I’ve rented aircraft with Mode C capable transponder, but no encoder. It was labeled as such. I don’t know what it would transmit if turned to “Alt”. I never did so. Mode A only.
Yes; that seems to be the common situation. Loads of Mode C but no encoder. I guess the transponder treats the encoder value as invalid if it is not connected, and returns no altitude even if ALT is selected.
Presumably in Germany such a bodge would not get the annual IFR signoff?
How much is an encoder?
Trig has inbuilt encoder, and just needs a tube to the static – we put a T piece into the altimeter tube.
What I don’t undestand is the rationale for avoiding the cost of the encoder alone, given that it removes the altitude return, and thus increases the chances of the customer getting killed. It’s a curious “culture”.
boscomantico wrote:
Hein? Then how can it squawk Mode C?
Because Mode C is the interrogation mode, not the output data. A transponder without an altitude encoder (or a transponder switched to ON not ALT) will respond to a Mode C all-call interrogation used by TCAS, but will return an unknown altitude.
As was discussed earlier in the thread, classic transponders are all “Mode A/C” transponders, regardless of whether they have an altitude encoder attached.
I am well aware of that. What I was getting that is that an aircraft is not “Mode C” if it doesn’t have an encoder. A Mode C capable transponder is not enough.
I presume it is that can that with no encoder connected, it in effect is transmitting 0000 which doesn’t resolve to any altitude.
Any avionics shop person who has a Mode C transponder tester can answer this one. Maybe there are pullups which drive the value to FL600 or some such?
TAS equipment definitely does not filter out a zero pressure altitude; I see planes sitting at the EGKA (elevation 12ft) holding point when I am on final and get proximity warnings off them, and if QNH is 1013 they will be radiating a zero pressure altitude. And regardless of QNH they will be radiating something close to mine when I am nearly on the ground.
But on top of that I reckon ATC radars might be filtering out obviously wrong stuff.
I didn’t say zero pressure altitude, I said 0000 as in binary 000000000000
0 pressure altitude is:-
Gillham:- 000011010000
Octal:- 0620
See here:-
boscomantico wrote:
What I was getting that is that an aircraft is not “Mode C” if it doesn’t have an encoder. A Mode C capable transponder is not enough.
So lemme get this straight:
A “Mode C aircraft” is one that replies to a Mode C interrogation with altitude information.
A “Mode A aircraft” is one that replies to a Mode C interrogation without altitude information.
If you find that terminology helpful, feel free. Just don’t expect me to use it.
