Aircraft manufacturers have been making many tests to new aircrafts after from production processes. Manufacturer companies tests structures, systems, components of their aircrafts according to all conditions or all situations. Thus If there are any problems in designs or productions, manufacturers learn. This is general purpose of tests.
Then manufacturer company deliveries aircraft to operators after everything for aircraft are determined to be seamless.
Some companies try different ways for tests. One of these ways is allocate a test aircraft by manufacturer.
When companies produce aircraft, they try structures, systems, softwares of their aircraft on the test aircraft for avoid risks and fuel save.
Different aircraft that providing more fuel efficient are used for tests instead of directly testing new produced aircaft.
Considering a company that produces a lot of aircraft, this methods provide to more fuel saving for hundreads of tests.
In the same time, thus manufacturers learn problems (if there are) of structures, systems or components before completing of production. In this way they get rid of extra workload and overspending. Engineers closely monitoring to operations through this method.
There are two B757 testbed aircraft that looking interesting and used this way.
Lockheed Martin has been using a B757 as test aircraft for F22-RAPTOR aircraft. Wings and nose of F22 are located as exactly the same on the B757-FTB. And most softwares of F22 can be tried on this aircraft. Registration of interesting looking aircraft is N757A.Other name of this aircraft is’Catfish‘.
This aircraft is operating for Lockheed Martin on behalf of Boeing Company. This B757 was produced in 1982. So 38.6 years old. Lockheed Martin is using this B757 as a test aircraft for F22 since the its first day. It flew with RR RB211 engines in the first year. Then it had continued to fly with PW2000 engines by changing engine modification in the 1983.
Another B757 has been operating as test aircraft for Honeywell Company.
Engines that produced by Honeywell have been testing on this aircraft. (as indepently from main engines) Registration of aircraft is N757HW. It was produced in the 1983. So 37.6 years old. This B757 was used by for Eastern Airlines, Airtours Airlines, My Travel Airlines (as commercial aircraft). And it has been operating for Honeywell Company at the moment. (as test aircraft)
Well-established and large companies think and design these methods. Aircraft like that B747, B737, A330 and such can be test aircraft.
For example; Lockheed Martin company has been using a B737 for tests of F35.