Mht controller experienced operational error at 4000 ft when air carrier on approach slowed descent rate and conflicted with other traffic being vectored.

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Attributes
ACN	719211
Time
Date	200611
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : mht.airport
State Reference	NH
Altitude	msl single value : 4000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : mht.tracon
Operator	common carrier : air carrier
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer & glide slope : 6
Flight Phase	descent : approach
Route In Use	arrival : on vectors
Flight Plan	IFR
Aircraft 2
Controlling Facilities	tracon : mht.tracon
Operator	general aviation : corporate
Make Model Name	PC-12
Operating Under FAR Part	Part 91
Flight Phase	cruise : level
Route In Use	enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : approach controller : radar
Qualification	controller : military controller : radar
Experience	controller military : 4 controller radar : 23
ASRS Report	719211
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	conflict : airborne less severe non adherence : required legal separation
Independent Detector	atc equipment : conflict alert other controllera
Resolutory Action	controller : issued alert controller : issued new clearance flight crew : took evasive action
Consequence	faa : investigated
Miss Distance	horizontal : 7260 vertical : 900
Supplementary
Problem Areas	ATC Human Performance
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

B737 was inbound for ILS runway 6 mht from the west. PC12 sbound at 4000 ft from northwest of mht airport. B737 was issued clearance for ILS runway 6 approach and had a good rate of descent. The B737 slowed his descent as I was distraction by a landline problem with mht tower. The B737 leveled at 5200 ft for approximately 1 min then started descent again and was now in conflict with the PC12. I instructed the PC12 to turn left 090 degree heading and the B737 to turn right 180 degree heading. The PC12 acknowledged the B737 did not acknowledge 3 calls or comply with his instructions. The B737 continued on the ILS approach as I climbed the PC12 to 5000 ft. In the future I will not assume aircraft will continue climbing or descending at current rates and try to minimize distrs from faulty equipment. I believe pilots should be required to advise ATC if they intend to drastically change rates of descent or climb.

Original NASA ASRS Text

Title: MHT CTLR EXPERIENCED OPERROR AT 4000 FT WHEN ACR ON APCH SLOWED DSCNT RATE AND CONFLICTED WITH OTHER TFC BEING VECTORED.

Narrative: B737 WAS INBOUND FOR ILS RWY 6 MHT FROM THE W. PC12 SBOUND AT 4000 FT FROM NW OF MHT ARPT. B737 WAS ISSUED CLRNC FOR ILS RWY 6 APCH AND HAD A GOOD RATE OF DSCNT. THE B737 SLOWED HIS DSCNT AS I WAS DISTR BY A LANDLINE PROB WITH MHT TWR. THE B737 LEVELED AT 5200 FT FOR APPROX 1 MIN THEN STARTED DSCNT AGAIN AND WAS NOW IN CONFLICT WITH THE PC12. I INSTRUCTED THE PC12 TO TURN L 090 DEG HDG AND THE B737 TO TURN R 180 DEG HDG. THE PC12 ACKNOWLEDGED THE B737 DID NOT ACKNOWLEDGE 3 CALLS OR COMPLY WITH HIS INSTRUCTIONS. THE B737 CONTINUED ON THE ILS APCH AS I CLBED THE PC12 TO 5000 FT. IN THE FUTURE I WILL NOT ASSUME ACFT WILL CONTINUE CLBING OR DSNDING AT CURRENT RATES AND TRY TO MINIMIZE DISTRS FROM FAULTY EQUIP. I BELIEVE PLTS SHOULD BE REQUIRED TO ADVISE ATC IF THEY INTEND TO DRASTICALLY CHANGE RATES OF DSCNT OR CLB.

Data retrieved from NASA's ASRS site as of January 2009 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.