Narrative:

I would like to share this event which took place at cruise with no deviation and no consequences. We flew a constant mach .78; it was smooth at altitude FL400; during the flight; using a ci 40; I decided to check a ci that would match my current cruise speed of mach .78 (if it was even feasible); and I went on the performance page (init page); and entered a ci of 25; without executing it; (that I recall); and as the system was trying to provide a cruise speed for ci 25; the footer/header disappeared and the aircraft went into control wheel steering level flight (cws); while the F/D on both sides disappeared. I was surprised that the system would do that; perhaps because at a ci of 25; hypothetically we would no longer be within the optimized cruise altitude of FL400.anyways I erased the attempt; and as soon as I had done this; within 30 seconds VNAV/LNAV was available again; and I re-established normal VNAV-LNAV; with no consequences.my first thought is do not mofidy the ci at high cruise altitude; keep it where it is; and educate crews on the interface between the ci and automation (its effect on automation). I don't think there was anything wrong with the automation; this was a pilot induced error. [The] aircraft changed into cws mode while attempting to change ci'.

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Original NASA ASRS Text

Title: B737-800 Captain reported experimenting with the FMC Cost Index (CI) in cruise at FL400. Without executing a CI modification; a change from 40 to 25 transitioned LNAV/VNAV to Control Wheel Steering while the PFD header and footer speed limitations were removed.

Narrative: I would like to share this event which took place at cruise with no deviation and no consequences. We flew a constant Mach .78; it was smooth at altitude FL400; during the flight; using a CI 40; I decided to check a CI that would match my current cruise speed of Mach .78 (if it was even feasible); and I went on the Performance Page (Init page); and entered a CI of 25; without executing it; (that I recall); and as the system was trying to provide a cruise speed for CI 25; the footer/header disappeared and the aircraft went into Control Wheel Steering level flight (CWS); while the F/D on both sides disappeared. I was surprised that the system would do that; perhaps because at a CI of 25; hypothetically we would no longer be within the optimized cruise altitude of FL400.Anyways I erased the attempt; and as soon as I had done this; within 30 seconds VNAV/LNAV was available again; and I re-established normal VNAV-LNAV; with no consequences.My first thought is DO NOT MOFIDY the CI at high cruise altitude; keep it where it is; and educate Crews on the interface between the CI and automation (its effect on automation). I don't think there was anything wrong with the automation; this was a Pilot induced error. [The] aircraft changed into CWS mode while attempting to change CI'.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.