During cruise at FL390; the flight director bars were not displayed anymore and 'cws' appeared on th...

Narrative:

During cruise flight at FL390; I noticed on the pfd that the flight director vertical bar disappeared and the message 'cws' was on. Looked around and saw both fmcs data were blank and the menu page showed ACARS but no FMC. I put my hands on the yolk and noticed that I had control thru the cws. Lost about 100 feet of altitude and returned the airplane to level flight manually. The first officer and I looking around the cockpit to access the situation but kept my main focus on flying the airplane. The first officer got out the QRH; but there was no procedure for what we had except invalid navigation data. Tried a couple of things but nothing helped...then out of the blue; everything came back and I reengaged the autopilot; then reselected LNAV and VNAV because it dropped out both modes. A couple of minutes later; it did it again. Same displays; approach was trying to give us clearance to a fix and I informed them that we were have FMC problems and could not accept the clearance. They gave us a heading to the ILS. Everything came back a second time and then it did it again the third time; this time I just shut everything off and hand flew the airplane to a normal landing and taxi to the gate. Wrote the event in the aml and maintenance took it from there.

Original NASA ASRS Text

Title: During cruise at FL390; the flight director bars were not displayed anymore and 'CWS' appeared on the PFD. The crew discovered both FMS' were intermittently blank and; with no QRH procedure to address the faults; the crew hand flew to their destination.

Narrative: During cruise flight at FL390; I noticed on the PFD that the Flight Director vertical bar disappeared and the message 'CWS' was on. Looked around and saw both FMCs data were blank and the MENU page showed ACARS but no FMC. I put my hands on the yolk and noticed that I had control thru the CWS. Lost about 100 feet of Altitude and returned the airplane to level flight manually. The First Officer and I looking around the cockpit to access the situation but kept my main focus on flying the airplane. The First Officer got out the QRH; but there was no procedure for what we had except invalid NAV Data. Tried a couple of things but nothing helped...then out of the blue; everything came back and I reengaged the autopilot; then reselected LNAV and VNAV because it dropped out both modes. A couple of minutes later; it did it again. Same displays; Approach was trying to give us clearance to a fix and I informed them that we were have FMC problems and could not accept the clearance. They gave us a heading to the ILS. Everything came back a second time and then it did it again the third time; this time I just shut everything off and hand flew the airplane to a normal landing and taxi to the gate. Wrote the event in the AML and Maintenance took it from there.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1278984
Time
Date	201507
Local Time Of Day	1801-2400
Place
Locale Reference	ZZZZ.ARTCC
State Reference	FO
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	B737-800
Operating Under FAR Part	Part 121
Flight Phase	Cruise Initial Approach
Flight Plan	IFR
Component
Aircraft Component	FMS/FMC
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Less Severe

During cruise at FL390; the flight director bars were not displayed anymore and 'cws' appeared on the pfd. The crew discovered both FMS' were intermittently blank and; with no QRH procedure to address the faults; the crew hand flew to their destination.