A cj-6A pilot, cruising at 3500 ft, was informed by phl approach control that his altitude readout indicated 3000 ft, conflicting with departure traffic climbing to 3000 ft.

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Attributes
ACN	558586
Time
Date	200208
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : ttn.airport
State Reference	NJ
Altitude	msl single value : 3500
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : phl.tracon tower : ord.tower
Operator	general aviation : personal
Make Model Name	Military Trainer
Operating Under FAR Part	Part 91
Flight Phase	cruise : level
Flight Plan	None
Aircraft 2
Controlling Facilities	tracon : phl.tracon
Operator	general aviation : corporate
Make Model Name	Beechcraft, Beech Aircraft Corp Undifferentiated or Other Model
Operating Under FAR Part	Part 91
Flight Phase	climbout : intermediate altitude
Person 1
Affiliation	other
Function	flight crew : single pilot
Qualification	pilot : atp
Experience	flight time last 90 days : 250 flight time total : 9500 flight time type : 50
ASRS Report	558586
Person 2
Affiliation	company : corporate
Function	flight crew : captain oversight : pic
Events
Anomaly	aircraft equipment problem : critical conflict : airborne critical non adherence : clearance other anomaly other
Independent Detector	atc equipment : conflict alert atc equipment other atc equipment : radar aircraft equipment : tcas other controllera
Resolutory Action	controller : issued advisory flight crew : returned to assigned altitude flight crew : took evasive action other
Consequence	faa : reviewed incident with flight crew other
Miss Distance	horizontal : 3000 vertical : 500
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Aircraft

Narrative:

CJ6 aircraft heading south at 3500 ft MSL with flight following. Had traffic pointed out by approach climbing out of 2500 ft at 10 O'clock position and 2 mi. Hawker traffic was sighted and not a factor. Approach then asked for CJ6's altitude which was still 3500 ft MSL. Controller stated that the altitude readout had dropped to 3000 ft and that the hawker had to descend back down to 2500 ft from his assigned altitude of 3000 ft. I believe an altitude encoder error may have sent an erroneous altitude deviation to both ATC and TCASII. Altitude change of 500 ft down in 5 seconds = 6000 FPM rate of descent, then 500 ft back up during the next sweep = 6000 FPM climb -- both outside the abilities of the aircraft. The aircraft never strayed from 3500 ft MSL. Altimeter and encoder are in the process of being evaluated.

Original NASA ASRS Text

Title: A CJ-6A PLT, CRUISING AT 3500 FT, WAS INFORMED BY PHL APCH CTL THAT HIS ALT READOUT INDICATED 3000 FT, CONFLICTING WITH DEP TFC CLBING TO 3000 FT.

Narrative: CJ6 ACFT HDG S AT 3500 FT MSL WITH FLT FOLLOWING. HAD TFC POINTED OUT BY APCH CLBING OUT OF 2500 FT AT 10 O'CLOCK POS AND 2 MI. HAWKER TFC WAS SIGHTED AND NOT A FACTOR. APCH THEN ASKED FOR CJ6'S ALT WHICH WAS STILL 3500 FT MSL. CTLR STATED THAT THE ALT READOUT HAD DROPPED TO 3000 FT AND THAT THE HAWKER HAD TO DSND BACK DOWN TO 2500 FT FROM HIS ASSIGNED ALT OF 3000 FT. I BELIEVE AN ALT ENCODER ERROR MAY HAVE SENT AN ERRONEOUS ALTDEV TO BOTH ATC AND TCASII. ALT CHANGE OF 500 FT DOWN IN 5 SECONDS = 6000 FPM RATE OF DSCNT, THEN 500 FT BACK UP DURING THE NEXT SWEEP = 6000 FPM CLB -- BOTH OUTSIDE THE ABILITIES OF THE ACFT. THE ACFT NEVER STRAYED FROM 3500 FT MSL. ALTIMETER AND ENCODER ARE IN THE PROCESS OF BEING EVALUATED.

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.