Date: 201511
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Weather elements / visibility: Icing; Rain; 5
Light: Daylight
Aircraft: Learjet 60
Operator: Air Taxi
Operating under: Part 91
Flight plan: IFR
Mission: Test Flight / Demonstration
Flight phase: Climb
Airspace: Class E ZZZ
ATC / advisory: TRACON ZZZ
Reporter organization: Air Taxi
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown; Situational Awareness
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Clearance; Flight Crew Became Reoriented
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: Conducting a maintenance test flight to determine proper operation of landing gear. We needed to get the plane up above 10;000 and go fast; then bring the power up and down sharply to see if this changed hydraulic pressure or caused problem to reappear (intermittent problem had caused nose gear to come unlocked; or completely lower uncommanded). Flight was under Part 91 IFR. Heavy rain. I was First Officer; non-flying pilot. We departed on a round robin IFR flight plan; we were cleared for departure. Initial climb was to 3;000 feet. After checking in with approach; they climbed us to 4;000. Shortly thereafter; approach said to turn direct to [waypoint] ZZZZZ and maintain 250 knots. I repeated this to the controller; then looked down to the FMS boxes to set them both to go direct to ZZZZZ. I also turned on the weather radar at this point on both sides of the cockpit; I was heads down for a few seconds; and when I looked up we were in a steep climb. The Captain (flying pilot) had dialed in 10;000 feet on the altitude selector and was climbing like a rocket (the plane was at a light weight; only about 2;500 LBS of fuel and cold conditions; so it will climb about 6;000 to 10;000 FPM easily at low altitude). I had barely enough time to get the words out of my mouth that I didn't think we were cleared above 4;000; when the Controller asked what altitude we were going to because we were cleared to 4;000. The Captain responded on radio at this point that he was climbing to 10;000. The Controller said to level off at 9;000. There was no TCAS alert and no conflict. The flight continued as normal and we returned to base a few minutes later.I believe contributing factors were:-Being distracted by monitoring gear; hydraulics because of intermittent problems on several flights before. The plane had been written up 3 of the last 4 flight legs for nose gear extending in flight; it had been repaired by 3 different shops; actuators replaced and rebuilt; switches replaced.-Bad weather; strong winds and lots of bumps and ice once we got into the clouds. -The flying pilot says he 'could have sworn the controller said to climb and maintain 10;000; and maintain 250 KTS; when he only said maintain 250 KTS'. Sometimes those 2 things do go together; so I think he heard what he wanted to hear; but that is why we repeat the altitude back to the Controller. In this case the Controller did not say 10;000; I did not repeat back 10;000; or set 10;000 on the altitude selector. The Captain set this himself and pitched to plane up while I was dealing with avionics and checklists.-I wasn't feeling pressure to get this flight done quickly; but looking back; I think the Captain was in a hurry to get this flight done quickly for cost reasons; and the factory maintenance people that had just repaired the plane were waiting for us to get back. Our procedure for changing altitude is for the non-flying pilot to repeat back the altitude to the controller; set the altitude; then say the altitude again; and wait for a verbal response from the flying pilot; verifying the altitude; then the flying pilot starts climbing or descending. While I was programming the FMS; the Captain skipped this; because I was overloaded with other tasks; set the altitude himself and started climbing without my knowledge.-Human factors: we were both completely soaking wet from the waist down from rains; the rain was coming down sideways while fueling the plane; and this could have slowed me from catching the other pilot.




Synopsis: Learjet 60 First Officer reported the Captain started a climb after misunderstanding the Controller's clearance.