Altitude deviation altitude excursion flight crew turns off mode C while correcting back to assigned...

Attributes
ACN	205485
Time
Date	199203
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : zzz
State Reference	US
Altitude	msl bound lower : 32500 msl bound upper : 33000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zzz artcc : ztl
Operator	common carrier : air carrier
Make Model Name	Heavy Transport, Low Wing, 4 Turbojet Eng
Navigation In Use	Other
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Experience	flight time last 90 days : 200 flight time total : 9000 flight time type : 1500
ASRS Report	205485
Person 2
Affiliation	company : air carrier
Function	flight crew : captain observation : company check pilot oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance non adherence : far other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other controllera other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course other
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

The captain left his seat to go to the restroom. The check airman (company) who was administering a line check got in the captain's seat so I wouldn't need to wear my oxygen mask. Approximately 3 mins after the check captain got in the seat, we got an altitude warning horn -- we were 500 ft low and descending. He started to correct back to FL330 when ATC called advising us to maintain FL330 -- we turned off mode C until we were back at FL330. The captain and I think the check captain was fiddling with the 'vertical speed, mach hold, airspeed hold' knob and inadvertently activated one of those modes which took the autoplt out of the altitude hold mode and allowed an imperceptible descent to begin. The altitude hold knob may have been inadvertently moved during the seat swap, however, its location makes that highly unlikely. All of us were awake and alert and all highly experienced and qualified in the airplane.

Original NASA ASRS Text

Title: ALTDEV ALT EXCURSION FLC TURNS OFF MODE C WHILE CORRECTING BACK TO ASSIGNED ALT.

Narrative: THE CAPT LEFT HIS SEAT TO GO TO THE RESTROOM. THE CHK AIRMAN (COMPANY) WHO WAS ADMINISTERING A LINE CHK GOT IN THE CAPT'S SEAT SO I WOULDN'T NEED TO WEAR MY OXYGEN MASK. APPROX 3 MINS AFTER THE CHK CAPT GOT IN THE SEAT, WE GOT AN ALT WARNING HORN -- WE WERE 500 FT LOW AND DSNDING. HE STARTED TO CORRECT BACK TO FL330 WHEN ATC CALLED ADVISING US TO MAINTAIN FL330 -- WE TURNED OFF MODE C UNTIL WE WERE BACK AT FL330. THE CAPT AND I THINK THE CHK CAPT WAS FIDDLING WITH THE 'VERT SPD, MACH HOLD, AIRSPD HOLD' KNOB AND INADVERTENTLY ACTIVATED ONE OF THOSE MODES WHICH TOOK THE AUTOPLT OUT OF THE ALT HOLD MODE AND ALLOWED AN IMPERCEPTIBLE DSCNT TO BEGIN. THE ALT HOLD KNOB MAY HAVE BEEN INADVERTENTLY MOVED DURING THE SEAT SWAP, HOWEVER, ITS LOCATION MAKES THAT HIGHLY UNLIKELY. ALL OF US WERE AWAKE AND ALERT AND ALL HIGHLY EXPERIENCED AND QUALIFIED IN THE AIRPLANE.

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.

Altitude deviation altitude excursion flight crew turns off mode C while correcting back to assigned altitude.