MD-11 flight crew reported an Autopilot Disconnect Warning in cruise. This was followed by numerous Airspeed Warnings. The flight crew ran the QRH and elected to continue to destination airport.
Synopsis
MD-11 flight crew reported an Autopilot Disconnect Warning in cruise. This was followed by numerous Airspeed Warnings. The flight crew ran the QRH and elected to continue to destination airport.
Narrative
A lot was going on in a short period of time. The following is a narrative of events in a linear flow but during the event a lot was happening at the same time.During cruise at FL360; Captain; PF (Pilot Flying); we had an Autopilot Disconnect Warning. I grabbed the yolk and throttles and checked PFD. Captain's PFD showed we were white across the board. Manual on speed; altitude and heading. We were in a slight decent and airspeed was slowing into the lower ft. FO (First Officer) was showing slight decent and airspeed increasing. I made the statement it looked like Airspeed Unreliable. We then had Multiple Level 2 Alerts pop up. We then started to get Over Speed Warnings. We determined that it looked like the FO's info was the more accurate and I slowed down just enough to discontinue the Over Speed Warning. Captain's Airspeed was continuing to decrease (showing 180 kts). I directed the FO to look at the first few steps of one of the 3 Level 2 Alerts. I said the checklist should have a statement that if we have the 3 Level 2 alerts showing it is an Airspeed Unreliable. We confirmed that was correct and started the Airspeed Unreliable checklist.At some point ATC called us and we confirmed we were having major issues. We told them we were correcting to altitude and course. We also said we would get back to them.While the FO started the checklist I was back on altitude and trying to correct to course. I was having trouble getting on course. I then realized that my side was commanding a heading to course based on incorrect winds. My airspeed was down to 80 kts. and showing a wind out of the southeast at over 200 kts. FO was showing winds near flight planned approximately 180 degrees and 120 kts. I contacted ATC and told them we were lost a lot of info and would like a heading. He gave us a heading and offered ZZZ for the nearest airport. I told him we would get back to him.During the Airspeed Unreliable checklist; it showed that our aircraft was equipped with an AUX CADC. Per the checklist we selected it to the Captain's Side. We finished the checklist and got back almost everything we had lost except: SEL FADEC ALT Level 2 was still showing on all 3 engines. We ran the QRH and corrected that Warnings.The only Warning still showing was an ALT Comparator Alert. We discussed the weather at ZZZ1 and felt that with weather at 1;500 ft. we could make a safe ILS approach. Captain's; FO's and Standby Altimeter were all with in 150 ft.We then contacted Dispatch and told them what we had. We also confirmed with them our plan to continue to ZZZ. They saw no problems with continuing to ZZZ.
Second reporter narrative
During cruise at FL360; the Autopilot disconnected. Captain was Pilot Flying and grabbed the controls. My instruments showed us in a descent with increasing airspeed. Captain's instruments were showing a descent and his airspeed was decreasing. Captain said that it looked like an unreliable airspeed. My instruments started showing an overspeed condition; and we started getting the associated warning with that. We also had multiple level 2 Alerts displayed so I started the checklist for the Select FADEC ALT (The first Level 2 Alert displayed); which along with the other Level 2 Alerts; directed me to the Unreliable Airspeed checklist; which I began. At that time we determined that it was my side's instruments that were correct; due to their correlation to the standby instruments. The Captain's Airspeed Indication dropped to as low as 90 kts. Captain then slowed down to stop the Over Speed. ATC queried us about being off on our altitude and I told them that we were working an instrumentation issue but we were working on getting back on course and altitude and we would get back with them. Captain was trying to maintain course towards the fix; but because of his incorrect data we were not going the right direction. We requested a heading assignment from ATC while we continued to work the issue; which they gave us. At some point around this time; ATC offered us ZZZ as the nearest airport if we needed to divert. The checklist directed us to switch the AUX CADC to the Captain's side. When we did that his airspeed data returned to normal. The Select FADEC ALTN level 2 Alert was still Illuminated; so we ran that checklist and it cleared the Alert. We were able to get back on course and reengage the Autopilot after that. The only remaining issue was an Altitude Comparison Advisory; because there was about a 150 ft. difference between my and the Captain's Altitude reading. We discussed how to proceed and neither one of us thought that the small altitude split would prevent safely continuing to ZZZ1; so we continued. We contacted company as well and they saw no issue with continuing on to ZZZ1. Remainder of the flight continued without incident.I believe this event was caused by the Captain's pitot tube becoming blocked by ice crystals; as we were in the tops of clouds at high altitude for an extended period of time. This caused the loss of useful airspeed data on his side; which caused the autopilot disconnect; leading the Captain to attempt to maintain proper speed; course and altitude with bad airspeed data.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.