B737-700
Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Human-Machine Interface; Situational Awareness; Workload
Aircraft Equipment Problem Critical; Deviation - Altitude Overshoot; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Overcame Equipment Problem
Aircraft; Human Factors
We were given a descent to FL290 from FL350. Around FL320 I noticed the pitch mode had switched out of VNAV PATH to CWS; and my airspeed was slowly decreasing through about 240 knots. We were still descending at 1000 fpm. I vocalized my displeasure at the VNAV disengaging and attempted to re-engage the VNAV; it would not engage--neither would the Vertical Speed. As I looked down again at my airspeed indication; the speed was then rapidly decreasing through about 190 knots and rapidly approaching the yellow band on the airspeed indicator. I feared an imminent stall; so I pushed the nose down; the speed was still decreasing. Then I added a lot of power. The airspeed was still decreasing. At that point the PM (Pilot Monitoring) said 'Hey! The airspeed is going through 300 knots and increasing!' My airspeed was still decreasing and now in the yellow band through about 170 knots. I quickly looked over at the First Officer's airspeed indication and then quickly looked at the STBY airspeed indication (something I don't think I would have intuitively done if I had not gone through the Airspeed Unreliable scenario in [training]); and saw that the FO's airspeed and the STBY airspeed indications matched. I pointed at his IAS and then the STBY IAS and said 'Ok; your IAS matches the STBY IAS; so yours is probably correct. We have an Airspeed Unreliable situation.' I then quickly brought the nose up and reduced power and re-established a 1000 fpm descent and called for the Airspeed Unreliable Checklist. (In my head I meant to say the QRC Airspeed Unreliable Checklist; but; in the heat of the moment; I only said Airspeed Unreliable Checklist). The autopilot was still engaged in CWS pitch mode. Then I set 76% N1; figuring that was a good power setting for a 1000 fpm descent; I also switched to the B Autopilot since his side had the good indications. The Vertical Speed mode now engaged. It wasn't until that time that I saw the IAS DISAGREE below my IAS indicator. As we approached FL290; moments later; the PM was still searching for the Airspeed Unreliable Checklist and looked up and said 'Hey I don't think the altitude captured at FL290.' I looked at his altimeter and it was almost 100 feet below FL290. I disengaged the autopilot and leveled off at FL290; then re-engaged the B autopilot in ALT HOLD. At that time I noticed that the ALT DISAGREE was also illuminated on my side and my altimeter was a few hundred feet different from the FO's and the STBY altimeter. Almost immediately after leveling off at FL290 we were given another descent to FL240. I dialed in a 1000 fpm Vertical Speed descent. Then; miraculously; all my instruments blinked and returned to normal. IAS and ALT all matched. We were so busy and task saturated just trying figure out what was happening and fly this aircraft to this point (it all happened very quickly) that we still had not even performed the Airspeed Unreliable Checklist. As we were about to finally go over the Airspeed Unreliable Checklist we noticed the ENG on the Master Caution annunciator was illuminated. We looked up and saw that both EECs (Electronic Engine Controls) were in the Standby mode. So; since the airspeed now appeared to be normal; we ran the EEC Standby Mode Checklist. Now; we took a step back; looked at everything; talked to each other about how we were doing; how the aircraft was doing; and tried to return to the Green. We finally looked at the Airspeed Unreliable Checklist in the QRC and talked about what actually happened and what we could/should have done differently-and when. The PM told me that he had not yet done the [training program] with Stall Training/Airspeed Unreliable training; but he would now be looking forward to it and have a much higher respect to what he would gain from the training. Bottom line; we did not make a hole in the ground. Thank you training. I never really look forward to training; but it may have just saved my bacon this time; and probably many times before.Wow; training; training; training! If I had not seen this scenario in training; I am scared to think of the possible different outcomes. Here are some things we talked about; that we could/should have maybe done differently: 1) Be careful what checklist you call for. I called for the Airspeed Unreliable Checklist. I should have called for the QRC Airspeed Unreliable Checklist. This may seem like a small detail; but it was huge! Instead of pulling up a quick easy card which would have pointed us to a more detailed checklist; the PM was busy trying to find this checklist in the QRH (the PM had not yet been through [the training program] and done the Airspeed Unreliable training).2) Once realizing the FO's instruments were correct; and mine were not; I should have transferred aircraft control to his side.3) Since we never got around to running the 'QRC' Airspeed Unreliable Checklist; we did not turn OFF both Flight Directors. Which meant; even though we switched to the 'B' autopilot; the Captain-side was still the master; so the autopilot was still looking at the Captain-side altitude for level off and altitude capture. When the Captain-side altimeter went into ALT DISAGREE; the autopilot was still looking for the Captain-side altitude to level off. Luckily; the PM saw the altitude discrepancy and voiced it. We were lucky not to have an altitude deviation problem.4) Talking to each other is big! Even after a four-hour flight at midnight local; we talked to each other. Told each other what we were seeing. This also is what we are trained to do. This may be my biggest request. In every other aircraft I have flown; if the VNAV or any other important system changes modes (i.e. goes from VNAV PATH to VNAV SPD; or even just changes to CWS; popping out of VNAV PATH) there should be an aural warning or at the very least a Master Caution; something to alert the crew of the change!
B737-700 Captain reported unreliable airspeed indication in descent that self corrected after a few minutes.