Incorrect TKE indication in the B 737-500 CDU Nav page.
Variation is 10 degrees west.
Track true is 90°, track magnetic is 100°
Aircraft is on track.
TKE shows an incorrect R 10, it should be R/L 0.
Aircraft is 10 degrees left of track, the Track Made Good is 90 magnetic. To get back on track its current track is 140 magnetic and the bearing to the next waypoint is 130 magnetic.
TKE should show L 10, the difference between Track and Track Made Good.
TKE actually shows the combination of the R 10 degree magnetic variation error and the difference between the current track and the bearing of the NEXT waypoint which is L 10 to give an incorrect indication of L/R 0 degrees.
South East of Detroit, USA. Magnetic variation 10 degrees west in the Aerofly database. The (95 degrees) track approximates to 100 degrees magnetic.
No cross track error but track error incorrectly reported as R 10 degrees.
Ten degrees/7.12 miles of cross track error north of the track, track error incorrectly reported as zero degrees.