This is from an earlier RNP discussion on these forums.
http://forums.flightinfo.com/showthread.php?t=61796
This is from the July 31, 2005 AFS-410 document revising the Aircraft Equipment Suffix Table. LINK /W aircraft are still expected to have limited RNAV capability, just not the Advanced RNAV to comply with Type A or B Terminal RNAV procedures.
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Aircraft with RNAV Capability. For flight in RVSM airspace, aircraft with RNAV and RVSM capability, but not Advanced RNAV capability, will file “/W”. Filing “/W” will not preclude such aircraft from filing and flying direct routes in enroute airspace.
With the KLN-90A, we file /W and routinely get cleared direct as if we were GPS equipped, which we are, but just for enroute ops. The TSO-C129 I think says something about our unit being class A2 (from the AIM), which is technically approved for enroute and terminal ops, however, the CDI sensitivity is locked on 5nm, so that is clearly not right.