Here is how it is supposed to be done.
"A09W0160: The WestJet Boeing 737-700, registration C-FWAQ, operating as flight 229, was on the approach to runway 16 at Calgary in VMC conditions. The first officer was the pilot flying (PF) and through 1000 feet he disengaged the autopilot to hand fly the approach. His left hand was on the thrust levers as required by WestJet SOPs and at approximately 150 feet above ground he felt the thrust levers retarding to the idle position. The airspeed was observed to be slightly below the target airspeed of 133 knots and the Flight Mode Annunciator (FMA) was commanding RETARD on the N1 display. The PF disengaged the autothrottle and applied additional thrust to correct the low energy state, and the aircrft landed without further incident. Flight data was downloaded from the aircraft by the operator and Boeing was asked to provide assistance in reviewing the data. The operator provided the following information to the TSB after the data review: 1. The #1 RAD ALT indicated -6 feet at approximately 147feet AGL. The #2 RAD ALT indicated between 88feet-136feet at this time. 2. The autothrottle commanded RETARD as displayed on the FMC at approximately 94feet AGL. A reduction in Thrust Lever Angle is noted at approximately 77feet AGL. For approaches flown in VNAV or V/S, the A/T will retard the throttles at 27 feet RA. On all 737s, the autothrottle logic uses left radio altimeter data if left radio altitude is being displayed, regardless of the autopilot selected. On the 737NG, if the left RA failure flag is displayed, the autothrottle will use the right RA data. No failures of the RAD ALT system were recorded in the FDR data. It appears that the #1 RAD ALT displayed erroneous indications which was not recognized as a system failure. The RA failure flag did not display which would have enabled the autothrottle system to default to RAD ALT #2. The autothrottle continued to respond to the incorrect data provided from RAD ALT #1 and initiated a thrust RETARD command. The #1 RAD ALT was removed from the aircraft and sent to the vendor for teardown and investigation."