A Captain and First Officer report that while they were in a holding pattern at FL360; their EMB-145LR experienced failure of both primary airspeed indicators within minutes of each other; with the same presentation of decreasing airspeed; causing them to question whether the single remaining standby airspeed indication was even accurate and reliable. Aircraft diverted.
Synopsis
A Captain and First Officer report that while they were in a holding pattern at FL360; their EMB-145LR experienced failure of both primary airspeed indicators within minutes of each other; with the same presentation of decreasing airspeed; causing them to question whether the single remaining standby airspeed indication was even accurate and reliable. Aircraft diverted.
Narrative
While holding in IMC at FL360 at 200 KIAS; we observed an increasing discrepancy between the #1 Airspeed indicator and the #2 airspeed indicator; along with the appearance of the amber IAS airspeed comparator annunciation on the PFDs. The #2 airspeed was relatively constant around 200 KIAS; while the #1 airspeed indicator (ASI); indicated decreasing airspeed to around 180 KIAS; at which time the #1 airspeed data was lost (red X on #1 airspeed indicator). Simultaneously; the autopilot disconnected with an AUTOPILOT FAIL master warning as the aircraft was turning in the holding pattern; resulting in a loss of altitude. The Captain leveled the aircraft while hand flying; and we coupled the flight guidance to the First Officer side and I began hand flying with my airspeed #2 indication still functional. As we continued in the holding pattern and completed the flight manual procedure for the loss of #1 airspeed indication; the airspeed #2 indicator began to indicate decreasing airspeed. We initiated a descent and increased thrust; but the Airspeed #2 indication continued to decrease while the Integrated Standby ISIS airspeed indication increased. We stabilized the descent with Integrated Standby Instrument System (ISIS) indicating about 240 KIAS and airspeed #2 indicating below 180 KIAS. There were several other intermittent EICAS cautions related to integrated computers (IC) and weight on wheels (WOW) faults. Also a Pack #2 OVHT caution and PACK #2 VLV CLSD message appeared while leveled at FL330. Descending through about FL280; while selecting the new landing elevation on the pressurization controller; the airspeed #2 indication suddenly increased from about 180 KIAS to match the ISIS displayed speed of 240 KIAS. While maneuvering for approach to landing; the airspeed #1 indication removed the red X and displayed a speed that coincided with the #2 and ISIS displayed speeds; and then also continued to operate normally. The aircraft ice detectors never indicated an ice condition throughout the incident; nor was ice accumulation observed from the cockpit. There were no EICAS messages associated with the heated probes. For reasons yet undetermined; the aircraft experienced the insidious failure of both primary airspeed indications within minutes of each other; each with the same presentation of decreasing airspeed that made it not immediately clear; what the actual airspeed condition of the aircraft was. The patterned failure of the two independent and redundant airspeed indicating systems caused me to question whether the single remaining standby airspeed indication (ISIS); was even accurate and reliable.
Second reporter narrative
ZZZ Airport had all inbound traffic in holding due to weather. We were in holding at FL360 in IMC conditions. Monitoring ZZZ weather and figuring our fuel reserves and alternate plan options. We had what seemed an ADC-1 (Air Data Computer) failure that presented initially as subtle PFD-1; IAS and Altitude Indicator failure followed by an Autopilot Fail message on our EICAS. Captain was Pilot Flying (PF) and recovery was initiated. There was up to an 800 FT altitude loss during the recovery as our position in the holding pattern was in a turn at the moment of failure. PF was switched to PNF (Pilot Not Flying) side; Flight Manual procedure followed; and ATC notified of our flight computer failure. ADC revision seemed to fix problem. ATC was notified that the problem had been an Air Data Computer failure and that our problem was solved. Indications of the failure were an uncharacteristic decrease in PFD indicated airspeed and altitude as well as an EICAS message (msgs); Autopilot Fail; and momentary Integrated Computer (IC) # 1-Weight on Wheels( WOW) INOP. The red x that is supposed to flag the failure is momentary; if at all; and late with regards to the need to control the Aircraft in todays' IMC environment. Autopilot Fail was understandable due to the ADC-1 information corruption. Additional and momentary was an 'IC # 1-WOW INOP' EICAS msg. We continued to evaluate our situation; ZZZ weather not improving; IMC conditions; and Fuel reserve status. Weather had moved through YYY earlier and their weather had improved. Diversion to YYY was decided; Dispatch was notified of our status and intentions and ATC was requested to divert us to YYY. The diversion was without incident; except that during the diversion and descent the PFD-2 airspeed decreased in conflict with the standby ISIS system and then rematched in indication after the cabin pressurization; Cabin Pressurization Acquisition Unit (CPAM) unit; was reset to our new landing altitude. During arrival process at YYY; in VMC conditions and warmer air; deselecting ADC revision showed normal ADC function. ADC revision was reselected for arrival to avoid further problems. Arrival and landing were completed without incident. There were no injuries at any time although there were a few cases of air sickness due to the flight conditions we had experienced.Altitude deviation; caused by transition from autoflight to manual flight during failure of Air Data Computer #1. PFD failure indications began subtly. At the moment of failure I was already on the thrust levers trying to both maintain our airspeed in the turn as well as economize our fuel consumption due to extended holding. Had I not been paying close attention at the moment of failure; the outcome could have been different. The ADC manufacturer needs to give us better notice of this type of failure. I spoke with another Captain who had experienced such a failure in a descent and the PF fixated on the instrument; to the exclusion of the overspeed warning. If this type of failure were to occur in a lower altitude environment; such as on a missed approach procedure; a hull loss might be unavoidable.
NASA callback
Reporter stated the newer EMB-145LR they flew had the Integrated Standby Instrument System (ISIS); instead of the older individual standby gauges; also known as 'steam' gauges. The ISIS is a self contained unit that combines the attitude (with it's own gyro); airspeed; magnetic heading and altitude; into a single LCD display and mounted just to the left of the center forward panel EICAS display unit; just above the radio management unit (RMU). Reporter; who was the First Officer; stated he did not have a full failure of his primary Airspeed Indicator (ASI-2) showing a red 'X' as the Captain's ASI-1 had displayed. The ISIS has approximately 45 minutes of availability; when using the aircraft's main battery. Reporter stated he believes the ISIS system is not tied in to the Air Data Computers. But whatever was affecting the #1 and #2 ASI's airspeed indications; seems to have started affecting the #3 Standby Airspeed Indication readings on the ISIS display. The aircraft has two integrated computers (IC); that are involved with multiple systems. Reporter stated they did not see any signs of ice accumulation. But as they descended to lower altitudes all airspeed indications eventually returned. He believes that; had they not descended to a warmer altitude; even the #3 ASI in the ISIS unit; would have started to show speed degradation.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.