B737-700
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 193
Troubleshooting; Confusion; Communication Breakdown; Situational Awareness
Aircraft Equipment Problem Critical
General Maintenance Action
Manuals; Procedure; MEL; Human Factors; Aircraft
[I was] involved with originating plane swap in ZZZ. New aircraft MEL 34-35 Inertial Reference System (IRS) for left IRU (Inertial Reference Unit) applied by previous Captain; restricted to VMC day only operation. Reviewed operational weather restrictions with Dispatch and was satisfied our leg to our destination could be conducted VMC. [We] reviewed MEL 34-35 IRS with previous Captain. All required placarding was done correctly by previous Captain. [We] reviewed MEL 34-35 IRS procedural step and autopilot restrictions with previous Captain. First Officer and I were satisfied that we understood and were in compliance with MEL 34-35; inoperative LEFT IRS to 'Off;' IRS select switch moved from Normal to both on RIGHT. Engine start and accompanying System Recall showed no inoperative items. Taxi out System Recall showed no inoperative items. Takeoff uneventful; but shortly thereafter TCAS showed inoperative. In descent First Officer selected autobrakes and received an Auto Brake Disarm light. All Landing Braking Levels generated same light. RTO did not generate [Do Not Arm] light on preflight; nor in the air. Complied with QRH guidance and selected autobrakes 'Off.' System Recall from Descent Checklist generated Master Caution cap 'Flight Cont' Speed Trim Fail; single channel. [We] complied with QRH guidance. Landing uneventful [we] contacted Maintenance Control regarding the system failures. Maintenance Control grounded aircraft. [We] stayed with aircraft until repaired. Mechanics replaced left IRU and conducted BITE check on the three systems that failed en route. All systems functional after left IRU replacement. Mechanics traced left IRS input signals to all three systems. I believe we followed the MEL 34-35 IRS correctly; but believe that the MEL is possibly incomplete in identifying all the associated systems that are affected by the left IRU. While it is possible that this is a single isolated event; the ramifications of a systemic problem with the B737-700/-800 fleet may warrant further investigation. MEL [is] incomplete regarding affected associated systems.
Reporter stated the left Inertial Reference Unit (IRU) was MEL'd in the 'OFF' mode and the MEL did not require any circuit breaker to be pulled. The MEL does not list that the AUTOBRAKES Landing Levels selection would be affected or that TCAS would be affected; or the RTO 'Do Not Arm' light may come 'On;' which did occur prior to landing. None of the three systems were mentioned in the MEL when the left IRU was deferred. If the assumption is that the functioning right IRU would take over; or continue to operate the three systems he mentioned that had malfunctioned; then what actually occurred in flight does not agree with that assumption.Reporter stated that since the MEL does not list the AUTOBRAKES; TCAS or RTO Systems as possibly being affected by the deferral of the left IRU and the assumption is the right IRU will takeover those system functions; perhaps a System Test of the right IRU should be accomplished prior to dispatch with an inoperative left IRU; to confirm those systems are in fact functioning when in the Right ADIRU Selection. Otherwise; the affected systems should be placarded as inoperative. His Air Carrier has not made any changes to their MEL addressing the incomplete deferral of an IRU.
We departed ZZZ after complying with an MEL. On descent to our destination; the Autobrake Fault light came on when selecting autobrakes to [position]-2. Also; when passing FL180 and pressing Recall; there was a single channel fault (I believe it was the Air Speed Trim). After talking to Maintenance on the ground; they said these issues were a result of the MEL we had.
Two pilots report their B737-700 aircraft MEL does not list the AUTOBRAKES; TCAS or RTO Systems as possibly affected by a deferral of the Left Inertial Reference Unit (IRU). Captain's suggestion to verify that all three systems are functioning prior to dispatch was also raised.