B737-700
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 220
Distraction
Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude
General Maintenance Action; General Declared Emergency
Aircraft
In Cruise flight at 40;000 in dark conditions about 30 minutes into flight we received an intermittent altitude warning and a red cabin altitude light. After a second or so I dawned on my mask. The Captain ask for lower altitude. We were given FL240. Next controller gave us 10;000 feet. We ran QRH. Gained control of cabin pressure in manual mode and removed masks below cabin pressure of 10;000 as briefed prior to departure.Cabin pressure was 9;500 feet and climbing. Cabin climb rate was excessive. Masks did not deploy in cabin. Other than ear pressure comfort no one in the cabin took any issue or notice. Flight Attendants (FA) did not hear anything unusual. As the 'Cabin Altitude' action was placed prior to 'emergency descent' we did not run the emergency descent check list. We did descend without delay to 10;000 feet at VMO and speed break out. Landing was without incident. Maintenance met the aircraft.Reference in the 'cabin altitude' QRH should reference the emergency descent check list. There was no issues however the seat belt sign was in off position until I noticed it later after the event. There was no turbulence in the descent.
When the flight crew of a B737-700 received a cabin altitude alert warning they performed an emergency descent to below 10;000 feet MSL before the passenger O2 masks deployed and landed without further incident.