B737 crew had controller change altitude clearance.

Attributes
ACN	440901
Time
Date	199906
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : iad.ils
State Reference	VA
Altitude	msl bound lower : 4700 msl bound upper : 5000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : iad.tracon
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer only : 19r other
Flight Phase	descent : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : multi engine pilot : commercial pilot : atp pilot : instrument
Experience	flight time last 90 days : 240 flight time total : 10500 flight time type : 2000
ASRS Report	440901
Person 2
Function	flight crew : first officer
Qualification	pilot : multi engine pilot : instrument pilot : commercial
Events
Anomaly	other anomaly other
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance flight crew : returned to assigned altitude flight crew : returned to original clearance none taken : anomaly accepted
Supplementary
Problem Areas	ATC Human Performance
Primary Problem	ATC Human Performance

Narrative:

First officer hand-flying. Frequency change. I checked in 3 times to iad approach, no response. Finally ATC states 'roger air carrier X, cleared to maintain 4000 ft.' I read back and verified with first officer 4000 ft as we descended out of 5000 ft ATC stated 'negative 4000 ft, maintain 5000 ft air carrier X.' we had descended to 4700 ft indicated before climbing back up to 5000 ft. Rest of flight uneventful. The problem appears that ATC changed his mind very quickly even though we had read back the clearance and followed said clearance.

Original NASA ASRS Text

Title: B737 CREW HAD CTLR CHANGE ALT CLRNC.

Narrative: FO HAND-FLYING. FREQ CHANGE. I CHKED IN 3 TIMES TO IAD APCH, NO RESPONSE. FINALLY ATC STATES 'ROGER ACR X, CLRED TO MAINTAIN 4000 FT.' I READ BACK AND VERIFIED WITH FO 4000 FT AS WE DSNDED OUT OF 5000 FT ATC STATED 'NEGATIVE 4000 FT, MAINTAIN 5000 FT ACR X.' WE HAD DSNDED TO 4700 FT INDICATED BEFORE CLBING BACK UP TO 5000 FT. REST OF FLT UNEVENTFUL. THE PROB APPEARS THAT ATC CHANGED HIS MIND VERY QUICKLY EVEN THOUGH WE HAD READ BACK THE CLRNC AND FOLLOWED SAID CLRNC.

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.