Light Transport; Low Wing; 2 Turbojet Eng
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 45; Flight Crew Total 8700; Flight Crew Type 220
Confusion; Workload; Human-Machine Interface
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Flight Crew Returned To Clearance; Flight Crew Took Evasive Action; Flight Crew Overcame Equipment Problem
Chart Or Publication; Procedure; Equipment / Tooling; Aircraft
UNIVERSAL UNS1-E FMS. Cruising at FL230 with an ATC airspeed restriction of 250 KTS we were cleared for GIBBZ TWO RNAV ARRIVAL after passing HIROY Intersection. Set in the final level-off altitude of 7;000 FT. Had to initiate descent manually as the FMS VNAV had not yet activated. It finally locked in approaching JIMVE Intersection. ATC had us increase to 280 KTS with instructions to slow to 250 KTS at MOSLE Intersection. VNAV profile was progressing normally and autopilot initiated 11;000 foot level-off approaching MOSLE Intersection. Shortly thereafter aircraft started a slow climb. Deactivated VNAV Mode and used manual descent mode to get aircraft headed down again for 10;000 foot restriction passing MONNK Intersection. At this point the Non Flying Pilot went to obtain ATIS. The Non Flying Pilot returned immediately upon noticing aircraft was not going to level at 9;000 FT at BMICH as described below.Selecting VNAV mode again; aircraft pitch increased dramatically into a steep descent profile; continuing through the 9;000 foot minimum altitude published for crossing BMICH Intersection. Had to disconnect the autopilot and climb back up to meet 9;000 foot crossing restriction. Re-engaged autopilot and VNAV. Autopilot was able to fly a normal descent profile to the THZMN Intersection. There never seems to be an uneventful outcome to this RNAV Profile Descent with any of the flight crews in our department. There is a multitude of reports by all types of aircrew illuminating problems flying this particular RNAV Profile Descent. Possibly the speed changes; altitude restrictions; or equipment inabilities all play into the outcome. It seems evident that a simpler design is needed and soon.
A Corporate pilot flying an aircraft with a Universal UNS1-E FMS described the difficulty flying the GIBBS TWO RNAV with this FMS and states' flying this RNAV profile never seems to have uneventful outcome.