Gulfstream IV / G350 / G450
Flight Crew Multiengine; Flight Crew Flight Instructor; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Flight Crew Last 90 Days 100; Flight Crew Total 4300; Flight Crew Type 1500
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued Advisory / Alert
Weather; Aircraft
We departed runway 24 on the RUUDY 5 departure. The aircraft leveled off to cross the WENTZ intersection at 1500 feet. When checking in with New York Departure; we were told to descend immediately to 1500 feet. We immediately cross checked our altimeters and verified we had 3 altimeters showing us level at 1500 feet. We were confused and thought there must be some mistake. Again the controller contacted us and described the departure procedure to us and stated we must be unfamiliar with the procedure. The controller was made aware that we flew the departure regularly without incident and had done so for many years and that all 3 of our altimeters showed us at 1500 feet. The controller stated he showed our aircraft at 2100 feet. I immediately switched transponders and requested a Mode C readout. He again stated that he still showed us at 2100 feet and that we needed our equipment checked. I made him aware that we had just deiced; which may have caused the problem. We were given direct ELVAE and climbed to 6000 feet. I requested another Mode C readout from the next controller; who then elected to wait until he got us through the departure corridor. Soon after; we checked both transponders and both were functioning normally. At this time the First Officer made me aware that when I called '400 feet flaps up' that he looked at his altimeter and it was descending in the climb. About 30 minutes later; the #2 digital air data computer failed. The First Officers airspeed; altimeter; AOA (Angle of Attack); and VSI were all failed. We immediately switched to DADC (Digital Air Data Computer) #1 on his side and continued the flight without incident.
G-IV Captain was informed by ATC that he had leveled off at 2100 feet on the RUUDY5 while all three aircraft altimeters show 1500 feet. A few minutes later the aircraft altimeters agree with the Mode C read out. Thirty minutes into the flight the number two air data computer failed. The aircraft had been deiced just prior to departure.