ARTCC controller alleged aircraft's mode C readout was 4700 MSL when flight crew of small transport ...

Attributes
ACN	126609
Time
Date	198910
Day	Sat
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : lby
State Reference	MS
Altitude	msl bound lower : 4700 msl bound upper : 5000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	other
Make Model Name	Small Transport, Low Wing, 2 Turboprop Eng
Flight Phase	descent : approach other
Route In Use	enroute : on vectors enroute airway : zhu
Flight Plan	VFR
Person 1
Affiliation	government : military
Function	flight crew : captain instruction : instructor oversight : pic
Qualification	pilot : military
Experience	flight time last 90 days : 100 flight time total : 10000 flight time type : 600
ASRS Report	126609
Person 2
Affiliation	government : military
Function	instruction : trainee
Qualification	pilot : military
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude other anomaly other
Independent Detector	other controllera
Resolutory Action	none taken : anomaly accepted
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

While on a VFR training flight, asked for and received an IFR clearance to go direct to libby VOR and hold at 5000' prior to commencing a VOR 36 approach to pib (pinebelt regional). After passing the VOR and reporting to ZHU 'holding on the VOR at 5000'', the controller complained that the mode C readout was 4700', or 300' below assigned. After verifying that we had the correct altimeter setting and that the flight director was on the 'altitude hold' mode, and still indicating 5000' on our altimeter, we turned the mode C off. The controller seemed quite upset about us being at 4700', but we were level at 5000! It seems odd that ZHU would start off by accusing us of not being at the assigned rather than asking 'what is your altimeter showing, we show you at XXXX ft?' well, this goes to prove 2 things: 1) mode C encoders can and do fail. 2) some controllers could be a little more polite and less confrontational. The solution to this problem of failed altitude encoders would be to have 2 mode C encoders aboard our aircraft so we could switch from #1 to #2 in case of failure.

Original NASA ASRS Text

Title: ARTCC CTLR ALLEGED ACFT'S MODE C READOUT WAS 4700 MSL WHEN FLT CREW OF SMT SHOWED 5000 ON ACFT ALTIMETER.

Narrative: WHILE ON A VFR TRAINING FLT, ASKED FOR AND RECEIVED AN IFR CLRNC TO GO DIRECT TO LIBBY VOR AND HOLD AT 5000' PRIOR TO COMMENCING A VOR 36 APCH TO PIB (PINEBELT REGIONAL). AFTER PASSING THE VOR AND REPORTING TO ZHU 'HOLDING ON THE VOR AT 5000'', THE CTLR COMPLAINED THAT THE MODE C READOUT WAS 4700', OR 300' BELOW ASSIGNED. AFTER VERIFYING THAT WE HAD THE CORRECT ALTIMETER SETTING AND THAT THE FLT DIRECTOR WAS ON THE 'ALT HOLD' MODE, AND STILL INDICATING 5000' ON OUR ALTIMETER, WE TURNED THE MODE C OFF. THE CTLR SEEMED QUITE UPSET ABOUT US BEING AT 4700', BUT WE WERE LEVEL AT 5000! IT SEEMS ODD THAT ZHU WOULD START OFF BY ACCUSING US OF NOT BEING AT THE ASSIGNED RATHER THAN ASKING 'WHAT IS YOUR ALTIMETER SHOWING, WE SHOW YOU AT XXXX FT?' WELL, THIS GOES TO PROVE 2 THINGS: 1) MODE C ENCODERS CAN AND DO FAIL. 2) SOME CTLRS COULD BE A LITTLE MORE POLITE AND LESS CONFRONTATIONAL. THE SOLUTION TO THIS PROBLEM OF FAILED ALT ENCODERS WOULD BE TO HAVE 2 MODE C ENCODERS ABOARD OUR ACFT SO WE COULD SWITCH FROM #1 TO #2 IN CASE OF FAILURE.

Data retrieved from NASA's ASRS site as of August 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.

ARTCC controller alleged aircraft's mode C readout was 4700 MSL when flight crew of small transport showed 5000 on aircraft altimeter.