Air carrier flight crew reported unreliable airspeed indicator during descent in severe weather. Diverted to alternate airport in VMC and landed uneventfully.
Synopsis
Air carrier flight crew reported unreliable airspeed indicator during descent in severe weather. Diverted to alternate airport in VMC and landed uneventfully.
Narrative
On Descent into ZZZ on our 6 hour ZZZ1-ZZZ flight; Captain's airspeed became unreliable causing multiple systems to fail. Due to severe weather at ZZZ airport our arrival was changed by ATC while we were already on it descending through 15000 feet with intended ILS approach to runway XXR. During this change the Captain's Airspeed Indicator failure occurred. Captain flying; I (First Officer) executed QRH for Airspeed Unreliable. After failing to restore captain's airspeed indicator we diverted to ZZZ2 where the weather was VMC. Executed ILS approach in visual conditions to runway XXL; landed and taxiied to parking where maintenance determined that there was significant water in the Captains pitot-static system. On descent into ZZZ we were assigned the ZZZZZ RNAV arrival and rwy XXR. We setup and briefed an approach to that runway via that arrival. Captain entered ILS XXL in secondary flight plan. We had been tracking a major weather system that was moving West to East and that was forecast to be over ZZZ at the time of our arrival. We had briefed this weather prior to the flight and noted our alternate fuel plus 10000 lbs contingency gave us options even beyond our filed alternate of ZZZ2. Captain suggested ZZZ3; for example. Also; this plane had autoland -MEL'd.; so we were even more prepared to divert if conditions required. During the last hour of flight we skirted severe turbulence and precipitation that was associated with this weather system; and at top of descent we planned for an anti-ice on descent through the weather; which was now painting large swaths of yellow and red; between us and the field; with some magenta on the weather radar display. During our initial descent we were advised of heavy precipitation on the arrival and asked if we needed a deviation. We declined at that time. Further into the descent (approximately 30 miles from the field and descending through 15000 feet; we observed large areas of red between us and the field. The controller suggested that previous aircraft had changed from the ZZZZZ or other arrivals to the ZZZZZ1 RNAV arrival. We asked for that and were assigned direct ZZZZZ2. We put ZZZZZ2 in the fix page and Captain turned towards it. We were advised we could divert as far left of our track to ZZZZZ2 as was required to avoid weather; so Captain chose a heading to the North and East of the weather that kept us clear of the red areas on our radar display. As I was head down programming the arrival; the autopilot disconnected. I looked up and observed several amber Level 2 alerts on the system display. Select Elevator Feel Manual; etc. I had not completed the reroute; so prior to acting on the alerts I inserted direct ZZZZZ2 in the Control Display Panel. At that point I was not aware of the unreliable airspeed. I pulled out the QRH and began looking at flight control checklists when Captain said 'Airspeed Unreliable.' I turned to that checklist and began reading and acting on it. As we did that we entered some light to moderate turbulence. We were advised that the ZZZZZ1 arrival was not available to us. I [advised ATC] that we had some failed instruments. They queried if we were [requesting priority handling] and needed assistance. I said 'negative; just advisory; we have a failed airspeed indicator.'We were assigned 10000 feet; and we descended and maintained that altitude with Captain flying.I worked through the Airspeed Unreliable checklist; but omitted an important step that would have restored the captain's instrument display. The captain asked for me to ask for a left 360; present position to give us time to work through the checklist. The radio was quite busy; and I was unable to make this request before the captain looked left (East) towards ZZZ2 and observed VMC conditions to the horizon. Captain asked to proceed to ZZZ2; and advised ATC that we were required to maintain VFR conditions and requested direct to ZZZ2.There were many other planes in the ZZZ vicinity; and the weather was changing rapidly and unpredictably; with runway assignments changing as well. We turned towards ZZZ2; asked for direct ZZZZZ3 on the ILS XXL approach; executed the approach and landing; and taxiied to the gate.As the PM it was my duty to accurately execute the QRH. I omitted a critical step on page X of the QRH. This would have restored the captain's instruments and allowed us to proceed to our filed destination. I believe I followed the wrong thread; or I confused the Static Air Switch items on pages Y and X. I was task saturated with ATC; helping captain maintain a course and airspeed; and with reading the checklist. My own inexperience with the plane was a factor; as well.In the moment; the following were contributing factors (in no