B737-700
Flight Crew Last 90 Days 170
Confusion; Distraction; Human-Machine Interface; Situational Awareness; Training / Qualification; Workload
Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Loss Of Aircraft Control
Flight Crew Regained Aircraft Control; Flight Crew Became Reoriented; Flight Crew Took Evasive Action
Procedure; Aircraft; Human Factors
Captain (pilot flying) and First Officer (pilot not flying). During climbout in a light weight -700 aircraft (only 19 passengers) on a cold morning; we were given a climb to FL230. The autopilot was engaged at approximately 8;000 FT MSL and; because of being light weight and cold outside air temperatures; our climb was greater than 5;000 FPM. After our climb check passing through 10;000 FT; we were given normal speed and direct to a down track fix. I selected LNAV and VNAV on the MCP. I checked that both were engaged and the airspeed was holding at the climb speed of 267 (+/- 4 KTS). Passing through 17;000 FT; we were given a climb to FL410. I set FL410 in the MCP and it was confirmed by the First Officer. At this point; we set the altimeter setting of 29.92; turned off the landing lights; and proceeded to put our departure plates in the book. We were climbing fast and the First Officer looked up and said; 'Watch your speed.' At this time I looked at the airspeed indicator and the airspeed was rapidly slowing below 210 KTS. I clicked off the autopilot and realized that VNAV was no longer working properly. I leveled the aircraft but we did get to a speed of about 180 KTS; which was just inside of the amber minimum maneuver speed bar. At level flight; we did get a one second of aircraft buffet so I lowered the nose slightly; got our speed out of the amber bar; again leveled off for a few seconds; and then selected on V/S a 500 FPM climb. We got back to our climb speed of 267 and continued on the flight to altitude. At first we thought that something was wrong with the VNAV system since I checked that it was engaged and working properly. The rest of the flight was uneventful. On our second flight; we were going to watch the system closely to see if was going to do it again. It did not. After further discussion with my First Officer; we decided that the system must have come out of VNAV and into V/S when we put FL410 in the MCP. Since the climb out at the time of the incident was above 5;000 FPM; as we passed through 17;000 FT and selected FL410; we believe that the system had already captured the FL230 that was set. If it had; then selecting FL410 would have put the aircraft in V/S mode. Because the climb was approximately 5;000 FPM; the aircraft continued that climb rate and the airspeed bled off rapidly. Doing other tasks as we passed FL180 took our attention away from the airspeed and the mode that was selected. Lesson learned is anytime you move or select a switch on the MCP you should check the FMA display to make sure that you are in the mode that you want to be in.
A lightly loaded B737-700 climbing through 17;000 FT at greater than 5;000 FPM had already captured the MCP altitude of FL230 and so when the new altitude of FL410 was set; VNAV had already reverted to V/S which allowed the airspeed go as low as 180 KTS before recovering to cruise speed.