SBR1 crew had an erroneous transponder altitude readout in ZKC class a.

Attributes
ACN	526044
Time
Date	200109
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : mad.vor
State Reference	MO
Altitude	msl bound lower : 31000 msl bound upper : 30900
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zhu.artcc artcc : zkc.artcc
Operator	general aviation : corporate
Make Model Name	Cessna Aircraft Undifferentiated or Other Model
Operating Under FAR Part	Part 91
Flight Phase	climbout : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : corporate
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 80 flight time total : 13000 flight time type : 3000
ASRS Report	526044
Person 2
Affiliation	company : corporate
Function	flight crew : first officer
Events
Anomaly	aircraft equipment problem : less severe other anomaly other
Independent Detector	other controllera
Resolutory Action	none taken : detected after the fact other
Consequence	faa : investigated other
Supplementary
Problem Areas	Aircraft ATC Human Performance
Primary Problem	Aircraft

Narrative:

On climb out from sus en route to lrd we had been given a heading for climb and cleared to climb to FL310. Heading and altitude were affirmed. As we approached FL310 a clearance to go direct to cedar creek VOR was given. At this time we received another transmission to 'maintain FL310.' it seemed repetitious and looked up to see that we were at FL309 and noticed the transponder on #2. I switched it to #1. Nothing else was mentioned and we were later cleared to FL350 and then later to FL390. It wasn't until later in the trip that ZHU advised us to call ZKC. I immediately recalled the second 'maintain FL310,' transmission. I asked center what our mode C read on the #2 transponder and they advised that we were showing FL435 when we were actually at FL430 as confirmed by the same controller after returning the transponder to #1. I called ZKC when arriving at lrd and then advised that the switch showed a loss of separation between us and another aircraft. Herein lies the problem. I feel the ZKC controller should have asked us what altitude we were displaying and reconfirmed that we were in fact at FL310. He never advised us that our mode C had shown a higher altitude. The fact that I saw the selector switch on #2 and switched it to #1 did not tell me I had an inaccurate transponder. I always have the transponder on #1 since I advised the first officer (the PF) to level off on my altimeter. The ZKC area supervisor told me that the paperwork had already been sent for continued investigation to our FSDO. I acknowledged and he also said that from my account of the event that it appeared to be an equipment error. I MEL'ed the #2 transponder per our procedures and flew back to sus later that evening. On 2 occasions I checked the mode C with ZKC on return to confirm the discrepancy. If the controller had asked to verify our altitude we could have ended and answered any questions there at that time. The area supervisor did say that their equipment had shown FL314 on one 'tick' and the next 'tick' at FL308. This was obviously from selecting the #1 transponder mode C. The next day maintenance found the #2 altimeter to be in error approximately 300-400 ft and replaced the unit.

Original NASA ASRS Text

Title: SBR1 CREW HAD AN ERRONEOUS XPONDER ALT READOUT IN ZKC CLASS A.

Narrative: ON CLBOUT FROM SUS ENRTE TO LRD WE HAD BEEN GIVEN A HEADING FOR CLB AND CLRED TO CLB TO FL310. HEADING AND ALT WERE AFFIRMED. AS WE APCHED FL310 A CLRNC TO GO DIRECT TO CEDAR CREEK VOR WAS GIVEN. AT THIS TIME WE RECEIVED ANOTHER XMISSION TO 'MAINTAIN FL310.' IT SEEMED REPETITIOUS AND LOOKED UP TO SEE THAT WE WERE AT FL309 AND NOTICED THE XPONDER ON #2. I SWITCHED IT TO #1. NOTHING ELSE WAS MENTIONED AND WE WERE LATER CLRED TO FL350 AND THEN LATER TO FL390. IT WASN'T UNTIL LATER IN THE TRIP THAT ZHU ADVISED US TO CALL ZKC. I IMMEDIATELY RECALLED THE SECOND 'MAINTAIN FL310,' XMISSION. I ASKED CTR WHAT OUR MODE C READ ON THE #2 XPONDER AND THEY ADVISED THAT WE WERE SHOWING FL435 WHEN WE WERE ACTUALLY AT FL430 AS CONFIRMED BY THE SAME CTLR AFTER RETURNING THE XPONDER TO #1. I CALLED ZKC WHEN ARRIVING AT LRD AND THEN ADVISED THAT THE SWITCH SHOWED A LOSS OF SEPARATION BTWN US AND ANOTHER ACFT. HEREIN LIES THE PROB. I FEEL THE ZKC CTLR SHOULD HAVE ASKED US WHAT ALT WE WERE DISPLAYING AND RECONFIRMED THAT WE WERE IN FACT AT FL310. HE NEVER ADVISED US THAT OUR MODE C HAD SHOWN A HIGHER ALT. THE FACT THAT I SAW THE SELECTOR SWITCH ON #2 AND SWITCHED IT TO #1 DID NOT TELL ME I HAD AN INACCURATE XPONDER. I ALWAYS HAVE THE XPONDER ON #1 SINCE I ADVISED THE FO (THE PF) TO LEVEL OFF ON MY ALTIMETER. THE ZKC AREA SUPVR TOLD ME THAT THE PAPERWORK HAD ALREADY BEEN SENT FOR CONTINUED INVESTIGATION TO OUR FSDO. I ACKNOWLEDGED AND HE ALSO SAID THAT FROM MY ACCOUNT OF THE EVENT THAT IT APPEARED TO BE AN EQUIP ERROR. I MEL'ED THE #2 XPONDER PER OUR PROCS AND FLEW BACK TO SUS LATER THAT EVENING. ON 2 OCCASIONS I CHKED THE MODE C WITH ZKC ON RETURN TO CONFIRM THE DISCREPANCY. IF THE CTLR HAD ASKED TO VERIFY OUR ALT WE COULD HAVE ENDED AND ANSWERED ANY QUESTIONS THERE AT THAT TIME. THE AREA SUPVR DID SAY THAT THEIR EQUIP HAD SHOWN FL314 ON ONE 'TICK' AND THE NEXT 'TICK' AT FL308. THIS WAS OBVIOUSLY FROM SELECTING THE #1 XPONDER MODE C. THE NEXT DAY MAINT FOUND THE #2 ALTIMETER TO BE IN ERROR APPROX 300-400 FT AND REPLACED THE UNIT.

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.