On Dec 9, 11:12*am, Darryl Ramm wrote:
Some Mode S transponders are designed to revert to behaving as a pure
old style Mode A/C transponder if they lose the ICAO address.

The MOPS is actually more specific than that. It specifies that if
the transponder 24 bit discrete address is detected as all ones, or
all zeros, at start up then this error condition shall be indicated
to the pilot and the transponder shall either transition to Standby
State, Power OFF condition, or operate as a pure mode A/C transponder.
(ref RTCA DO 181D para 2.2.13.1.1. a) (1).

So reverting to Mode A/C is an allowed response to a null address
being set.

As you said, who knows what the TT21 was designed to do.

Before you jump on it again though I do see a great distinction
between entering a null address and entering an incorrect but valid
ICAO identifier. In either case it's all hypothetical since I'm NOT
paranoid.

Damn, where are my meds?

Andy