A TRACON controller described a loss of heavy separation when post-review equipment indicated 3.93 w...

Narrative:

I turned air carrier X; a heavy B747; onto the localizer behind another heavy. The preceding heavy was assigned 160 KTS. When I turned air carrier X on [course] they were assigned 180 KTS. I was just starting to notice that aircraft were picking up about 30 KTS on base. I still had more than four miles and told air carrier X to slow to their slowest practical [speed] and asked what that speed would be; they said 140 KTS. I knew that speed would not hold up inside the marker; so I decided to vector air carrier X out for re-sequence. I never lost four miles; but when I arrived today at work I was told I only had 3.93 in stead of four miles. Well what is seven hundredths of a mile? I don't want to do the math; but I believe it's about 380 ft. I'm looking at a range of about 75 miles on my scope; so I say it is impossible to judge 380 ft. Also; on my radar scope when I use the slew ball measuring technique; every time I try to measure five miles I get either 4.93 or 5.06 miles. So; if management's equipment is so accurate; how come I'm not using it? Or is their equipment that accurate? When was the last time it was calibrated? Who calibrated it? Give me a break; I should be getting a pat on the back for recognizing that the four miles wasn't going to hold up and re-sequencing air carrier X; which caused me to spin three aircraft on my downwind. [I] recommend using excessive spacing just in case.

Original NASA ASRS Text

Title: A TRACON Controller described a loss of heavy separation when post-review equipment indicated 3.93 when four miles was required. The reporter expressed concern that determining this level of spacing is impossible on RADAR.

Narrative: I turned Air Carrier X; a heavy B747; onto the LOC behind another heavy. The preceding heavy was assigned 160 KTS. When I turned Air Carrier X on [course] they were assigned 180 KTS. I was just starting to notice that aircraft were picking up about 30 KTS on base. I still had more than four miles and told Air Carrier X to slow to their slowest practical [speed] and asked what that speed would be; they said 140 KTS. I knew that speed would not hold up inside the marker; so I decided to vector Air Carrier X out for re-sequence. I never lost four miles; but when I arrived today at work I was told I only had 3.93 in stead of four miles. Well what is seven hundredths of a mile? I don't want to do the math; but I believe it's about 380 FT. I'm looking at a range of about 75 miles on my scope; so I say it is impossible to judge 380 FT. Also; on my RADAR scope when I use the slew ball measuring technique; every time I try to measure five miles I get either 4.93 or 5.06 miles. So; if Management's equipment is so accurate; how come I'm not using it? Or is their equipment that accurate? When was the last time it was calibrated? Who calibrated it? Give me a break; I should be getting a pat on the back for recognizing that the four miles wasn't going to hold up and re-sequencing Air Carrier X; which caused me to spin three aircraft on my downwind. [I] recommend using excessive spacing just in case.

Data retrieved from NASA's ASRS site as of April 2012 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	970387
Time
Date	201109
Local Time Of Day	0001-0600
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Light	Night
Aircraft 1
Make Model Name	B747 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Flight Plan	IFR
Person 1
Function	Approach
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	ATC Issue All Types Deviation - Procedural Published Material / Policy

A TRACON controller described a loss of heavy separation when post-review equipment indicated 3.93 when four miles was required. The reporter expressed concern that determining this level of spacing is impossible on radar.