Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 169; Flight Crew Type 6143
Confusion; Communication Breakdown; Distraction; Time Pressure
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown
Air Traffic Control Provided Assistance; Flight Crew Overcame Equipment Problem; Flight Crew Requested ATC Assistance / Clarification; Flight Crew FLC complied w / Automation / Advisory
Aircraft; Human Factors
While cruising at FL400; with a weight of approximately 130;000 pounds and about a 30 knot buffer between Red Line and top of the yellow arc; the PLI (Pitch Limit Indicator) displayed 1 above our Pitch Attitude. I did not realize that it was only on my (Captain) side ADI (Attitude Director Indicator); I asked the FO (First Officer) to request FL380 and I initiated a shallow descent with the AP (Autopilot) off. I believe that the PLI disappeared but then reappeared approaching FL380 with the PLI still indicating only 1 above our Pitch Attitude. I continued descending to FL360 and asked [the FO] to request FL360. It took ATC a short time to respond and issue clearance to FL360 and I may have deviated from our cleared altitude trying to ensure that we did not enter a stalled condition. The PLI did appear a 3rd time; briefly at FL360 but then disappeared for good. I then switched the AP to the B AP. We had been in the tops of clouds and I initially thought the turbulence was causing the PLI display but after leveling off at FL360; [the FO] informed me that the PLI never displayed on his ADI. We worked with Maintenance Control who informed us of a previous write up 7 days earlier of the #1 Stall Management Yaw Damper (SMYD). After a short time we returned to FL400; and continued to destination uneventfully.
During cruise at FL400 at approximately 130 knots; CA (Captain) stated he had 'whiskers' (a.k.a. the PLI Pitch Limit Indicator); disconnected the autopilot (Command A); requested that I ask ATC for FL380; and initiated a descent. By the time ATC responded with authorization; we had passed FL397. CA then stated that PLI went away passing FL390; but as we approached FL380; the PLI returned and he had me coordinate for FL360 while he continued the descent. Again ATC was late in responding and we were past FL377 by the time we received approval. During this second descent; I noted to CA that I didn't have a PLI indication on my ADI (Attitude Director Indicator); we had sufficient airspeed stall buffers; and it appeared to be a system malfunction. CA leveled the aircraft at FL360; his PLI went away and we began to troubleshoot. No other malfunctions were noted and QRH was not applicable. A review of system in flight manual and discussion with [Maintenance Control] identified the potential cause of the erroneous PLI to a malfunction in the SMYD #1 system (previous write up noted by [Maintenance Control]). CA re-selected autopilot with Command B and flight resumed without incident.
B737 flight crew reported the Captain's Pitch Limit Indicator came into view during cruise and a descent was initiated before the full clearance was received from ATC.