Mdt gains 900 ft when autoplt pitches up violently.

Attributes
ACN	198116
Time
Date	199201
Day	Fri
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : zzz
State Reference	US
Altitude	msl bound lower : 37000 msl bound upper : 37900
Environment
Flight Conditions	Mixed
Light	Night
Aircraft 1
Controlling Facilities	artcc : zdv
Operator	general aviation : corporate
Make Model Name	Medium Transport
Navigation In Use	Other
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : cfi
Experience	flight time last 90 days : 60 flight time total : 6000 flight time type : 500
ASRS Report	198116
Person 2
Affiliation	Other
Function	flight crew : first officer
Qualification	pilot : cfi pilot : atp
Experience	flight time last 90 days : 20 flight time total : 6000 flight time type : 200
ASRS Report	198205
Events
Anomaly	aircraft equipment problem : critical altitude deviation : excursion from assigned altitude
Independent Detector	other flight crewa
Resolutory Action	flight crew : overcame equipment problem flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

The aircraft was established in level cruise flight at FL370. Copilot flying aircraft with autoplt coupled to copilot's flight director. With no warning aircraft pitched up violently and instantly gained 800-900 ft of altitude. I immediately disconnected autoplt and returned aircraft to level flight at FL370. While returning to level flight, both pilots realized copilot's ADI had failed (30 degree pitch change) and flight director had responded. I've never seen an instantaneous failure like this before. We responded properly and I can't think of anything which might have prevented this incident. I'm thankful it didn't occur at low airspeed, on an approach as the pitch up was violent enough that it probably would have caused a wing to stall. Supplemental information from acn 198205: neither pilot could have anticipated this or prevented this from happening.

Original NASA ASRS Text

Title: MDT GAINS 900 FT WHEN AUTOPLT PITCHES UP VIOLENTLY.

Narrative: THE ACFT WAS ESTABLISHED IN LEVEL CRUISE FLT AT FL370. COPLT FLYING ACFT WITH AUTOPLT COUPLED TO COPLT'S FLT DIRECTOR. WITH NO WARNING ACFT PITCHED UP VIOLENTLY AND INSTANTLY GAINED 800-900 FT OF ALT. I IMMEDIATELY DISCONNECTED AUTOPLT AND RETURNED ACFT TO LEVEL FLT AT FL370. WHILE RETURNING TO LEVEL FLT, BOTH PLTS REALIZED COPLT'S ADI HAD FAILED (30 DEG PITCH CHANGE) AND FLT DIRECTOR HAD RESPONDED. I'VE NEVER SEEN AN INSTANTANEOUS FAILURE LIKE THIS BEFORE. WE RESPONDED PROPERLY AND I CAN'T THINK OF ANYTHING WHICH MIGHT HAVE PREVENTED THIS INCIDENT. I'M THANKFUL IT DIDN'T OCCUR AT LOW AIRSPD, ON AN APCH AS THE PITCH UP WAS VIOLENT ENOUGH THAT IT PROBABLY WOULD HAVE CAUSED A WING TO STALL. SUPPLEMENTAL INFO FROM ACN 198205: NEITHER PLT COULD HAVE ANTICIPATED THIS OR PREVENTED THIS FROM HAPPENING.

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.