An mdt air carrier had a TCASII RA when an small aircraft varied its altitude by about 200 ft.

Attributes
ACN	213450
Time
Date	199206
Day	Sat
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : iad
State Reference	VA
Altitude	msl bound lower : 6000 msl bound upper : 6600
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : iad
Operator	common carrier : air carrier
Make Model Name	Medium Transport, High Wing, 2 Turboprop Eng
Flight Phase	descent : approach
Route In Use	arrival other
Flight Plan	IFR
Aircraft 2
Make Model Name	Small Aircraft, High Wing, 1 Eng, Fixed Gear
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 5400 flight time type : 3000
ASRS Report	213450
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : airborne less severe other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other controllera other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course flight crew : took evasive action
Consequence	Other
Miss Distance	vertical : 900
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation other

Narrative:

Traffic was pointed out by dulles approach, small aircraft 12 O'clock, 4 mi at 5500. About 2 mins later, target appeared to climb to 5700 on TCASII and then we received an RA to climb. The first officer climbed out of 6000 to 6600. Dulles approach was notified of the TCASII and the altitude leaving. We did not have the aircraft in sight until the climb through approximately 6300. Solution: provide vertical separation of at least 1000 between all aircraft that are not in sight, even in a TCA.

Original NASA ASRS Text

Title: AN MDT ACR HAD A TCASII RA WHEN AN SMA VARIED ITS ALT BY ABOUT 200 FT.

Narrative: TFC WAS POINTED OUT BY DULLES APCH, SMA 12 O'CLOCK, 4 MI AT 5500. ABOUT 2 MINS LATER, TARGET APPEARED TO CLB TO 5700 ON TCASII AND THEN WE RECEIVED AN RA TO CLB. THE FO CLBED OUT OF 6000 TO 6600. DULLES APCH WAS NOTIFIED OF THE TCASII AND THE ALT LEAVING. WE DID NOT HAVE THE ACFT IN SIGHT UNTIL THE CLB THROUGH APPROX 6300. SOLUTION: PROVIDE VERT SEPARATION OF AT LEAST 1000 BTWN ALL ACFT THAT ARE NOT IN SIGHT, EVEN IN A TCA.

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.