B747-400
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Communication Breakdown
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Maintenance
Flight Crew Overcame Equipment Problem; General Maintenance Action
MEL; Logbook Entry; Procedure; Aircraft; Human Factors
Arrived at the airplane and there were 5 deferred maintenance items on the logbook and aircraft; which matched the OFP (Operational Flight Plan) paperwork. One of those items was for the Left RA (Radio Altimeter); MEL 34-XX-XX-XX. We looked at the DDG (Dispatch Deviations Guide) and noted the Maintenance actions; which there were only 2. There was only one radar altimeter required for our flight due to our routing and weather. I looked at previous logbook entries and noticed it was written up as intermittent operation; so it was still working occasionally. I believe there was a writeup stating that both the Left and Right RAs were intermittent. The maintenance action was they tested good on the ground; but the logbook entry never cleared the initial DMI (Deferred Maintenance Instruction Manual) for the Left RA. There was an operations note in the DDG that stated for One RA inoperative; status message NO LAND 3 will be displayed. We did not see this message; however since the Left RA was intermittent and not totally inoperative; I figured that why we didn't see the NO LAND 3 status message. The Left RA seemed to be working on the ground as well with no erroneous information on the PFD or random GPWS alerts.Shortly after reaching cruise; the Left PFD started showing random heights for radio altimeter ranging from 400 feet to 2;500 feet. We began to get 1;000 feet callouts and terrain warnings from the GPWS; even a 'Pull-UP' advisory at FL340. Even though we were at FL340; this became extremely annoying and unsafe as the frequency of the callouts was increasing over the speaker. It was obvious the Left RA was feeding erroneous data to the GPWS; as verified by the random numbers being displayed on the Left PFD in the radio altimeter display. The First Officer and I agreed we couldn't continue receiving the erroneous data to the GPWS and talked about removing the L RA from the picture by pulling the circuit breaker. We decided to contact Dispatch via Satcom who phone patched us with Maintenance Control. I described what was happening with the L RA and the associated GPWS alerts. They recommended pulling the L RA circuit breaker located at B3 on the overhead panel. I pulled the breaker; with confirmation from the First Officer; and the erroneous left radio altimeter indications stopped as well as the associated GPWS alerts; as expected. Upon arrival; the left and right PFDs displayed accurate radio altimeter information and all auto callouts (eg. 1;000 feet) and GPWS alerts were normal. The action of pulling the L RA circuit breaker at B3 was written up in the logbook. Upon researching the maintenance history online; it appears the DMI for MEL 34-XX-XX-XX was cleared according to Maintenance; however this was not the case according to the logbook; aircraft stickers; flight plan and fact that the Left RA was not working properly.First; I suggest that if a radio altimeter is installed; but not working properly; that it be disabled by pulling the associated circuit breaker to avoid erroneous data being sent to the GPWS; PFDs; or Autopilot/Flight Directors and allow for correct data to be sent to those systems. I would assume this would be a Maintenance step under the MEL 34-XX-XX-XX. Second; there must have been a disconnect between the person performing the maintenance entry in the logbook for the writeups concerning the L RA and the person at Maintenance Control making the entry into the aircraft history online. The crew does not have time to go online and verify DMIs online in the aircraft maintenance history; sometimes they are cleared before we arrive at the aircraft; sometimes they are not. It would be good if the ground maintenance staff would do this prior to the crew arriving at the airplane to ensure that what is listed as a DMI in the airplane logbook concurs with what is recorded in the aircraft maintenance history online.
B747 Captain reported an aircraft was dispatched with a faulty radio altimeter; causing erroneous GPWS warnings.