that the opposite magneto has been properly grounded out.  Complete cutting out of the engine when switching from  “both” to “off” indicates that both magnetos are grounded  properly. While in the single magneto position, failure to  obtain any rpm drop, or failure of the engine to cut out while  switching to off, indicates that one or both ground connections  are faulty. This indicates a safety problem; the magnetos are  not secured at shut down and may fire if the propeller is turned. 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: 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: 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  1.  Control position check 2.  Cowl flaps (if equipped)—open 3. Mixture—rich 4.  Propeller—high rpm 5.  Carburetor heat—cold 6.  Check propeller according to propeller manufacturer’s  instruction. 7.  Open throttle to the run-up rpm setting as per  manufacturer’s instructions (specified RPM and  manifold pressure). 8.  Ignition system operational check. 1.  Proper timing of each magneto. 2.  General engine performance as evidenced by smooth  operation. 3.  Additional check of the proper connection of the  ignition leads. Ground Check 10-27