Regional Jet 700 ER/LR (CRJ700)
Flight Crew Air Transport Pilot (ATP)
Troubleshooting; Human-Machine Interface; Confusion; Workload
Aircraft Equipment Problem Critical; Inflight Event / Encounter Weather / Turbulence
Aircraft Equipment Problem Dissipated; Flight Crew FLC complied w / Automation / Advisory; Flight Crew FLC Overrode Automation; Flight Crew Diverted; Flight Crew Landed in Emergency Condition; General Declared Emergency; General Maintenance Action
Aircraft; Weather
At cruise; FL370; in IMC Stall Fail Caution appeared briefly 3 or so times and would self-clear each time after a couple of seconds. Consulted QRH; message had cleared at this point. At this time we were deviating through and area of isolated thunderstorm the IMC conditions were partly the result of these cells as the 'tops' were dispersed in the upper FL300's. We were no closer than 10-15 NM of the cell cores at any time. Approximately 10 minutes later same Stall Fail Caution scenario occurred except the Stall Fail Caution remained posted. Per QRH we selected Stall PTCT Pusher to OFF. Shortly thereafter Stall Fail caution self-cleared and with agreement of First Officer; I decided that I was more comfortable turning the Stall PTCT switches back ON we could apply QRH guidance again if Stall Fail Caution returned (it did not). ATC advised we could expect holding for our filed destination and were given a clearance limit. We consulted our enroute chart to determine if a published holding pattern was depicted. Received holding instructions and were instructed that speed to our holding fix (approximately 40 NM ahead) was at our discretion. We began to slow. Soon after; stabilized at our new speed; in continuous light turbulence; the First Officer noticed that the indicated airspeed on PFD 2 was continuing to show a slow decrease. Both PFD 1 and Standby Airspeed were stable relative to it; allowing for the continuous light turbulence. [I] directed the First Officer to consult the QRH to assist with troubleshooting when EFIS COMP MON caution posted. Applied the QRH guidance and concurred that we appeared to have an ADC 2 failure. [We] selected Air Data source selector switch to ADC 1. First Officer reiterated the requirement to advise ATC of RVSM status. I noted it and said we would. Reviewing ADC Failure QRH procedure; as directed by QRH; we had brief discussion in relation to whether we had a single or a dual ADC failure. Things started to happen quickly from this point. My recollection was that at this juncture we received a MACH TRIM Caution. I then observed that indicated airspeed on my PFD 1 was slowly degrading. I increased thrust; but indicated speed and trend vector maintained slow indicated decrease. ISI speed was steady. I selected climb power and told First Officer to declare an emergency and advise that we would be descending. As Captain's indicated speed on PFD 1 continued to degrade; the low speed cue began to appear. ISI indicated speed was stable. As we began descent with autopilot disengaged low speed cue and high speed cue quickly converged. The sticker shaker activated and stayed engaged for some time. I believe we were descending when we received the Rudder Limiter Caution. [I] directed the First Officer to consult that QRH procedure. I was concerned with rudder travel issues at altitude with an expedited; no Mach Trim and autopilot disengaged descent. IMC conditions still prevailed. First Officer was exploring diversion airport options with ATC and backing me up. Soon after beginning descent ISI speed began to decrease. We were in a descent with high power set. I asked the First Officer if he would cross-check my analysis as I now felt that I could no longer rely upon my ISI speed either. He agreed. I maintained my approximately 2.5 degree nose down attitude and rolled out on ATC assigned heading for a vector to a divert airport. With wings level I observed MFD ground speed indicating approximately 520 KTS. The First Officer was busy so I think I notified him that I was going to reduce power. When his workload allowed I asked him to query ATC as to our ground speed. Their response was consistent with our ground speed readout at that time. Around this time we lost all ADC display info on both PFD's (red flags for ALT; V/S and IAS). First Officer pointed out that we might want to consider reselecting BOTH position for ADC's on source selector panel. Given the hope that we may recover at least one ADC at some point and/or that we might have somehow misdiagnosed our problem I concurred. We had no usable ADC data on either PFD at that point anyway. In the region of 24;000 FT or so ISI airspeed seem to recover. Shortly thereafter PFD 1; then PFD 2 air data recovered. Consulting the ISI; [I] asked the First Officer to query ATC as to current ground speed. ISI indicated speed was more credible at this point. Assigned FL190 and descending through approximately FL210 we agreed that ADC info for ISI and PFD 1 appeared to be reliable but airspeed for PFD 2 was still indicating a 'slower' speed. We elected to deem PFD 2 info invalid and per QRH procedures selected PFD 1 data on the Source Selector panel. All other previous caution messages had self-cleared. At this point we felt confident enough to attempt to reengage the autopilot. Having initially chosen a nearby divert airport with ATC input as to weather conditions and runway length there and at a major regional airport; we revised our destination to the larger airport with updated weather now indicating no gusts and similar ceiling to the closer airport. [We] notified flight attendants of approximately 6 minutes to landing. We had initially notified them of our intention to divert earlier. First Officer notified Operations of our impending arrival and got a gate assignment. First Officer made PA to passengers notifying them of our intention to divert. Remainder of flight was without further incident. Parked at what later transpired was a remote international gate. [We] assisted Agent with passenger queries. Were led to a vacant Customs quarantine area which necessitated 'rescue' by airport officials 25 minutes later. Returned to aircraft and followed-up with Dispatch; notified Maintenance Control and contacted Chief Pilot on-call.
