B737-700
Flight Crew Last 90 Days 210
Confusion; Workload
Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Wake Vortex Encounter
Flight Crew Became Reoriented; Flight Crew Overcame Equipment Problem
Aircraft; Procedure
We were departing SNA on a VMC day. We thoroughly briefed the noise abatement takeoff procedures from SNA. All operations and procedures occurred normally through the completion of the Before Takeoff Checklist. The aircraft was configured correctly for the special procedures; flaps were at 10 degrees; N1's in manual and set at 75.8%. THR RED set at 3;000 FT AGL; flight directors ON; LNAV armed; autothrottles armed; with the departure SID (CHANL 1) loaded in the FMC with the takeoff Runway 19R. After being cleared for takeoff by the Tower; the Captain pushed the power up and let the N1's stabilize at 40%; then released the brakes and pressed the TO/GA button. The throttles successfully advanced to the MAX power setting; the flight mode annunciator (FMA) said TO/GA; and the command bars came up on the PFD/ND. At sometime during the takeoff roll; the command bars dropped out of view from the PFD/ND. I noticed during the takeoff rotation that we no longer had command bars; and the TO/GA mode had dropped out from the FMA. The white LNAV armed had also dropped out from the FMA. The Captain continued to fly using pre-briefed pitch attitude. As the pilot not flying; I re-engaged LNAV at 400 FT AGL; announced 'approaching cutback' at 600 FT AGL; and performed the throttle cutback noise abatement at 800 FT AGL. The Tower then told us to contact Departure at 1;000 FT MSL. However; with no vertical mode engaged (TO/GA had dropped out); the command bars would not come back up. At one mile; we turned left to heading 175 with the help of the map display and the LOC DME. I didn't want to re-engage a vertical mode by hitting the TOGA button. I was afraid this would override THR HOLD and cause the power to go to go-around setting and trigger a noise sensor on the departure corridor. I decided to select LVL CHG and verified the throttles remained in THR HOLD with the current cutback power setting. We also encountered minor wake turbulence at 2;000 FT with the power still at cutback setting. An Airbus A320 had taken off approximately two minutes prior to our departure. We successfully flew and navigated the remainder of the CHANL 1 Departure; including the cleanup and power-up procedures at 3;000 FT. After climbing through 10;000 FT and completion of the Climb Checklist; the Captain and I discussed the whole incident. We verified that we had all the buttons and switches in the correct positions and modes for the prescribed SNA close-in noise abatement procedure. We still cannot understand why the TO/GA and LNAV modes dropped out during the takeoff roll. Again the autothrottles successfully set MAX N1's. We flew the CHANL 1 Departure and the noise abatement procedure correctly. However; in an already high task saturation environment; the minor mode malfunctions increased our already high workload significantly. I do not know what we could have done differently. However the new procedures that require us to execute the cutback procedure even earlier during the climbout (800 FT AGL) are even more unsafe and create higher cockpit task saturation than the old cutback (1;000 FT AGL) procedure. We have to prioritize noise considerations with navigation/automation malfunctions. In the big picture; this whole close-in noise abatement procedure at SNA creates a potential safety hazard; and when a minor navigation/automation malfunction occurs; it multiplies the already high cockpit workload. In the interest of safety; we need to examine the necessity of this arcane procedure that exists purely for the benefit of questionable noise issues in the local Orange County area.
B737-700 First Officer reports auto flight anomalies during a noise abatement departure from SNA. The procedure is successfully flown manually but the crew does not understand what caused the flight director bars to retract from view; auto thrust to disengage; and LNAV to be no longer armed.