B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 80
Human-Machine Interface; Situational Awareness
Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance; Flight Crew Overcame Equipment Problem; Flight Crew Requested ATC Assistance / Clarification
Aircraft; Human Factors
We had a normal takeoff and I called for LNAV at 400 ft. The PM could not engage it after several attempts. The flight directors disappeared. The HSI did not provide course guidance even though the HSI selectors were set up for LNAV prior to takeoff. Because of the navigation errors; we missed the immediate turn to SAPEE on the WEVIC 4 SID. The frequency was busy; the PM and I were trying to solve the navigation issue and still retract the flaps when the Controller asked if we were following the RNAV SID. We told the controller we were unable RNAV and obtained the vector. When the PM tried to line select a fix; the FMC did not respond on the right but I was finally able to select a fix on the left FMC. We received clearance direct MUSAW and the LNAV and VNAV worked properly for the remainder of the flight. I believe this navigation error occurred because the route wasn't activated and executed on the ground prior to takeoff. I didn't see evidence of this problem on the ground when I looked at the Route pages or LEGS pages during my preflight and briefings. Additionally; in hindsight; I would have tried to tell ATC that we were unable RNAV sooner since the initial turn is immediate. But we had to fly the aircraft first and then communicate. Thankfully; we didn't forget to retract the flaps in the climb as we were trying to problem solve.
I was the PM on this leg from SLC. We were cleared on the EDETH 3 RNAV Departure. We briefed the full departure in accordance to standard operating procedures to include mode selection; routing; speed and altitude limits. After takeoff; the PF called for LNAV approaching 400 ft. Since we could not select LNAV prior to departure. After I selected LNAV; I verified LNAV mode in on the AFDS flight mode annunciator and saw the flight director bars indicate guidance. Then just a few seconds later; the flight director bars went away in both the PF and PM's HSI's. I rechecked and LNAV was still indicating on the AFDS flight mode annunciator but we were also not getting any guidance from either the ADI (flight director bars) or HSI. Both NAV switches were in NAV. I attempted to reselect the next point on the departure (SAPEE) on my MCDU. I could not select anything. I tried cycling the F/D switches. Still nothing. I also knew the departure required a relatively quick turn to the west. As I went to notify SLC Tower of our issue and to request a vector to allow us to continue; the Tower queried me as to why we had not turned west. I informed him of our issue and he subsequently gave us a vector of 280 and cleared to climb to 10;000 ft. He switched us to Departure where I informed him of our heading and altitude. It was at this time after cycling the flight director switches and with the PF selecting the next point on her MDCU; we were able to resume the departure with full guidance and functionality. I informed SLC Departure that we had resolved the issue and were able to resume the EDETH 3. Departure gave us a clearance to climb; delete the speeds in the departure and cleared direct to MUSAW. The flight continued with no additional problems from the LNAV/VNAV or MCDU.
B737 flight crew reported being unable to use LNAV after takeoff from SLC. A vector was requested from ATC and once cleared direct to MUSAW the FMC and LNAV returned to normal operation.