SQL Server 2012

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CREATE TABLE TestBed 
(
     Aircraft_ID INT, 
     Serial VARCHAR(50),
     MDS VARCHAR(50)
)
INSERT INTO TestBed (Aircraft_ID, Serial, MDS) 
VALUES (85, '56-1965', 'T-37B'),
       (85, '56-1967', 'T-37B'),
       (85, '56-3547', 'T-37B'),
       (85, '56-3577', 'T-37B'),
       (85, '57-2265', 'T-37B'),
       (85, '57-2272', 'T-37B'),
       (85, '58-1915', 'T-37B'),
       (85, '58-1925', 'T-37B'),
       (85, '59-0249', 'T-37B'),
       (85, '59-0273', 'T-37B'),
       (85, '59-0299', 'T-37B')
11 rows affected

SELECT tb1.aircraft_id,
       stuff((SELECT concat(', ', tb2.serial)
              FROM testbed tb2
              WHERE tb2.aircraft_id = tb1.aircraft_id
                    AND tb2.mds = tb1.mds
              FOR XML PATH ('')), 1, 2, '') serial,
       tb1.mds
       FROM testbed tb1
       GROUP BY tb1.aircraft_id,
                tb1.mds;


aircraft_id
serial
mds


85
56-1965, 56-1967, 56-3547, 56-3577, 57-2265, 57-2272, 58-1915, 58-1925, 59-0249, 59-0273, 59-0299
T-37B