P3 autoplt began uncommanded climb from cruise altitude.

Attributes
ACN	431292
Time
Date	199903
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	navaid : swl.vortac
State Reference	MD
Altitude	msl single value : 25000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zdc.artcc
Operator	other
Make Model Name	Orion (P3)
Operating Under FAR Part	Part 91
Navigation In Use	other vortac
Flight Phase	cruise : level
Route In Use	enroute airway : j174.airway
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : zdc.artcc
Operator	other
Make Model Name	Military
Operating Under FAR Part	Part 91
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Function	flight crew : captain oversight : pic
Qualification	pilot : military pilot : multi engine
Experience	flight time last 90 days : 50 flight time total : 2700 flight time type : 2000
ASRS Report	431292
Person 2
Affiliation	government : military
Function	flight crew : first officer
Qualification	pilot : military
Events
Anomaly	aircraft equipment problem : critical altitude deviation : excursion from assigned altitude conflict : airborne less severe non adherence : clearance
Independent Detector	other controllera other flight crewa other flight crewb
Resolutory Action	controller : issued advisory flight crew : overcame equipment problem flight crew : returned to original clearance
Miss Distance	horizontal : 6000 vertical : 1000
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Cruising at FL250, autoplt began a climb without any warnings or noticeable change in aircraft attitude. Center called and questioned us about our altitude. Now at FL255, we quickly descended back to FL250. A few mins after we leveled at FL250, another aircraft passed over us at FL260. Pilot at controls should have been scanning altimeter more closely. Also, this 'old model' autoplt for the P3 (model pb-20N) does not have warning system for this type of altitude deviation malfunction.

Original NASA ASRS Text

Title: P3 AUTOPLT BEGAN UNCOMMANDED CLB FROM CRUISE ALT.

Narrative: CRUISING AT FL250, AUTOPLT BEGAN A CLB WITHOUT ANY WARNINGS OR NOTICEABLE CHANGE IN ACFT ATTITUDE. CTR CALLED AND QUESTIONED US ABOUT OUR ALT. NOW AT FL255, WE QUICKLY DSNDED BACK TO FL250. A FEW MINS AFTER WE LEVELED AT FL250, ANOTHER ACFT PASSED OVER US AT FL260. PLT AT CTLS SHOULD HAVE BEEN SCANNING ALTIMETER MORE CLOSELY. ALSO, THIS 'OLD MODEL' AUTOPLT FOR THE P3 (MODEL PB-20N) DOES NOT HAVE WARNING SYS FOR THIS TYPE OF ALTDEV MALFUNCTION.

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.