Captain of a corporate jet cruising at high altitude shut off the mode C portion of the transponder.

Attributes
ACN	311474
Time
Date	199507
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : flm
State Reference	KY
Altitude	msl bound lower : 39000 msl bound upper : 39000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zid
Operator	general aviation : corporate
Make Model Name	Beechjet 400
Operating Under FAR Part	Part 91
Navigation In Use	Other Other
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : first officer
Qualification	other other : other pilot : atp
Experience	flight time last 90 days : 60 flight time total : 5530 flight time type : 40
ASRS Report	311474
Person 2
Affiliation	Other
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe non adherence : far other anomaly other
Independent Detector	other controllera
Resolutory Action	flight crew : overcame equipment problem other
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

ATC began asking us to say altitude. I looked down at mode C and it was in the off position. PIC was hand flying at FL390. Once I noticed encoder off, I turned it back on immediately. I think PIC, PF, turned encoder off for some reason, I would suspect he may have deviated off of altitude even though I did not notice. I never noticed us off our assigned altitude at any point. I then asked PIC if he turned off the mode C. He said yes, but he wouldn't tell me why. This type of careless action would be typical of his ego. I can only hope that we did not deviate without me noticing. I told him that you never turn off an encoder, period. He said he's done it before and never had a problem. He should not be hand flying a swept wing jet at FL390 anyway, it's too sensitive and susceptible to upset.

Original NASA ASRS Text

Title: CAPT OF A CORPORATE JET CRUISING AT HIGH ALT SHUT OFF THE MODE C PORTION OF THE XPONDER.

Narrative: ATC BEGAN ASKING US TO SAY ALT. I LOOKED DOWN AT MODE C AND IT WAS IN THE OFF POS. PIC WAS HAND FLYING AT FL390. ONCE I NOTICED ENCODER OFF, I TURNED IT BACK ON IMMEDIATELY. I THINK PIC, PF, TURNED ENCODER OFF FOR SOME REASON, I WOULD SUSPECT HE MAY HAVE DEVIATED OFF OF ALT EVEN THOUGH I DID NOT NOTICE. I NEVER NOTICED US OFF OUR ASSIGNED ALT AT ANY POINT. I THEN ASKED PIC IF HE TURNED OFF THE MODE C. HE SAID YES, BUT HE WOULDN'T TELL ME WHY. THIS TYPE OF CARELESS ACTION WOULD BE TYPICAL OF HIS EGO. I CAN ONLY HOPE THAT WE DID NOT DEVIATE WITHOUT ME NOTICING. I TOLD HIM THAT YOU NEVER TURN OFF AN ENCODER, PERIOD. HE SAID HE'S DONE IT BEFORE AND NEVER HAD A PROB. HE SHOULD NOT BE HAND FLYING A SWEPT WING JET AT FL390 ANYWAY, IT'S TOO SENSITIVE AND SUSCEPTIBLE TO UPSET.

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.