"The media and airlines tell us that N591UA and N612UA crashed on Sept 11, but FAA records say the planes are still valid and registered. And one UA employee let slip in his private website that he travelled on N591UA after sept 11, apparently not realising the significance of the plane’s ID"
The central implication here is that because the FAA records show these registration numbers are still valid, it means the planes still exist. There is, of course, another alternative -- no-one may have moved to have them de-registered.
Is this possible? It seems so. Here's an example.
On July 26, 2002, FedEx flight 1478, a Boeing 727-232F (N497FE) struck trees on final approach to Tallahassee Regional Airport at 5:37 a.m. The flight had originated in Memphis, Tennessee. The captain, first officer and flight engineer were seriously injured, and the airplane was destroyed by impact and resulting fire.
Destroyed, then? So then we visit the FAA Registry, enter the number, and this record pops up as of 17th April 2005:
N497FE is Assigned
Manufacturer Name BOEING
Model 727-232 Status Valid
FEDERAL EXPRESS CORPORATION
On January 1, 2002, about 1802 eastern standard time, a Piper PA-31-250, N3525Y, registered to Taurus Wings Inc., and operated by Air Taxi Inc., as a Title 14 CFR Part 135 on demand air taxi flight, ditched in the Atlantic Ocean, near Hollywood, Florida.
An FAA Registry check on 10th March 2006 tells us:
N3525Y is Assigned
Model PA-31-350 Status Valid
Registered Owner: TAURUS WINGS INC
Interestingly, the model here is listed as a PA-31-350, while the NTSB report calls it a PA-31-250. It’s the same owner, though, so we’d suggest it’s probably the same plane. (And if it isn’t, this only shows the N-numbers can be reassigned, which doesn’t support the 9/11 conspiracy case, either).
Then we went for a third, and quickly found this:
On October 18, 2001, at 1543 Alaska daylight time, a Bell 206L helicopter, N400EH, impacted the waters of Cook Inlet about six-tenths of a mile west of the shoreline off the approach end of runway 06 at Ted Stevens Anchorage International Airport, Anchorage, Alaska. The pilot, who held a commercial pilot's certificate, expired as a result of the accident sequence.
And yet the FAA record as of 10th March 2006 says:
N400EH is Assigned
Manufacturer Name BELL
Here’s an accident from 1987:
Type: Douglas DC-9-14
Operator: Continental Airlines
Airplane damage: Written off
But as of July 7th 2006, it’s still showing with a “Valid” status, assigned to Continental Airlines:
N626TX is Assigned
Manufacturer Name DOUGLAS
Maybe these planes were “recycled”... Or perhaps it’s more likely that the FAA database isn't a reliable indicator of whether a plane was destroyed in an accident, after all.
But what about the "UA employee" who let slip he'd flown on the plane? Well, he isn't an employee of United Airlines, and he doesn't have much interest in conspiracy theories. In fact his page ends with the comment:
If you have a problem with the data on my page, well, you probably need to find more outside hobbies"
Still, he does note that he flew on a flight with the registration number N591UA on 4/10/2003. Could this be true? We checked at www.bts.gov, and it seems not. The plane taking that flight is on record as being N594UA.
A simple transcription error seems the most likely explanation here.