The ground check is performed to evaluate the functioning of the engine by comparing power input, as measured by manifold pressure, with power output, as measured by rpm or torque.
The engine may be capable of producing a prescribed power, even rated takeoff, and not be functioning properly. Only by comparing the manifold pressure required during the check against a known standard is an unsuitable condition disclosed. The magneto check can also fail to show shortcomings, since the allowable rpm dropoff is only a measure of an improperly functioning ignition system and is not necessarily affected by other factors. Conversely, it is possible for the magneto check to prove satisfactory when an unsatisfactory condition is present elsewhere in the engine.
The ground check is made after the engine is thoroughly warm. It consists of checking the operation of the powerplant and accessory equipment by ear, by visual inspection, and by proper interpretation of instrument readings, control movements, and switch reactions. During the ground check, the aircraft should be headed into the wind, if possible, to take advantage of the cooling airflow. A ground check procedure is outlined below:
- Control position check
- Cowl flaps (if equipped)—open
- Propeller—high rpm
- Carburetor heat—cold
- Check propeller according to propeller manufacturer’s instruction.
- Open throttle to the run-up rpm setting as per manufacturer’s instructions (specified RPM and manifold pressure).
- Ignition system operational check.
In performing the ignition system operational check (magneto check), the power-absorbing characteristics of the propeller in the low fixed-pitch position are utilized. In switching to individual magnetos, cutting out the opposite plugs results in a slower rate of combustion, which gives the same effect as retarding the spark advance. The drop in engine speed is a measure of the power loss at this slower combustion rate.
When the magneto check is performed, a drop in torquemeter pressure indication is a good supplement to the variation in rpm. In cases where the tachometer scale is graduated coarsely, the torquemeter variation may give more positive evidence of the power change when switching to the individual magneto condition. A loss in torquemeter pressure not to exceed 10 percent can be expected when operating on a single magneto. By comparing the rpm drop with a known standard, the following are determined:
- Proper timing of each magneto.
- General engine performance as evidenced by smooth operation.
- Additional check of the proper connection of the ignition leads.
Any unusual roughness on either magneto is an indication of faulty ignition caused by plug fouling or by malfunctioning of the ignition system. The operator should be very sensitive to engine roughness during this check. Lack of dropoff in rpm may be an indication of faulty grounding of one side of the ignition system. Complete cutting out when switching to one magneto is definite evidence that its side of the ignition system is not functioning. Excessive difference in rpm drop off between the left and right switch positions can indicate a difference in time between the left and right magnetos.
Sufficient time should be given to the check on each single switch position to permit complete stabilization of engine speed and manifold pressure. There is a tendency to perform this check too rapidly with resultant wrong indications. Operation as long as 1 minute on a single ignition system is not excessive.
Another point that must be emphasized is the danger of sticking tachometer. The tachometer should he tapped lightly to make sure the indicator needle moves freely. In some cases using older mechanical tachometers, sticking has caused errors in indication to the extent of 100 rpm. Under such conditions, the ignition system could have had as much as a 200 rpm drop with only a 100 rpm drop indicated on the instrument. In most cases, tapping the instrument eliminates the sticking and results in accurate readings.
In recording the results of time ignition system check, record the amount of the total rpm drop that occurs rapidly and the amount that occurs slowly. This breakdown in rpm drop provides a means of pinpointing certain troubles in the ignition system. This can reduce unnecessary work by confining maintenance to the specific part of the ignition system that is responsible for the trouble.
Fast rpm drop is usually the result of either faulty spark plugs or faulty ignition harness. This is true because faulty plugs or leads, take effect at once. The cylinder goes dead or starts firing intermittently the instant the switch is moved from “both” to the “right” or “left” position.
Slow rpm drop usually is caused by incorrect ignition timing or faulty valve adjustment. With late ignition timing, the charge is fired too late (in relation to piston travel) for the combustion pressures to build up to the maximum at the proper time. The result is a power loss greater than normal for single ignition because of the lower peak pressures obtained in the cylinder. However, this power loss does not occur as rapidly as that which accompanies a dead spark plug. This explains the slow rpm drop as compared to the instantaneous drop with a dead plug or defective lead. Incorrect valve clearances, through their effect on valve overlap, can cause the mixture to be too rich or too lean. The too rich or too lean mixture may affect one plug more than another, because of the plug location and show up as a slow rpm drop on the ignition check. Switch from “both” to “right” and return to “both.” Switch from “both” to “left” and return to “both.” Observe the rpm drop while operating on the right and left positions. The maximum drop should not exceed that specified by the engine manufacturer.
Fuel Pressure and Oil Pressure Check
Fuel pressure and oil pressure must be within the established tolerance (green arc) for the engine.