For those of you that don't have access to the FCOM Bulletin.
There have been several events where the indicated airspeed shown to the pilots
dropped rapidly (within a few seconds) to below 50kts. These events are
suspected to have been caused by temporary blockage of multiple pitot tubes
when near weather systems with high moisture content. In all events, the
blockage cleared within several seconds. During some events, the FLIGHT
CONTROL MODE, AUTOPILOT DISC, and other messages were shown
(along with a flight control reversion to secondary mode). In one event, the PFD
automatically switched to AOA speed; and in another event, the AIRSPEED
UNRELIABLE message was shown. Depending on the duration and nature of
the pitot blockage, FLIGHT CONTROL MODE, NAV AIRDATA SYS, NAV
AIRSPEED DATA, AIRSPEED UNRELIABLE, AUTOPILOT DISC,
AIRSPEED LOW EICAS messages can show during similar events.While, in all these events, the air data system recovered and the fault detection
system provided alert level EICAS messages where expected, a possible flight
crew response to an initial sudden drop in indicated airspeed could be to apply
pitch inputs which may result in ‘G’ excursions. If the crew applies strong nose
down force to the yoke without disconnecting the autopilot, when the autopilot
disconnects (as a result of the manual force applied), a sudden nose down
command would be applied to the airplane. This nose down input could be
amplified by the low column forces being fed back to the pilot as a result of the
low airspeed being indicated.
As a result of the above events, Boeing is making improvements to the flight
control system and force feedback to mitigate the negative effect of large control
inputs in these scenarios, and is also making improvements to the EICAS Alert
logic related to air data faults. In addition, Boeing is continuing to investigate the
pitot blockage event rate to determine if any design improvements are warranted.
In response to concerns about possible negative crew reaction to an initial sudden
drop in airspeed, the FAA intends to issue an Airworthiness Directive to require
operators to incorporate a procedure into their AFM which instructs crews to
avoid abrupt control inputs in response to sudden unrealistic drops in indicated
airspeed. This procedure also reminds pilots that if manual flight is desired, the
autopilot should be disconnected prior to any manual flight control inputs.