particular order): (1) the startle factor. I believe that the Captain thought I had disengaged the autopilot somehow. Because; very coincidentally; as I was reprogramming the arrival; his airspeed indication failed. (2) We were pre-determined to divert to ZZZ2 based on our knowledge of the incoming weather. (3) I am new to the airplane and my systems knowledge is not completely consolidated. (4) I was not assertive enough: I should have asserted that I take the controls so that the captain could review my QRH work. (5) The Airspeed Unreliable checklist is very long with many branches that require the pilot monitoring to effectively render the flying pilot as a 'single pilot'; man-handling a defective airplane with ambiguous instruments; in IMC; while the PM chases wordy and complex decision threads with many wordy notes and references to rarely used switches. (6) Our aircraft does not have AUX CADC switch(es) installed. So that portion of the checklist that refers to them is a time-consuming distraction. (7) Negative training in the sim: At my airline and at previous carriers that I recently flew at; we were never allowed to restore the inoperative system until we had executed the memory item: 4/10 degrees pitch up; 90% N1; etc. Often the exercise was terminated prior to successful restoration of the instruments; leaving me with an ambiguity as to how the situation would resolve. (8) Fatigue. We started work at XA:00 and this was 7 hours after that. (9) rapidly changing ATC instructions when we were already task saturated with maintaining the flight path and completing the QRH. (10) Weather. It appeared to be deteriorating from bad to worse. (11) This plane had autoland -MEL'd.; so we were already prepared to divert if conditions required.Suggestions: (1) Captains should delegate flying to the FO when situation awareness and systems knowledge are critical to the outcome of the flight. (2) The QRH checklists with multiple threads / decision trees should be color coded. Each thread/ branch with its own color. Important steps should be in bold to separate them from the various notes that are relevant; but not essential to the safe outcome of the flight.Diverting to ZZZ2 was an avoidable consequence; my inexperience; combined with the other major factors noted above resulted in the diversion. On the other hand; in the moment; we justifiably concluded that it was the safest course to proceed to our filed alternate; which was not far away. I did not advocate further troubleshooting for the reasons discussed above: (1) much traffic in ZZZ airspace; (2) deteriorating weather; (3) ambiguous system failure; (4) QRH directive to maintain VMC; (5) captain's very justifiable decision to divert to our previously briefed; nearby filed alternate.(1) Instrument failure due to water in the pitot static system. (2) Incorrect application/ execution of QRH resulting in avoidable diversion. (3) Overly complex QRH procedure requiring too much attention from crew to execute correctly and in a timely manner when faced with other primary attention requirements (flight path/ weather/ ATC).
Second reporter narrative
On descent to 10;000 feet; passing approximately 16000 ft; the F/O (First Officer) (PM) was heads down inputting our second cleared arrival for ZZZ. The auto pilot clicked off. The Captain (Pilot Flying) attempted to re engage the auto pilot to no avail. It was then I noticed the IAS (airspeed comparison alert) and scanned my airspeed; the standby and the FO's airspeeds; noticing that mine was 20-25 knots slower than the other two. The FO was done with the arrival and looking up to get situated again (he's been on the line for about X months). I called for the Airspeed Unreliable checklist; which with everything going on he didn't hear right away. He opened his QRH and started to go into the checklist for some of the alerts that were being displayed. I ran the first 4 steps of the checklist (as displayed) and then again got his attention and directed him to the proper checklist. That checklist is very convoluted just reading it in the crew room later; but with the arrival; weather; and inexperience in the plane; he missed some steps on the checklist. One of the notes in the checklist was to if possible find VMC. Looking east; towards our alternate of ZZZ2; it was VMC the whole way; so with the nasty weather still in ZZZ; we decided to proceed to the alternate.Somehow the Captain's pitot static system had water introduced into it which affected the system. I've flown through heavy rain plenty of times before and not had this happen. And we flew normally for the first 5 1/2 hours of the flight. It was a normal descent; but somehow under that descent angle; that heavy rain; and those winds; the water got into the Captain's system.I don't know that the water in the system could have been prevented; but the mis read checklist could have been prevented by me giving the plane to the FO to fly; while I; with more experience in the plane; accomplished the checklist.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.