G SERIES PILOT’S GUIDE .
INSIGHT
INSTRUMENT CORPORATION BOX 122, FORT ERIE, ONTARIO, L2A 5M6
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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.
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 inter-cylinder 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 G3, is virtually impossible with other EGT systems.
Using G3-4 On Take-off
The G3-4 can be used during takeoff to identify a very serious class of combustion problems that can result from poor fuel
distribution at take-off power settings.
The combustion phenomenon known as pre-ignition can do extensive damage in a matter of a few seconds if left unat-
tended. This combustion process produces abnormally high temperatures in the combustion chamber which results in im-
mediate 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.
Pre-ignition can be caused by red-hot cylinder deposits or overheated exhaust valves.
Regardless of cause, pre-ignition, once started, causes an extreme temperature rise in the combustion chamber and is self-
sustaining until engine failure occurs (often in a few seconds).
Broken connecting rods, melted pistons, and cylinder head separation are among the common pre-ignition induced failures.
A second type of pre-ignition that does not fit the previous definition is magneto induced pre-ignition.
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 pres-
sures which translate into higher CHT’s and lower EGT’s. 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 Take-off
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 G3-4.
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 (eg. 150° F rich of peak EGT) is specified as the takeoff power mixture guideline.
After some experience with the G3-4 to determine the location of peak EGT, the G3 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.
