B787 First Officer reported concerns over MEL procedures requiring removal of a disabled stabilizer brake assembly. The reporter was concerned that additional workload placed on the remaining functioning assembly could result in faster wear and possible failure of the assembly.

Narrative

Broken Stab R2 brake assembly removed and stowed in flight deck. Please review MEL XX-XX-X and XX-XX-X. The stabilizer has two EMCU (Electric Motor Control Unit) units L2 and R2. Their inputs are single path summed inputs to the HTAS (Horizontal Trimmable Stabilizer System) (Company name for the jack screw that moves the horizontal stabilizer). This MEL allows one of the two to be inoperable (and requires the entire EMCU and brake assembly to be removed); and for the plane to fly 10 flights before being fixed with no restrictions (such as no ETOPS; etc). In the last several years a high proportion of fatal aviation crashes have involved broken stabilizers or jack screws. A single failure of the remaining control unit (but breaking in a different way than unanticipated by Boeing) could create a very difficult to control airplane. The backup for the remaining stabilizer control unit is the elevators to provide pitch control--but they could be fighting against a non-faired stabilizer and CG both moving in the same direction against them. Issues with MEL: 1. Only a single failure of the other EMCU stabilizer motor control brake unit could create a priority situation. While the elevators could still control pitch; the non normal checklist for stabilizer requires a flaps 20 landing because elevators alone may be insufficient to overcome an out of position stabilizer. An example of this scenario is where the plane is loaded with an aft CG requiring nose down trim on the nose down limit for takeoff (what we had) and the stabilizer assembly breaks on transition from climb to cruise and locks in the climb attitude. You would have two strong forces pitching the nose up and the smaller elevator surfaces to bring it down again. Additionally this stabilizer position would create parasitic drag and increase fuel burn.2. Unlike other parts on the plane (for example the center fuel pumps; each of which can fully fuel both engines); the stabilizer control units are each designed for only half the load/work of moving the horizontal stabilizer. This is called single path summed inputs design. This is evidenced by the fact the trim rate drops to half with the MEL. When you remove a part of system and leave the remaining parts under a heavier than normal strain they are likely to break earlier and different than they otherwise might have. For connected systems the failure of one component can often cause damage to any attached system elements because as one brake assembly is breaking it is putting greater than normal load and strain on the remaining unit and everything else attached to it. The fact that this MEL could allow 100 hours plus of flight time on the remaining stabilizer assembly is increasing the chance that it breaks. Any wear statistics on the stabilizer assemblies are based on both working in concert each taking half the load. Does Boeing really have failure data on a single assembly? Especially flown over 100 hours; and after the other one was breaking first?3. Dispatch over ETOPS compounds the problems a single failure could create. Parasitic drag of the stabilizer could increase fuel burn. Alternates are far away.4. The EMCU R2 was removed and placed in the flight deck and not tied down. So created a hazard for the pilots.