Pilot of AA5 descends below assigned altitude while traversing bwi airspace under pct control.

Attributes
ACN	614336
Time
Date	200404
Day	Sat
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : bwi.airport
State Reference	MD
Altitude	msl bound lower : 2200 msl bound upper : 2500
Environment
Flight Conditions	VMC
Weather Elements	Turbulence
Light	Daylight
Aircraft 1
Controlling Facilities	tower : bwi.tower tracon : huf.tracon
Operator	general aviation : personal
Make Model Name	Cheetah, Tiger, Traveler
Operating Under FAR Part	Part 91
Flight Phase	cruise : level
Flight Plan	DVR
Person 1
Affiliation	other
Function	flight crew : single pilot
Qualification	pilot : cfi pilot : commercial pilot : instrument
Experience	flight time last 90 days : 5 flight time total : 1500 flight time type : 500
ASRS Report	614336
Person 2
Affiliation	government : faa
Function	controller : radar
Events
Anomaly	altitude deviation : excursion from assigned altitude inflight encounter : turbulence other anomaly
Independent Detector	atc equipment other atc equipment : mode c other controllera
Resolutory Action	controller : issued advisory flight crew : returned to assigned altitude
Consequence	faa : reviewed incident with flight crew
Supplementary
Problem Areas	Flight Crew Human Performance Weather Aircraft
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Moderate turbulence prevailed. I noted that I was slightly (150 ft) low and correcting. Had been asked to maintain 2500 ft crossing bwi. Controller asked altitude, which I confirmed as 2500 ft since I was nearly back to level. Controller indicated I was at 2200 ft. There must have been an equipment calibration error contributing to the problem, as I was showing 2500 ft level and the controller 2400 ft. Asked again for a mode C check with next potomac sector and he showed me level at 2500 ft. Problem was caused by difficulty maintaining altitude in turbulence, but the discrepancy in the equipment made the deviation appear much larger than actual.

Original NASA ASRS Text

Title: PLT OF AA5 DSNDS BELOW ASSIGNED ALT WHILE TRAVERSING BWI AIRSPACE UNDER PCT CTL.

Narrative: MODERATE TURB PREVAILED. I NOTED THAT I WAS SLIGHTLY (150 FT) LOW AND CORRECTING. HAD BEEN ASKED TO MAINTAIN 2500 FT XING BWI. CTLR ASKED ALT, WHICH I CONFIRMED AS 2500 FT SINCE I WAS NEARLY BACK TO LEVEL. CTLR INDICATED I WAS AT 2200 FT. THERE MUST HAVE BEEN AN EQUIP CALIBRATION ERROR CONTRIBUTING TO THE PROB, AS I WAS SHOWING 2500 FT LEVEL AND THE CTLR 2400 FT. ASKED AGAIN FOR A MODE C CHK WITH NEXT POTOMAC SECTOR AND HE SHOWED ME LEVEL AT 2500 FT. PROB WAS CAUSED BY DIFFICULTY MAINTAINING ALT IN TURB, BUT THE DISCREPANCY IN THE EQUIP MADE THE DEV APPEAR MUCH LARGER THAN ACTUAL.

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.