G SERIES PILOT’S GUIDE .
INSIGHT
INSTRUMENT CORPORATION BOX 122, FORT ERIE, ONTARIO, L2A 5M6
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After you have returned to the airport and shut down after your first G3-4 test flight, we recommend you remove the SD Card
and examine the data log using your favorite spreadsheet program. If you didn’t get a chance to set your EGT bar heights
as discussed earlier, you will get a good idea where it should be set for next time by looking at the EGT columns.
Using G3-4 On The Ground
The temperature range of the G3-4 extends lower than most traditional EGT systems to include temperatures normally en-
countered at start-up. Under normal engine operation at 1,000 to 1,200 rpm, the G3 will produce a white 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 G3-4 is its ability to detect abnormal combustion during the pre-take-off run-up.
The primary purpose of the pre-take-off 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 G3-4 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 G3-4 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 tempera-
tures of all cylinders should rise. (50 to 100° F).
Various problems can be detected easily during run-up with the aid of the G3-4. The absence of an rpm drop or EGT rise on
single-mag 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 switch-
ing 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 G3-4 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 G3-4 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 mis-
rigged, 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.
