Date: 201503
Local time of day: 1801-2400
Location: EWR.Airport, NJ
Flight conditions: IMC
Weather elements / visibility: Snow; Icing; Rain; 2
Light: Night
Aircraft: Learjet 35
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Mission: Ambulance
Flight phase: Final Approach
Airspace: Class B EWR
Aircraft component: Attitude Indicator(Gyro/Horizon/ADI)
Component problem: Malfunctioning
Reporter organization: Air Taxi
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Flight Instructor; Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown; Human-Machine Interface
Anomaly: Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Weather / Turbulence
Contributing factors: Weather
Primary problem: Weather

Narrative: The chain of events begins prior to the approach with the weather deteriorating in the NYC area.  Original intended destination was TEB; due to plowing operations and a potential short icy runway the decision was made to divert to EWR.  This being said the workload increased dramatically with being put onto an arrival procedure very quickly; customs coordination; ambulance coordination and the approach being changed three times enroute to EWR.  We descended into snow; then rain; and then freezing rain.  During the descent the pilot flying's artificial horizon began to show a lazy turn to the right; while the standby gyro and the pilot not flying gyro showed level.  This was discussed as something to watch. The aircraft was handling the ice very well up until the last couple turns onto the approach.  The aircraft entered an area of severe clear ice.  At this time our navigation instruments seemed to differ from each side entering the approach.  We got established on the approach just outside the final approach fix and descended on the ILS.  I did perform a Navigation (NAV) aid morse code check again to confirm we had the correct frequencies in and we did but my NAV aid was only showing slightly off course and the pilot flying was showing half scale.  The aircraft was severely iced up but performing ok with ref plus 20.  The pilot flying Attitude Indicator now showed a 20 degree turn and the standby Attitude Indicator and the pilot not flying Attitude Indicator seemed level.  We were off course but no more than 3/4 scale.  That's when we realized something was wrong with the pilot flying Attitude Indicator and I called out small turns to get corrected on the ILS.  The pilot not flying NAV systems were then realized to be on GPS mode instead of NAV.  It was following approach indications as loaded in the GPS.  This is what resulted in the discrepancy between NAV systems. My system was only slightly off due to RNP sensitivity and the ILS sensitivity.  I quickly switched modes and it then started to agree with the pilot flying and glide slopes were recaptured.  We exited the clouds at about 1500 feet and went visual to the runway.  The Attitude Indicator on the pilot flying side was at a 30 degree bank on landing.  We advised ATC of the severe icing after landing and another aircraft on the approach experienced the same ice conditions.  We wrote up the attitude indicator on the pilot's side at the conclusion of the flight.The loss of situational awareness on the approach and lack of good CRM evolved from the icing conditions; change of destination; and multiple changes of approaches encountered enroute to the alternate.  Adding to the complexity was the confirmed loss of the Attitude Indicator inside the FAF and pilot not flying mode of NAV source.  I had mentioned to the pilot flying my instruments were all working on my side but the decision was made to continue on his side since he was flying and was comfortable with his remaining instruments.  This still resulted in an ILS that had up to 3/4 scale deflections.  The largest factor affecting the human performance was task overload enroute to the alternate.  Looking back on all the decisions that were made we needed more time to get properly set up and stabilized.  This would have hopefully eliminated the confusion of aircraft indications and NAV sources.  The weather and change of destination created a false sense of urgency.  We reacted as fast as we could but ultimately it still lead to an approach that had deviations on the ILS until we went visual.




Synopsis: A Lear 35 flight crew diverted to EWR from their intended destination due to weather; icing conditions; and short runway.  Enroute flight conditions worsened and ice formed on the surfaces. In addition; the attitude indicator of the pilot flying (PF) gave evidence of anomalous behavior by showing a different bank attitude from both the standby and the reporter's indicators. The Captain; and PF; elected to continue as PF and flew a modestly flawed but successful approach and landing.  Upon landing they made a pilot report of their observations. Post flight; the flight crew realized the reporter's Navigation (NAV) systems were in the GPS vice NAV mode.