Originally Posted by
Capt Kremin
... the transponder was turned off from the flight-deck. That is known from the transmissions received from the transponder as the knob was turned through the various options available, to the “OFF” position.
...
Capt K, I would be somewhat cautious regarding turning what is essentially an untested, and frankly somewhat speculative, interpretation of the penultimate and final ADS-B transmissions received from MH370 into an unconditionally declarative statement of fact concerning whether the transponder was manually turned off.
We know from the data that the last two ADS-B transmissions received from MH370 (at 17:20:34.15 and 17:20:34.55 UTC) didn't have altitude data. There was subsequently some speculation that the absence of that altitude data may have been because the transponder was, at those times, in a mode that would suppress the transmission of said data, specifically ALT OFF.
It was noted that the ALT OFF position on the Transponder Mode Selector sits between STBY and the normal operating mode position, TA/RA. There was then further speculation that the process of manually moving the Transponder Mode Selector knob from TA/RA to STBY would cause the transponder to go into ALT OFF mode as the selector passed through that switch position.
Stitching all of that together, some observers contend that the ADS-B data shows that transponder must have been switched off manually.
A couple of things are worth noting. The physical process of turning the Transponder Mode Selector knob through 135° from TA/RA to STBY takes about 0.35 seconds, a bit less if the act is performed with a sense of purpose. The dwell time in the ALT OFF position is consequently fleeting, <0.1 seconds. To the best of my knowledge, nobody has actually checked whether the switch reader would even register that as a mode change.
Similarly, again to the best of my knowledge, there hasn't been much of an effort put into understanding the processing logic that sits under that mode selector switch. Once the switch has read a mode change, the processing logic will be such that different routines are commanded. In there somewhere will be instructions for what data needs to be be packaged and handed off for transmission. It would be useful to understand how that works.
And an understanding of basic processing times and latency would be needed to resolve the problem of timing. ADS-B transmits every 0.5 seconds, give or take. The physical act of turning the transponder off takes less than that. It would be an extraordinary piece of timing for the fleeting transition to ALT OFF to have been read and the data package for next ADS-B transmissions to have been amended accordingly before the final mode position, STBY, is read and the transponder responds by ceasing to transmit.
The missing altitude data may be an indication that the transponder was manually turned off but that is currently just speculation. Speculative interpretations being commonplace on this particular topic, I am sure that nobody is surprised that some other observers point to the exact same ADS-B data as being evidence of some sort of cascading system failure whereby altitude data was lost just prior to the transponder failing.