So in the question above, you would reduce power by 100 rpm. This is true for almost any piston plane you are likely to fly. If, for one hour before ETA through one hour after ETA, the ceiling and visibility are forecast to be less than 2000 feet and three miles, then an alternate needs to be determined and included in the flight plan. The heading indicator is a gyroscopically driven instrument. Precession during straight and level flight is caused by bearing friction.
Excessive precession indicates an incorrectly operating and unreliable instrument.
Loose Snow On Runway, Slush on Runway, Water On Runway, and Touch Down Zone Elevation. Dwight Eisenhower in 1957. Back during WWII, allied airplanes, when swept by friendly radar, sent back a sound like a parrot which identified them as friend, not foe.