B737-800
Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Human-Machine Interface; Distraction; Troubleshooting
Aircraft Equipment Problem Less Severe
Flight Crew FLC complied w / Automation / Advisory; Flight Crew Diverted; General Declared Emergency; General Maintenance Action
Aircraft
While enroute a 'Display Source' flag appeared on both outboard PFD's. Additionally; the First Officer's PFD had 'SPD LIM' flag and the low speed indicator was missing. We referred to the QRH and notified Maintenance Control and Dispatch via ACARS. After approximately 30 minutes we started losing additional equipment. We lost the First Officer's weather radar; both ECC's went into ALT mode; Captain's low speed limit; and the First Officer's pitch F/D. Based on what information we had lost I surmised we had lost the #2 Display Electronic Unit (DEU) and possibly some information from #1 DEU. I was monitoring the weather in our filed destination and at that time it was [900 FT] BKN; RA and 10 miles visibility. I referred to the Approach/Landing data for non-ILS approaches and noted we are required to have the Source in AUTO. I interpreted this to mean we need both DEU's. Also; in my judgment it would not be prudent to make an instrument approach with only one DEU operating. If we were to lose the second DEU we lose all 6 displays and would only have the standby for flight instruments and no engine or hydraulic indications. I then made the decision to declare an emergency and divert. We were about 100NM from our divert airport and it was VFR. We landed uneventfully and once on the ground maintenance ran a bite test and it indicated the #2 DEU had failed. Additionally; something did not test properly with the DEU interface. The mechanic pulled both DEU's out and swapped them and reinstalled and tested again. This time both DEU's passed. Maintenance released the aircraft with the remark of no previous history. I flew this aircraft 2 more legs and everything worked normally.
A B737-800 First Officer's Display Electronic Unit (DEU) failed in flight so the crew diverted to a VMC airport where Maintenance determined that both DEU's had faults.