B757-200
Flight Crew Air Transport Pilot (ATP)
Troubleshooting; Confusion; Communication Breakdown; Human-Machine Interface
Aircraft Equipment Problem Critical; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural FAR
Aircraft Automation Overrode Flight Crew; Flight Crew Took Evasive Action; Flight Crew FLC Overrode Automation; General Maintenance Action
Procedure; Aircraft
The previous crew could not get the aircraft to level off in VNAV mode.  Spoke with previous crew in Ops; was told the aircraft would not level off in VNAV mode on all 3 autopilots.  Crew also told me the aircraft could not track the localizer or glideslope without excessive movements.  I was very concerned about flying the aircraft so I called MOC and was told the write up by the pilot was very vague.  I expressed my concern whether it was really the VNAV or Altitude Hold function that was the problem and  was worried about RVSM airspace.  MOC agreed to contact Local Maintenance and request some testing.  I met the Local Mechanic at the airplane and found out the aircraft passed all its ground testing with the auto land functions a little weak.  I then voiced my concern as to how they determined that it was the MCP VNAV mode specifically.  My question could not be answered.  I then pointed out that maybe more than one mode of the MCP was failing; which would ground the aircraft per the MEL which required 3 of the 4 modes in working order.  After 30 minutes of conversation with the Mechanic I stated; 'I feel like we were test pilots;' since they were unwilling to get to the bottom of the real problem. Local Maintenance deferred the MCP VNAV mode. So I did speak with previous crew as well as Maintenance to voice my concerns.  We did an extensive pre-departure brief for a FLCH takeoff and talked about the speeds that we would set 215; 235; 250 knots respectively.  Then we did a FLCH T.O.; when Departure gave us a sudden clearance to level at 2;500 feet.  Upon reaching 2;500 feet the autothrottle didn't retard and airspeed continued to increase as so I called for Flaps 1 and pulled the throttles back for the first time. While the flaps where in transit from 5 to 1 the First Officer stated; 'Watch your airspeed.'  The same time the First Officer made his statement I was calling for flaps up and retarding the throttles manually for the 2nd time with the autothrottle still engaged; just then ATC issued a climb to 6;000 feet.  I tried to initiate a FLCH 250 knot climb immediately; but the autothrottles went to climb power and accelerated the aircraft while the FMA'S went to Alt Cap first; followed by Alt Hold still maintaining 2;500 feet while the aircraft accelerated to 265 knots and maintained its altitude at 2;500 feet.  I tried twice to re-initiate FLCH climb by pushing FLCH on the MCP.  The FLCH light on the MCP would light momentarily but FMA'S still went ALT CAP followed by ALT HLD and extinguished the FLCH MCP light.  At this point I disconnected automation and hand flew the aircraft in a normal climb at 250 knots. At cruise while discussing the situation the First Officer stated he saw a speed of 218 knots while flaps where transitioning from 5 to  1 and he also said he saw a momentary flash of the red flap speed tape. We did write the aircraft up in our destination for the FLCH/autothrottle problem as well as a possible flap overspeed even though the speed was under 220 knots.  Maintenance determined it was an autothrottle problem and deferred the autothrottle along with the VNAV MCP MEL still remaining MEL'd.  What I learned from this experience was I should have been more aggressive in disconnecting the automation.  I also initially had some confusion as to what the First Officer meant by; 'Watch your speed.'  I didn't realize the airspeed was catching the flaps speed while they where transitioning.  I was using the HUD and only had the chromatic green color so nothing jumped out at me at the time except the on coming of 250 knots.  I thought the First Officer was worried about the 250 knot speed limit; because that was my main concern at that time.   Maintenance deferred the autothrottle along with the previous deferred VNAV MCP.  We encountered a similar situation on T.O. on this second takeoff we were cleared to 12;000 feet and where climbing through 9;000 feet at 1;500 feet/minute when the FMA's went to ALT CAP followed by ALT HOLD and started leveling around 9;500 feet.  This was a higher altitude situation and much easier to manage because work load.  We were prepared this time and didn't have any problems.  I contribute this to the pre-takeoff briefing covering what we would do if this situation repeated itself.  The aircraft was written up for a third time.  In the future I will first be more demanding in getting to what the real mechanical problem is with the airplane with Maintenance and not let the pressure of the mission sway me.  I will also be a lot quicker to remove the automation when the aircraft when it's not doing what I want it to do.  I do believe both of our pre-departure briefings prior to our take offs made both situations more manageable and prevented a far worse outcome.
A B757 autoflight system would not govern autothrottles to MCP selected speed nor would it level at the selected MCP altitude so Maintenance MELed the autothrottles and VNAV.