Aircraft X had a pressurization problem, made an emergency descent through aircraft Y's altitude. Aircraft Y was issued traffic by ATC and was able to keep clear of aircraft X by using TCASII.

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Attributes
ACN	448121
Time
Date	199909
Day	Sun
Local Time Of Day	1801 To 2400
Place
Locale Reference	navaid : csn.vortac
State Reference	VA
Altitude	msl single value : 35000
Environment
Light	Dusk
Aircraft 1
Controlling Facilities	artcc : zdc.artcc
Operator	general aviation : personal
Make Model Name	Light Transport, Low Wing, 2 Turbojet Eng
Operating Under FAR Part	Part 91
Flight Phase	descent : intermediate altitude descent other
Route In Use	enroute airway : j42.airway
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : zdc.artcc
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Route In Use	enroute airway : j42.airway
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Experience	controller limited radar : 13 controller radar : 10
ASRS Report	448121
Person 2
Affiliation	other
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : critical altitude deviation : excursion from assigned altitude conflict : airborne less severe non adherence : clearance non adherence : required legal separation other anomaly other
Independent Detector	aircraft equipment : tcas other controllera other flight crewa other flight crewb
Resolutory Action	controller : issued advisory flight crew : diverted to another airport flight crew : declared emergency none taken : unable
Consequence	other
Miss Distance	horizontal : 24000 vertical : 400
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance ATC Human Performance
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

Aircraft X at FL370 declared emergency with pressure problems and was descending. ATC could hear no more verbal statements on xmissions until aircraft was below FL230. Aircraft Y was behind aircraft X by about 3-4 mi at FL350. Aircraft X needed descent to 15000 ft to stabilize. Aircraft X requested through use of the 'identify' feature, to go to iad. I gave clearance to iad and assistance. Aircraft X landed at iad. Communication was not established in descent from FL370 to 15000 ft. Aircraft Y had TCASII. I called traffic to aircraft Y who was able to use TCASII to locate aircraft X and was then able to establish visual contact.

Original NASA ASRS Text

Title: ACFT X HAD A PRESSURIZATION PROB, MADE AN EMER DSCNT THROUGH ACFT Y'S ALT. ACFT Y WAS ISSUED TFC BY ATC AND WAS ABLE TO KEEP CLR OF ACFT X BY USING TCASII.

Narrative: ACFT X AT FL370 DECLARED EMER WITH PRESSURE PROBS AND WAS DSNDING. ATC COULD HEAR NO MORE VERBAL STATEMENTS ON XMISSIONS UNTIL ACFT WAS BELOW FL230. ACFT Y WAS BEHIND ACFT X BY ABOUT 3-4 MI AT FL350. ACFT X NEEDED DSCNT TO 15000 FT TO STABILIZE. ACFT X REQUESTED THROUGH USE OF THE 'IDENT' FEATURE, TO GO TO IAD. I GAVE CLRNC TO IAD AND ASSISTANCE. ACFT X LANDED AT IAD. COM WAS NOT ESTABLISHED IN DSCNT FROM FL370 TO 15000 FT. ACFT Y HAD TCASII. I CALLED TFC TO ACFT Y WHO WAS ABLE TO USE TCASII TO LOCATE ACFT X AND WAS THEN ABLE TO ESTABLISH VISUAL CONTACT.

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.