B737-800 captain experiences a math error by his first officer during approach speed calculations. T...

Narrative:

Prior to the top of descent; the first officer calculated the fuel burn and subtracted it from the current weight on the init/reference 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/reference 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 first officer 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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1288331
Time
Date	201508
Local Time Of Day	1801-2400
Place
Locale Reference	PHX.Airport
State Reference	AZ
Environment
Light	Daylight
Aircraft 1
Make Model Name	B737-800
Operating Under FAR Part	Part 121
Flight Phase	Landing
Flight Plan	IFR
Component
Aircraft Component	FMS/FMC
Person 1
Function	Pilot Flying Captain
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 137
Events
Anomaly	Deviation - Procedural Published Material / Policy Deviation - Speed All Types

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.