The Reporter stated that the aircraft's ISI altimeter continued functioning; but no valid pitot statically powered airspeed or altitude data was available; including the ISI speed which became unreliable. The fact that the ADC lost input activated the stall warning which operated until airspeed was again available.
The Captain and I were discussing our fuel state and contingencies when I noticed decreasing IAS on PFD 2. I crosschecked with PFD 1 and the ISI and alerted the Captain to a growing discrepancy. We observed that PFD 1 was indicating 245 KIAS; the ISI 240 KIAS; and PFD 2 indicating 230 KIAS and decreasing toward the low speed tape. As we were evaluating the discrepancy we received the EFIS COMP MON caution message with IAS miscompare flags on PFD 1 and PFD 2. At this point we determined that PFD 2 had the erroneous indications and I ran QRH checklist for EFIS COMP MON message (AHRS) - IAS (Abnormal 12-3). Per the QRH I selected ADC 1 on the Source Selector Panel. I notified ATC of the failure and requested descent out of RVSM airspace. We were initially cleared to FL290 and the Captain began a descent. The QRH further directed me to the ADC Failure procedure (Abnormal 12-7). Based on cross checking ADC 1 indications with the ISI I was not sure that ADC 1 was functioning normally; although I don't recall the specific indications observed. I was about to query the Captain about whether he wanted to run the procedure for ADC 2 failure or consider the Dual ADC failure when we received a RUD LIMITER caution message. The Captain was concerned about the implications of the Rudder Limiter Failure at high altitude and directed me to run that QRH procedure. I scanned the remainder of ADC Failure QRH procedure and confirmed that we had completed the procedure for First Officer's (ADC 2) failure and noted that there were additional pressurization control issues if ADC 1 were to fail completely as well. While running the RUD LIMITER message QRH procedure (Abnormal 8-3) I determined from the flight control synoptic page that the rudder limiter was failed in the intermediate/limited position and began reviewing the items requiring Flaps 20 landing and increased landing distance. Before I could complete a landing distance calculation it became clear that the Captain's air data information was bad; with large disagreements between PFD 1 indications and the ISI. The IAS on PFD 1 continued to decrease despite downward pitch attitude and significant thrust setting. The stick shaker activated the high speed tape dropped quickly to match the low speed tape. At this point we were concerned that we may have misdiagnosed the ADC failure and that ADC 1 had failed. We switched the Air Data Source Selector back to NORM in the hopes of cross checking indications with the ISI to reconfirm which ADC had failed. Unreliable conditions remained and we were not able to crosscheck indications to determine which source was accurate. Shortly after we observed red flags posted on IAS; ALT; and V/S on both PFD 1 and PFD 2. At the Captain's direction I declared an emergency and reported a dual ADC failure. I told ATC that our remaining information was unreliable and asked about the nearest VMC conditions. ATC reported that we would need to be much lower; below 10;000 FT; before we could get below the ceiling. They assigned FL190 and provided weather conditions at two nearby airports.  We initially chose an airport that gave us the best chance to get to VMC with the most altitude to work with. I quickly checked the actual landing distance table and did a rough overestimation to confirm that we could land on the runway at Flaps 20. We were assigned a 230 heading and continued descent to FL190. During the turn we encountered increased precipitation; lightning; and large amounts of static on COMM 1. During this time we observed no radar information on MFD 1 or MFD 2; FMS position errors; MACH TRIM caution message; and possibly additional messages. I can't recall the exact sequence of these less critical error messages. Weather and high workload made communication difficult and I needed ATC to repeat communications often. We asked for vectors to avoid whatever weather Center was able to depict. We continued to distrust the ISI as low pitch and high power settings were resulting in decreasing IAS indications. We were frequently getting ground speed reports from ATC to estimate our airspeed. Once we were heading toward our divert airport I turned my attention to preparing for landing. Remembering that I had been in the middle of the RUD LIMITER message and ADC Failure QRH procedures; I returned to the RUD LIMITER message procedure to complete the Flaps 20 landing items. The Captain noticed that the