Learjet 40
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Provided Assistance; Flight Crew Landed in Emergency Condition; General Maintenance Action
Human Factors; Aircraft
I was assigned PIC on a flight carrying three passengers on a LR 40. The preflight was conducted in accordance with the checklist. Engine start and subsequent cockpit checks did not yield any abnormalities that would cause the flight to be cancelled/delayed. Takeoff; departure; and cruise were uneventful. During our descent we got an Amber EICAS message 'Rudder Boost INOP'. We ran the appropriate checklist which called for pulling a CB. That tripped off both yaw dampers (YD) and autopilot. The airplane started a roll to the left which I counteracted. The CB was set in accordance with the checklist. YD and AP were re-engaged and everything seemed to work normal again. No EICAS message at that time. A short time thereafter we got the same message again. Simultaneously we received a PRIM PITCH TRIM FAULT (white) and IAS (amber; PFD) message. The airspeed indications were unreliable; differing between both Pilot's PFDs up to 60 KIAS. The IAS on the PF/left side dropped at some point by about 130 KIAS before returning to a 'normal' value. At this point; as the indications seemed more 'reliable' the controls were transferred to that side. ATC was advised of our situation. While the SIC was flying I monitored the systems available as well as requesting Speed and ALT read outs from ATC (difference between cockpit and ATC was up to 80 KIAS). We were assigned block ALT 5500-7000. As we had no reliable indications with regards to speed I told the SIC to fly by power settings and don't rely on the airspeed tape.  To avoid any further distractions I requested to be taken off the arrival and being vectored straight to the ILS. ATC accommodated immediately. We flew the ILS; when handed off to Tower we were advised of our airspeed which was reported to be 110 KIAS while our cockpit instruments indicated 130 KIAS. To be safe PF added a little more power. From what I remember we landed a little faster than Vref but did not exceed any limitations.I am glad that we landed safely. As we had multiple issues to deal with; multiple EICAS messages at the same time; IMC; icing; it became apparent that we had to prioritize. Possible ADC (Air Data Computer) failure took precedent over everything else. No doubt; the decision to [advise ATC]was the best course of action. We were able to focus on the tasks at hand while ATC kept other planes 'away' from us. We made it safely on the ground. That's all that matters. No injuries; no damage. The fact that the PF cut off the engines after touchdown instead of engaging the thrust reversers is/was the least of our problems. Clear of the runway we waited for transportation for our passengers who didn't seem to be phased by the events as well as a tug to pull the plane to the facility. After the plane was parked I called Maintenance and the Chief Pilot and advised them of our experience. Consequent Maintenance write ups were filed and submitted. In our case proper training and great CRM made all the difference.
LR40 Captain reported 'Rudder Boost INOP' light illuminated followed by pitch trim and IAS faults; as well as unreliable airspeed indications.