S-Mode
 

When does S-Mode start transmitting ?

Strange question - mode S transponders have been around for some while in the UK ( and other parts of the world) so just turn the switch to Mode S. If you mean when will all ATC units be able to receive Mode S that will depend on when they install the appropriate equipment and train the operators..

from an operator's perspective - when you set your XPDR to any position other then OFF and STBY then you activate mode S also.

Even when the engines are not running ?

ha3x ! why you want to set it ON without engines running ? testing ?

jxk,i dont see why this is such a strange question !

Could (you) or anyone who "knows" tell me if there are a selftesting sequence prior to engine start, and if it will transmitt simply by switching it on to XPR while the engines are not running ?

A Mode S transponder itself does not know whether the engines are running or not. If it's on, it's on.

When does S-Mode start transmitting ?
Ans: As soon as you switch it on!!
OK???
But do not forget that not every radar can receive a S-mode, and the Mode S transponder reply only when interrogated.
Ciao

my understanding is that if the mode selector is set to XPNDR mode S is off and ATC is on but ATC does not transmit because the aircraft is on the ground. If TA or TA/RA is selected mode S is enabled and will transmit on the ground. Aircraft in flight will not generate TAs for aircraft between estimated ground level and estimated ground level plus 180 feet. These are declared 'on ground' by TCAS. The estimated ground level is determined from the difference between radio altitude and barometric altitude.

Sorry, greatwhitehunter, but your understanding is well wide of the mark. Although a Mode S transponder is required for TCAS 2, and although some avionics vendors muddy the water by combining the transponder switch with the TCAS switch, your Mode S transponder operates quite independently of TCAS.

When you are on the ground a Mode S transponder answers to a subset of the possible interrogation types, and reports the fact that it is on the ground. When you are airborne, it answers to all the interrogation types, including legacy Mode A/C interrogations, and reports that it is airborne.

And, to confirm the tongue in cheek replies to rockamora, this behaviour is independent of whether the engines are running, but assumes that the avionics bus has power :8 Thus, if you are airborne, but the engines are not running, you can squawk 7700 with confidence. :E

CJ

perhaps you would like to re-read my post. I did not say that that the two systems were dependant. The comments merely refered to the different positions on the Mode selector switch and which system would be likely to transmit.
If the comments are that wrong I'll return the training notes to the company school:O

Any mode S transponder is transmitting any time it's in GND, ON, or ALT, regardless of whether the engine is running or not. If it's got power and is selected in a mode that transmits, it's transmitting.

Just turn it on and watch the reply light. It'll blink, even on the ground.

Generally, unless you're at an airport with ground surveilance radar, ATC filters unwanted returns off the scope (I forget offhand if it's by altitude or by airspeed or some combination of both).

I also don't know of any modern unit that doesn't do some sort of basic internal self-test when the power is applied.

AS I understood it on an airliner for example the Mode S starts up in Gnd mode although it may indicate it is being interogated (flashing light) it does not respond. However once airborne it starts to transmit. This switch to ALT can be triggered by ground squat switches, ground speed or Alt input

Not quite. It does respond while on the ground, but it indicates in the reply that it is on the ground.