<< >> NASA ASRS Report 590683, August 2003

C172 pilot was unable to determine his altitude after the approach controller informed the pilot his mode C read 5500 ft rather than 8000 ft.

Attributes
ACN	590683
Time
Date	200308
Day	Sun
Local Time Of Day	0601 To 1200
Place
Locale Reference	navaid : odf.vortac
State Reference	GA
Altitude	msl bound lower : 5500 msl bound upper : 8000
Environment
Flight Conditions	IMC
Weather Elements	Turbulence other
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : ztl.artcc tracon : avl.tracon
Operator	general aviation : personal
Make Model Name	Skyhawk 172/Cutlass 172
Operating Under FAR Part	Part 91
Navigation In Use	other vortac
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	other
Function	flight crew : single pilot
Qualification	pilot : private pilot : instrument
ASRS Report	590683
Person 2
Affiliation	government : faa
Function	controller : approach
Events
Anomaly	aircraft equipment problem : critical altitude deviation : excursion from assigned altitude inflight encounter : weather other anomaly
Independent Detector	atc equipment other atc equipment : radar other controllera
Resolutory Action	controller : issued new clearance controller : provided flight assist flight crew : overcame equipment problem flight crew : regained aircraft control flight crew : exited adverse environment
Consequence	other
Supplementary
Problem Areas	Weather Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance

Narrative:

I departed IFR from avl, cleared to 8000 ft. At cruise, avl approach informed me that mode C read 5500 ft in an area of MOCA 8900 ft. In IMC, I was unable to determine altitude conflict resolution, I was assigned a block of 8000-10000 ft. I was handed off to ZTL, but was advised this was an area of poor coverage. I climbed to 10000 ft since I could not tolerate an error of 2500 ft in mountains and experienced updrafts such that I could not maintain altitude. Still in VMC, I finally was able to contact atl ATC, explained I was unable to determine altitude nor maintain level in updrafts. I was assigned a new heading/altitude twice. The controller and I agreed the altimeter was accurate and I was now VMC. I was cleared on course to destination and completed the flight without problem in intermittent IMC/VMC.

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Original NASA ASRS Text

Title: C172 PLT WAS UNABLE TO DETERMINE HIS ALT AFTER THE APCH CTLR INFORMED THE PLT HIS MODE C READ 5500 FT RATHER THAN 8000 FT.

Narrative: I DEPARTED IFR FROM AVL, CLRED TO 8000 FT. AT CRUISE, AVL APCH INFORMED ME THAT MODE C READ 5500 FT IN AN AREA OF MOCA 8900 FT. IN IMC, I WAS UNABLE TO DETERMINE ALT CONFLICT RESOLUTION, I WAS ASSIGNED A BLOCK OF 8000-10000 FT. I WAS HANDED OFF TO ZTL, BUT WAS ADVISED THIS WAS AN AREA OF POOR COVERAGE. I CLBED TO 10000 FT SINCE I COULD NOT TOLERATE AN ERROR OF 2500 FT IN MOUNTAINS AND EXPERIENCED UPDRAFTS SUCH THAT I COULD NOT MAINTAIN ALT. STILL IN VMC, I FINALLY WAS ABLE TO CONTACT ATL ATC, EXPLAINED I WAS UNABLE TO DETERMINE ALT NOR MAINTAIN LEVEL IN UPDRAFTS. I WAS ASSIGNED A NEW HEADING/ALT TWICE. THE CTLR AND I AGREED THE ALTIMETER WAS ACCURATE AND I WAS NOW VMC. I WAS CLRED ON COURSE TO DEST AND COMPLETED THE FLT WITHOUT PROB IN INTERMITTENT IMC/VMC.

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.