B737-800
Flight Crew Air Transport Pilot (ATP)
Confusion; Situational Awareness
Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Unstabilized Approach
General None Reported / Taken
Human Factors
On approach to Runway 13L; as PF; I lost VNAV guidance because I mistakenly hit the VNAV button. After multiple attempts I could not get VNAV to re-engage. Since we were on a visual approach; I elected to continue without it. Meanwhile; while trying to slow below 170 KIAS to approach speed; the throttles kept pushing themselves up. As I tried to manage the airspeed; the throttles were hampering my effort to reduce airspeed. I elected to disengage the autothrottles and continued the approach. During this time we had become high on the approach. I pitched over to get back on a visual glide path. At some point on the approach; we got a 'sink rate; sink rate' warning. I do not recall if we were below 500 feet in which case; we should have gone around. We determined we were in a safe position to land so we continued the approach and landed uneventfully.PF should coordinate with PNF to coordinate any MCP/FMC changes so as not to distract from flying.PM should be ready to be able to offer back-up visual guidance by use of V/S mode when primary means is lost. If we thought we were close to 500 feet when we hear the 'sink rate; sink rate' warning; we should have done the conservative thing and gone around.
Many of our pilots; myself included; have grown accustomed to flying with vertical and lateral guidance 99.9% of the time; and it's uncomfortable to say the least to fly without; or contrary to; that guidance. I believe it would be good practice to occasionally fly a takeoff approach and landing with no FD guidance in the Sim and the airplane (with PM using FD guidance).Be ready at any time to lose automation; and know your plan B ahead of time (for both PF and PM).Don't stay tied to the automation in the heat of battle. Know when to let go of it.
B737-800 flight crew reported their approach became unstabilized when VNAV was mistakenly de-selected.