Aircraft deviated from assigned cruise altitude.

Attributes
ACN	109496
Time
Date	198904
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : gve
State Reference	VA
Altitude	msl bound lower : 33000 msl bound upper : 33000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zdc
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	cruise other
Route In Use	enroute airway : zdc
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : instrument pilot : commercial pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 225 flight time total : 8000 flight time type : 1000
ASRS Report	109496
Person 2
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Events
Anomaly	other anomaly other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

First officer departed cockpit to use lav. After adjusting O2 mask in place I verified the annunciator (FMA) had us in the perf mode. We were about to cross gve, so I checked the chart to verify the outbnd course. As I put in the outbnd course I noticed the aircraft descending at 200 FPM. A quick check of the FMA showed us no longer in the perf mode. I checked off the autoplt and returned the aircraft to FL330. (We lost about 250'.) a maintenance entry to this effect was made in the log book. There are no audio or visibility cues to alert you that the system drops from perf cruise to vertical speed and not altitude hold; ie, a design or program oversight, and the subtleness of this could spell real problems.

Original NASA ASRS Text

Title: ACFT DEVIATED FROM ASSIGNED CRUISE ALT.

Narrative: F/O DEPARTED COCKPIT TO USE LAV. AFTER ADJUSTING O2 MASK IN PLACE I VERIFIED THE ANNUNCIATOR (FMA) HAD US IN THE PERF MODE. WE WERE ABOUT TO CROSS GVE, SO I CHKED THE CHART TO VERIFY THE OUTBND COURSE. AS I PUT IN THE OUTBND COURSE I NOTICED THE ACFT DSNDING AT 200 FPM. A QUICK CHK OF THE FMA SHOWED US NO LONGER IN THE PERF MODE. I CHKED OFF THE AUTOPLT AND RETURNED THE ACFT TO FL330. (WE LOST ABOUT 250'.) A MAINT ENTRY TO THIS EFFECT WAS MADE IN THE LOG BOOK. THERE ARE NO AUDIO OR VIS CUES TO ALERT YOU THAT THE SYS DROPS FROM PERF CRUISE TO VERT SPD AND NOT ALT HOLD; IE, A DESIGN OR PROGRAM OVERSIGHT, AND THE SUBTLENESS OF THIS COULD SPELL REAL PROBS.

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.