockpit controls with the engine. The phenomenon is easily observed in aircraft with fuel flow gauges. When the pilot pulls back on the mixture controls to lean the engines, fuel flow is reduced and the EGT rises as expected. But when the mixture controls are pushed forward to enrich the mixture, the fuel flow continues to drop and the EGT drops on the lean side of peak. Even though the mixture control is moved in the rich direction, leaning continues. It would appear that the function of the mixture control has temporarily reversed! Continued movement of the mixture control picks up the slack and normal mixture function resumes. The magnitude of this phenomenon varies from aircraft to aircraft, but we have observed transitions of up to 1.5 gph past peak before the fuel flow began to increase. Monitor the fuel flow gauge to identify this phenomenon in your aircraft.