H125 flight crew experiences failure of the autoplt to maintain altitude above FL370.

Attributes
ACN	703343
Time
Date	200607
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : zob.artcc
State Reference	OH
Altitude	msl bound lower : 37000 msl bound upper : 38000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zny.artcc artcc : zob.artcc
Operator	common carrier : charter
Make Model Name	HS 125 Series
Operating Under FAR Part	Part 91
Flight Phase	cruise : enroute altitude change
Flight Plan	IFR
Person 1
Affiliation	company : charter
Function	flight crew : captain oversight : pic
Qualification	pilot : commercial pilot : instrument pilot : atp pilot : multi engine pilot : cfi
Experience	flight time last 90 days : 130 flight time total : 7800 flight time type : 415
ASRS Report	703343
Person 2
Affiliation	company : charter
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 90 flight time total : 24000 flight time type : 600
ASRS Report	703349
Events
Anomaly	aircraft equipment problem : critical altitude deviation : excursion from assigned altitude non adherence : clearance
Independent Detector	other controllera other flight crewa other flight crewb
Resolutory Action	controller : issued advisory controller : issued new clearance flight crew : returned to assigned altitude
Consequence	faa : assigned or threatened penalties faa : reviewed incident with flight crew
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

Cruise at FL370. Descended (ATC request) to FL350; then subsequent controller change. Requested return to FL370; cleared to FL380 for traffic. Upon reaching FL380; the autoplt had a difficult time of leveling off at FL380. Diverging pitch oscillations ensued; to the point where the PF had to disengage the autoplt and manually trim for level flight. Autoplt was reengaged and oscillations slowly resumed. Neither altitude hold or altitude capture functions were effective in maintaining level flight. Altdevs were as much as +/-300 ft. Requested descent to FL370 when traffic had passed underneath us; but were denied until the next controller. During pitch oscillations and subsequent altitude excursions; observed divergent and opposite instrument indications from PF and PNF's instruments. When PF's instruments indicated a climb on the altimeter and vsi; the PNF's instruments often indicated opposite. Standby altimeter indicated as much as 500 ft difference from primary instrumentation. When handed over to ZNY; requested and received descent to FL370. Aircraft responded normally to level off at FL370 with autoplt engaged. We were informed by ny that ZOB wanted to talk to us when we reached destination and were given a telephone contact number.

Original NASA ASRS Text

Title: H125 FLT CREW EXPERIENCES FAILURE OF THE AUTOPLT TO MAINTAIN ALT ABOVE FL370.

Narrative: CRUISE AT FL370. DSNDED (ATC REQUEST) TO FL350; THEN SUBSEQUENT CTLR CHANGE. REQUESTED RETURN TO FL370; CLRED TO FL380 FOR TFC. UPON REACHING FL380; THE AUTOPLT HAD A DIFFICULT TIME OF LEVELING OFF AT FL380. DIVERGING PITCH OSCILLATIONS ENSUED; TO THE POINT WHERE THE PF HAD TO DISENGAGE THE AUTOPLT AND MANUALLY TRIM FOR LEVEL FLT. AUTOPLT WAS REENGAGED AND OSCILLATIONS SLOWLY RESUMED. NEITHER ALT HOLD OR ALT CAPTURE FUNCTIONS WERE EFFECTIVE IN MAINTAINING LEVEL FLT. ALTDEVS WERE AS MUCH AS +/-300 FT. REQUESTED DSCNT TO FL370 WHEN TFC HAD PASSED UNDERNEATH US; BUT WERE DENIED UNTIL THE NEXT CTLR. DURING PITCH OSCILLATIONS AND SUBSEQUENT ALT EXCURSIONS; OBSERVED DIVERGENT AND OPPOSITE INST INDICATIONS FROM PF AND PNF'S INSTS. WHEN PF'S INSTS INDICATED A CLB ON THE ALTIMETER AND VSI; THE PNF'S INSTS OFTEN INDICATED OPPOSITE. STANDBY ALTIMETER INDICATED AS MUCH AS 500 FT DIFFERENCE FROM PRIMARY INSTRUMENTATION. WHEN HANDED OVER TO ZNY; REQUESTED AND RECEIVED DSCNT TO FL370. ACFT RESPONDED NORMALLY TO LEVEL OFF AT FL370 WITH AUTOPLT ENGAGED. WE WERE INFORMED BY NY THAT ZOB WANTED TO TALK TO US WHEN WE REACHED DEST AND WERE GIVEN A TELEPHONE CONTACT NUMBER.

Data retrieved from NASA's ASRS site as of January 2009 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.