Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 222; Flight Crew Type 8000
Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance; Flight Crew Took Evasive Action
Human Factors; Weather
While at cruise altitude FL400; aircraft encountered severe turbulence. This caused the aircraft to momentarily exceed our Mach Max Operating speed along with aircraft flight path deviations. The aircraft then encountered a rapid decay in indicated airspeed while still encountering severe turbulence. We [advised ATC] and exercised Captain's emergency authority and began a descent to regain critical airspeed and give the aircraft a larger margin of safety in which to operate. We descended 4000 feet in order to regain our loss of airspeed; once the aircraft was no longer in severe turbulence and completely controllable; we coordinated a new cruise altitude with Air Traffic Control and cancelled our emergency status.We had our weather radar operating with a tilt below our flight path. Our radar indicated no weather on our flight path or within 50 miles of our flight track. Access to our internet weather program would have possibly given us a clearer picture of weather that was not being depicted on our aircraft weather radar.
In retrospect; the moon was not present so shadows of cumulus clouds were hard to make out. We inadvertently flew right over the top of a very small but forming cumulus. The radar did not pick it up; if we had the tilt lower it may have given us a heads up. Having used an auto tilt is of course safer; but we could have made more frequent sweeps of the tilt.
B737-700 flight crew reported encountering severe turbulence at FL400.