Transponder failure

[nlrobert]

Dear fellow DA40 aviators,

A few days ago I made a flight at the end of the morning with a DA40 TDI on a particular sunny day. An hour after a first uneventful flight I started my second flight of the day (at 11.30 am). During run up the OAT was almost 30 degrees Celsius (85 F). Engine was running fine and take off at MTOW from a grass strip didn't cause any troubles.

5 minutes after take off (still in climb to cruise altitude) we notified a smell of something burning/melting. Checking the instruments everything seemed to be fine (all green), and after a short time the smell was gone. Few minutes later the transponder code went from 7000 to FAIL and resetting didn't solve the issue (restarting in STBY works, but when switching to ALT mode it shows the FAIL code very soon).

Transponder is now in maintenance but I'm pretty that some internal wiring melted/a short circuit occurred.

All in all the plane had been outside in full sun for 2.5 hours since leaving the hangar in the morning.
How likely is it that this failure was caused by the heat of the sun?

Are instruments and engine suffering more from heat in this composite plane than metal ones like Cessna and Piper models? Several other planes where flying that day with me and none of them reported malfunctioning instruments.

I'm just wondering if I should blame myself for flying on this hot day (with maximum temperatures around 35C/95F in the afternoon, which is quite exceptional for my country, but still within the official operating limitations of 54C/129F) or that this failure was just inevitable waiting to happen and could've occurred on a cooler day as well?

[ihfanjv]

DA40s are flown in the American West and South in temperatures well exceeding 100F all the time with no ill effects. Your 95F day is a standard day in many parts of the USA for a large part of the year. But, I do know of a DA40 that blew some rack mounted LRUs when a fabric cabin cover mistakenly covered up the vents for the LRU rack on a very hot day (at least, that's what the Diamond Service Center said). I think the explanation is plausible.

[rwtucker]

Agreed, it is not on you Robert.

Your transponder is basically a receiver at 1030 Megahertz and a high power pulse transmitter operating at 1090 Megahertz (I think this is right). The transmitter's final output is not live in the STBY mode. The current drawn by the output transistor(s) is probably pretty high (the transmitters are usually around 200 watts, 20-40 times that of your COMM transmitters, but only in very short pulses). The current could go much higher if your transponder antenna shorted, or became de-tuned, or if a small component in the final transmitting circuit malfunctioned. That may be what you smelled (burned resistor, fried diode, etc.). I'm surprised that you didn't trip a breaker

Be sure to let us know what the problem turned out to be.

[BlackMammoth]

We operate a DA40-180 in temperatures passing 44C regularly with no ill effects (except to the people on board ).

We have loads of avionics, Aspen EFD, TAS, XM Wx, ADS-B in & out... the only failure we have had of an avionics component was the remote gyro for the King HSI, and that happened in the winter.

I wouldn't worry about the heat.

[nlrobert]

Thanks for your replies guys! When I know more about the cause of the failure I will post an update.