Date: 201708
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: Thunderstorm; Turbulence; 30
Light: Daylight
Aircraft: Learjet 31
Operator: Air Taxi
Operating under: Part 91
Flight plan: IFR
Mission: Ferry / Re-Positioning
Flight phase: Descent
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Elevator Trim System
Component problem: Improperly Operated
Reporter organization: Air Taxi
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Flight Instructor; Flight Crew Air Transport Pilot (ATP)
Second reporter function: First Officer; Pilot Flying
Anomaly: Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance
Contributing factors: Procedure
Primary problem: Procedure

Narrative: We were kept high at cruise FL380 for longer than expected. Center cleared us to descend to FL340. The First Officer was flying on autopilot and initiated a steep descent; since we were getting close to the destination. As we passed through FL360; I saw that the rate of descent was between 3;000-4;000 FPM; and stated that he should reduce the rate as we approached 1;000 feet above the assigned altitude. He disconnected the autopilot and tried to raise the nose to reduce the descent rate; also trimmed nose-up. The rate was still above 2;000 FPM as the altitude passed 34;500 feet and I coached him to level. We have flown together many times. He is very familiar with the aircraft and is an excellent pilot. I did not feel the need to take control from him. He seemed to be struggling with control pressures but leveled off at about 150 feet low from FL340... and I thought we were correcting by climbing back up. He could not seem to get it into trim. He relaxed his arms and the plane dropped an additional 200 feet immediately. The Controller then cleared us to FL330; obviously noticing our error.I figured out then that he had it completely out of trim... nose-down. I took control of the aircraft; fixed the trim and continued the descent to FL330. After a discussion; we figured out that he only pressed down on the trim barrel intermittently... he got confused as to whether the autopilot was on or not and started pushing the trim barrel without pressing the center button down. This is the way to change attitude when the Autopilot is engaged; but is ineffective with Autopilot disengaged. Since he occasionally trimmed correctly; I heard the trim clicker sound and thought he was getting the trim resolved. I would have taken control sooner if I had realized his error and will take over sooner in the future.


Second narrative: The autopilot had actually disengaged requiring the trim button be depressed to allow for the trim actuator to operate. I had to apply very heavy aft yoke pressure manually and was still unable to fully overcome the set trim position until I realized the issue. As I was trimming the aircraft and no trim input was occurring on the controls; I interpreted this as a trim failure and was troubleshooting the issue.


Synopsis: LR31 flight crew reported that an altitude overshoot on descent was due to the First Officer using improper trim procedure.