B737-800
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 137
Human-Machine Interface; Situational Awareness
Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Became Reoriented; Flight Crew Took Evasive Action
Human Factors; Procedure
Prior to the Top of Descent; the FO calculated the fuel burn and subtracted it from the current weight on the INIT/REF page. All normal so far. Then; he punched the calculated weight into both the performance computer and the FMC. He made a math error that I did not catch when reviewing the data. On final; the aircraft did not feel right. [It felt] sluggish and 'burbly.' I quickly hit the INIT/REF page to double-check the 30 flap speeds. Here's the problem. The weight that he punched into the FMC was still there - 111.8. That's about 20;000 pounds too light for the 737-800 we were flying. There was no way for me to double-check the speeds. I knew the speeds were wrong and flew fast to compensate. The landing was normal. He is not the first FO I've flown with that manually changes the aircraft weight in the FMC. By doing this; we gain nothing and lose the ability to double-check the speeds with the calculated data.
B737-800 Captain experiences a math error by his First Officer during approach speed calculations. The weight used is 20;000 pounds too light and the slow approach speed that results is detected by the feel of the aircraft.