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TECH SUPPORT #2
GEM Series  Pilot's Guide
GEM Leaning Procedure
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 (TM) 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. 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.
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