Date: 201810
Local time of day: 1201-1800
Location: TEB.Airport, NJ
Flight conditions: IMC
Light: Daylight
Aircraft: Small Transport; Low Wing; 2 Turbojet Eng
Operating under: Part 91
Mission: Passenger
Flight phase: Initial Approach
ATC / advisory: TRACON N90
Aircraft component: Autopilot
Component problem: Malfunctioning
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Human factors: Troubleshooting
Second reporter function: Captain; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew FLC Overrode Automation; Flight Crew Returned To Clearance
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: While intercepting ILS 19 Approach into TEB the aircraft did not capture the localizer and approach mode was re-engaged. At that time the aircraft climbed uncommanded while autopilot was engaged and in altitude hold. We both recognized the 'uncommanded pitch up' and corrected our altitude. ATC (NY TRACON) then queried our altitude and we replied with our current corrected altitude of 2;000 feet. I also noticed a yellow FD1 on my copilot PFD.


Second narrative: At 2;000 feet assigned; being vectored to ILS 19; cleared for approach. Avionics showed us ALT Hold; 2;000 feet; approach armed; TERM mode; with no error messages. Upon intercepting LOC course it was obvious that a switchover to 'green needles' was not occurring. I announce that I was switching manually and did so. Switchover to LOC (green needles) occurred and autopilot captured normally. At this time; on localizer; the aircraft pitched upward and began a climb. I've seen weird behavior from TEB ILS approaches before so I decided to give it to 2;100 feet to allow it to correct. It did not; in fact; the FD bars moved further upward and pitch seemed to be following. Pilot Monitoring (PM) called the deviation at this same time as I disconnected autopilot to correct- maximum deviation observed was 2;300 (300 feet above assigned 2;000). Length of deviation was only a few seconds and there was no traffic in the vicinity above us. When level at 2;000 again; ATC called and verified our altitude; said 'looked like you guys were at 2;400... be careful' We advised that we were in fact level at 2;000. PM at this time advised that he had a yellow 'FD-1 flag.' At this point; hand flying; level at 2;000; just inside UNVIL; on speed and on glide slope. Approach was stable so we continued approach. Autopilot was reengaged as there was not flags or abnormalities on the left side and we were stable on the approach. Approach and landing were normal; and the event was not noticed by passengers (no negative gee on the altitude return). We were never told to contact anyone nor were we advised of any possible deviation. Upon landing we wrote up the malfunction ('uncommanded pitch up on ILS approach') as per policy and notified Fleet. Again; we've seen weird stuff similar to this (mostly with the localizer) at TEB. Furthermore; there have been times when this exact scenario has occurred before (not switching to green needles and we manually switch). We have never seen this be accompanied by any type of uncommanded pitch. Also; we verified that the 'go around' button was not accidentally pushed (it would have sequenced FMS and disconnected autopilot if it had).


Synopsis: Small Transport flight crew reported an autopilot malfunction as they were capturing the localizer causing them to leave their assigned altitude.