B737 issued visual separation with preceding B747 slows and is then questioned by ATC.

Attributes
ACN	606779
Time
Date	200401
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : lax.airport
State Reference	CA
Altitude	msl single value : 3500
Environment
Flight Conditions	VMC
Aircraft 1
Controlling Facilities	tracon : sct.tracon
Operator	common carrier : air carrier
Make Model Name	B737-500
Operating Under FAR Part	Part 121
Flight Phase	descent : approach
Route In Use	approach : visual arrival : on vectors
Flight Plan	IFR
Aircraft 2
Controlling Facilities	tracon : sct.tracon
Operator	common carrier : air carrier
Make Model Name	B747 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	descent : approach
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Experience	flight time last 90 days : 210 flight time total : 9000 flight time type : 2000
ASRS Report	606779
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	conflict : airborne less severe other anomaly
Independent Detector	other flight crewa
Resolutory Action	controller : issued advisory controller : issued new clearance
Consequence	faa : reviewed incident with flight crew
Supplementary
Problem Areas	ATC Human Performance
Primary Problem	ATC Human Performance

Narrative:

On a vector of 065 degree heading and speed 210 KTS assigned. Approach asked us if we had the preceding B747 aircraft ahead of us in sight. We had the aircraft in sight and told approach we had them in sight. Approach control then cleared us to descend to 3500 ft and maintain visual separation with the B747. The B747 was slowing down, so we slowed to 190 KTS. ATC then asked us if we were still doing 210 KTS assigned and we reported that we had slowed to 190 KTS to maintain separation. The controller then cleared us for the visual runway 24R and said maintain normal speed. The controller acted as if he was upset by the fact that we slowed down. However, he did tell us to maintain visual separation and to do so we needed to slow down.

Original NASA ASRS Text

Title: B737 ISSUED VISUAL SEPARATION WITH PRECEDING B747 SLOWS AND IS THEN QUESTIONED BY ATC.

Narrative: ON A VECTOR OF 065 DEG HDG AND SPD 210 KTS ASSIGNED. APCH ASKED US IF WE HAD THE PRECEDING B747 ACFT AHEAD OF US IN SIGHT. WE HAD THE ACFT IN SIGHT AND TOLD APCH WE HAD THEM IN SIGHT. APCH CTL THEN CLRED US TO DSND TO 3500 FT AND MAINTAIN VISUAL SEPARATION WITH THE B747. THE B747 WAS SLOWING DOWN, SO WE SLOWED TO 190 KTS. ATC THEN ASKED US IF WE WERE STILL DOING 210 KTS ASSIGNED AND WE RPTED THAT WE HAD SLOWED TO 190 KTS TO MAINTAIN SEPARATION. THE CTLR THEN CLRED US FOR THE VISUAL RWY 24R AND SAID MAINTAIN NORMAL SPD. THE CTLR ACTED AS IF HE WAS UPSET BY THE FACT THAT WE SLOWED DOWN. HOWEVER, HE DID TELL US TO MAINTAIN VISUAL SEPARATION AND TO DO SO WE NEEDED TO SLOW DOWN.

Data retrieved from NASA's ASRS site as of July 2007 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.