Rudder Limiter caution message had cleared; so he instructed me to abandon that checklist and plan on a Flaps 45 landing. I reset GND PROX FLAP switch to OFF and began trying to complete landing data setup and the dual ADC failure items when the Captain reported air data information returning to PFD 1. We were approaching FL190 at this time. I crosschecked PFD 1; PFD 2; and the ISI and observed that the indications of PFD 1 and the ISI were beginning to agree again; but PFD 2 was still indicating significantly lower IAS. The Captain and I discussed the returning indications and agreed that ADC 1 appeared to have returned to normal operation. Therefore I reviewed the ADC 2 failure instructions and returned the Air Data Source Selector to ADC 1. I reported to ATC that we seemed to be regaining some air data information and they reported improved weather conditions at the other nearby divert airport. At this time that aircraft was 40-50 NM off our right wing. The Captain and I discussed the situation and agreed that improved weather; stabilizing flight instrumentation; and significantly better passenger handling capabilities made our second divert airport the best option. I requested diversion to the second airport from ATC and they provided a vector and lower altitude. With air data information restored and stable the Captain was now able to activate the Autopilot and covered COMM 1 while I notified the flight attendants of our situation; that we were diverting; and that we were expecting a normal landing. After that exchange I pulled out my approach plates and set up both RTUs for ILS 28R. As I was retrieving the airport diagram I noted the operations frequency and gave Operations a very brief heads up that we were diverting for an emergency and would call back on the ground for gate assignment and further coordination. I reported to the Captain that we were set up for the approach and asked if he wanted me to notify the passengers. As instructed I gave a brief PA stating that we were diverting due to weather and would have further information on the ground. We were about to descend below 10;000 FT; so I asked the flight attendants to prepare the cabin for landing during that PA announcement and gave 2 bells for good measure. At this point the Captain and I had a brief discussion about trying to contact Dispatch regarding the diversion. Because we were already below 10;000 FT and wanted to land ASAP in case ADC1 failed again; we elected to contact Dispatch on the ground and continued landing preparation with the Descent and Approach checklist. While running the checklist I realized that I had not had a chance to complete the dual ADC failure items once that condition had been confirmed; but remembered that it was generally manual pressurization control. I crosschecked the pressurization indications; noted that they seemed normal; and entered the landing elevation for new divert airport. During the checklist we also discovered that landing speeds had not been set. I attempted to set the speeds from the LM; but my ADRP was not functional due to the Source Selector panel setting. We transferred controls briefly while the Captain set the landing speeds; then transferred them back. We had picked up intermittent visual contact with the ground and were in and out of clouds from approximately 14;000 FT to break-out below the ceiling at approximately 8;000 FT. We were vectored for the ILS 28R but had the runway in sight for the approach. The approach and landing were normal. I notified ATC that the emergency condition had ended; received a gate assignment from Operations; and contacted Ground Control for taxi instructions. After parking and completing the shutdown check we pulled the CVR circuit breaker; the Captain attended to passenger handling; and I contacted Dispatch as instructed to provide general notice of our situation. Threats included multiple mechanical failures; weather; high workload; communication difficulty. Undesired states are likely given lack of air data reference. There is a high likelihood that we exceeded Vmo/Mmo. We experienced stick shaker activation with overspeed and underspeed tapes converged. There may have been errors in QRH application when swapping between procedures as new issues changed priorities.Prior to this event I was aware of a seemingly similar event that occurred recently on another CR7. If the events are indeed similar finding the root cause is obviously imperative.
A CRJ-700 in IMC at FL370 lost all air data computer information; resulting in the loss of all altitude; indicated airspeed; and VSI indications. A diversion was made to a nearby airport and while descending through FL240; Air Data Computer 1 systems returned to normal.