A320
Flight Crew Air Transport Pilot (ATP)
Confusion; Communication Breakdown; Human-Machine Interface; Situational Awareness; Training / Qualification; Troubleshooting
Aircraft Equipment Problem Critical
General Flight Cancelled / Delayed; General Maintenance Action
Aircraft; Manuals; MEL; Logbook Entry; Human Factors
The aircraft arrived with MEL 32-60-04-XX: Fault(s) Indicated by BRAKES N/WS MINOR FAULT Caution on ECAM E/WD; Not Confirmed by a Brake Pressure Transducer Fault??? which was added a couple days earlier. This MEL requires Alternate Braking System is checked 'operative' before the first flight of each day. Although it wasn't the first flight of the day for the aircraft; it was our first flight of the day on that aircraft; and for peace of mind; we decided to perform the Alternate Braking System Check described in the MEL. The test failed; the Alternate Braking System did not work correctly. This caused a five hour delay. It is very likely the aircraft flew several legs without an operational alternate braking system. It was subsequently discovered that the Left Normal (Green) Brake Pressure Transducer was faulty (it constantly sensed 2;000 PSI pressure regardless of the actual pressure). So it's unclear to me if it should have instead been dispatched under MEL 32-60-04-XY; 'Fault(s) Indicated by BRAKES N/WS MINOR FAULT Caution on ECAM E/WD'; following a Confirmed Fault on a Brake Pressure Transducer'. I believe several brake related MEL items are unsafe because they allow aircraft to be dispatched without an operational ALTERNATE Braking System. The MEL procedures related to faulty brake pressure transducers seem to be correct only when a faulty transducer does not sense pressure and indicates zero pressure. But the procedures don't make sense when a faulty transducer erroneously indicates a positive pressure. Even though MEL 32-60-04-XY was probably the correct MEL and 32-60-04-XX was applied instead; the situation would have been less safe and would not have been recognized if the correct MEL was applied; since 32-60-04-XY doesn't require an Alternate Brake Check at all. MEL 32-60-04-XX says the 'BRAKES N/WS MINOR FAULT' ECAM message may be displayed provided: A). BRAKES pressure indicators operate normally. B). Alternate Braking System is checked 'operative' before the first flight of each day; and C). It is checked prior to each departure that the following Centralized Fault Display System (CFDS) failure message related to BRAKES N/WS MINOR FAULT is not displayed: A fault on Alternate Pressure Transducer. Condition A and C were met on all flights. Condition B was met only on the first flight of each day. With a faulty 'Normal' brake pressure transducer erroneously sensing a positive pressure; the Alternate Braking System would test OK on the first flight of the day (if the aircraft electrical power was cycled off overnight); and all the conditions to dispatched under that MEL would be met. However; the Alternate Braking System would subsequently not be operational. That's because after powering up the aircraft; the braking system is defaulting to ALTERNATE and will pass the test described in the MEL. But once the 'Normal' Braking System is used (the Green Hydraulic System is pressurized and the brake pedals are pressed); the BSCU's won't automatically revert to ALTERNATE braking in case of loss of Green hydraulic pressure if they sense pressure from a 'Normal' brake pressure transducer (the ECAM WHEEL page will indicate the Braking System is in Alternate; but there will be no brakes available unless the A/SKID and N/W STRG switch is turned 'Off' or the parking brake is used). Similarly; MEL: 'BRAKES Pressure Indicators' allows a flight to be dispatched with a faulty alternate pressure transducer. But I'm not certain it should [be] allowed to do so with a pressure transducer erroneously sensing a positive pressure. Was the BSCU logic taken into account for this case? (Specifically; after landing gear extension; when the BSCU tests the braking systems.) I think MEL 32-60-04 and probably MEL 32-44-03 should not apply when a brake transducer erroneously senses a positive pressure. I think the 'Alternate Brakes Check procedure in MEL 32-60-04-XX should begin with steps to pressurize the Green Hydraulic System and apply 'Normal' brake pressure; then test the automatic reversion to Alternate Braking System after losing Green hydraulic pressure. Also; I think MEL 32-60-04-XY should require an Alternate Brakes Check. Flight delay. Procedures inadequate.
Reporter stated the initial trigger was the earlier deferral of Faults Indicated by BRAKES N/WS MINOR FAULT Caution on ECAM E/WD applying MEL 32-60-04-XX. He decided to perform the Alternate Braking System Test at the gate on their A320 aircraft by activating the Yellow Hyd System Electric pump and pressurizing the Green Hyd System using the Power Transfer Unit (PTU). Then positioning the PTU switch to 'Off' and viewing the ECAM Hyd System Page showing the loss of the Green Hyd System pressure. He then applied the brakes and should have seen Yellow Hydraulic System pressure to the Alternate brakes on the Triple Indicator; but no pressure was indicating. At that point the Alternate Braking System was not functional as required per MEL 32-60-04 and the aircraft took a five hour delay. Reporter stated further troubleshooting by Maintenance during that time noted a Brake Pressure Transducer error code that corresponded to the Normal Green Hydraulic System. The pressure transducer had failed to a 'positive' hydraulic pressure; always showed 2000 psi of pressure; preventing the automatic reversion from Normal Green hydraulic pressure to Yellow System pressure for the Alternate Braking System. Maintenance swapped the faulty Green Hyd System transducer with the good Yellow System pressure transducer to verify the Green pressure transducer was showing a constant pressure on the Yellow System Brake Pressure Triple Indicator even if with Yellow System pressure off; which it did.  Maintenance then performed another test of the Alternate Braking System that was successful because the 'good' pressure transducer from the Yellow System that was installed in the Green System; allowed the automatic reversion of the Alternate Braking System to occur. The Green and Yellow Braking Systems seem to function properly then. That's when MEL 32-44-03 was applied showing a pressure indication deferral in the Yellow System.Reporter stated the issue of deferring a Green Hyd System pressure transducer that has faulted to a constant 'positive 'pressure reading under MEL 32-60-04 should not be allowed because automatic reversion to the Alternate Braking System will not occur; even if the Green System fails; because the Normal Green pressure transducer will continue to show a 'positive' pressure in the Green System.
A Captain reports that several brake related MEL items are unsafe because they allow A320 aircraft to be dispatched without an operational ALTERNATE Braking System. Deferring a faulty Brake Pressure Transducer that erroneously indicates a 'positive' hydraulic pressure in the 'Normal' Green Hydraulic System under MEL 32-60-04 would prevent automatic reversion to the Alternate Braking System even after losing Green hydraulic pressure.