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After September 11th, the FAA required all Part 135 air carrier flights to prefix their registration number with a "T" and use the "Tango November" prefix on the radio.
Every once in a great while (about once to twice a year) I still hear someone doing this, but surely there are more 135 flights than this.
Is it (or was it ever) required to use the "T" prefix for Part 135 flights?
Since you're asking if it's required then I believe the answer is no.
First, just to include the information from another answer, the AIM 4-2-4 only says "should", not "must" (my emphasis):
4. Air Taxi or other commercial operators not having FAA authorized call signs should prefix their normal identification with the phonetic word "Tango."
EXAMPLE- Tango Aztec Two Four Six Four Alpha.
Second, the FAA's Joint Order defining ATC regulations says in section 2-4-20 (my emphasis):
Air taxi and commercial operators not having FAA authorized call signs. State the prefix “TANGO” on initial contact, if used by the pilot, followed by the registration number. The prefix may be dropped in subsequent communications
That's the only mention of this prefix in the entire ATC regulations, therefore it's safe to say that controllers don't need the prefix and aren't required to do anything when they hear it.
On the AOPA forums there are a couple of unsupported comments that the prefix was used to give commercial flights priority in the past, and that it's still used for gathering statistics on flights today (although it seems like a very unreliable mechanism for that).
AIM 4-2-4, part A.
4. Air Taxi or other commercial operators not having FAA authorized call signs should prefix their normal identification with the phonetic word "Tango."
EXAMPLE- Tango Aztec Two Four Six Four Alpha.
It was pre-911 but not often used. It is just a way for operators who have no callsign to let ATC know that they are a charter operation. If appropriate, it may be dropped after initial contact, just like shortening your tail number.
The "T" (Tango) designation is required to be used by controllers, as per the Air Traffic Controller's Handbook:
Section 4. Radio and Interphone Communications
2−4−20. AIRCRAFT IDENTIFICATION
Use the full identification in reply to aircraft with similar sounding identifications. For other aircraft, the same identification may be used in reply that the pilot used in his/her initial callup except use the correct identification after communications have been established. Identify aircraft as follows:
...
- Air taxi and commercial operators not having FAA authorized call signs. State the prefix “TANGO” on initial contact, if used by the pilot, followed by the registration number. The prefix may be dropped in subsequent communications. EXAMPLE− “Tango Mooney Five Five Five Two Quebec.” “Tango November One Two Three Four.”
[emphasis mine]
AIM Section 2-4-2 is a little more difficult to parse because it uses the term should instead of must:
- Call signs should never be abbreviated on an initial contact...
3. Civil aircraft pilots should state the aircraft type, model or manufacturer's name, followed by the digits/letters of the registration number.
4. Air Taxi or other commercial operators not having FAA authorized call signs should prefix their normal identification with the phonetic word "Tango."
[emphasis mine]
Since we know item 3 is not optional even though it uses the term should, we must question whether the use of should in item 4 is optional, and it is my belief that not using tango on the initial call up would violate item 1.
