Cross feed

I can’t think of a realistic scenario where one side is contaminated and the other not.

It depends on the airfield security, and whether somebody dislikes you sufficiently but is not thorough enough to put the sand in both tanks

Realistic.

I can’t think of a realistic scenario where one side is contaminated and the other not.

Bad o-ring on fuel cap on one wing and lot of rain

…. in flight necessitating cross feed? Come on guys.

In DA42 when cross-fed, the temperature of fuel in non-used tank goes down because of no warmed fuel inflow from the engine to tank, so I guess that prolonged cross-feed is not recommendable if really not needed.

Wouldn’t that depend on whether you are using Diesel of Jet-A? If I remember correctly the takeoff fuel temperature for Diesel is +5°C whereas for Jet-A it is -22°C. The starting temperatures are lower (-5 and -30°C respectively?)

On one engine you would drift down to 8000’ where, unless you are flying around the South Pole, the temperatures would be above -22°C. most of the year.

…. in flight necessitating cross feed? Come on guys.

Ever done any training or checkrides in a twin? I have not yet flown anything that does not require some amount of crossfeeding after 15-30 minutes of asymmetric flight.

Lots. Every tried varying the engines you fail during simulated asymmetric flight? :)

Anyway, my comments were in response to the OPs post:

I am interested in circumstances of engine failure or rough running. Its possible it could be fuel contamination which could potentially transfer to the good engine.

Lots. Every tried varying the engines you fail during simulated asymmetric flight? :)

During training you could do that, during checkrides it’s up to the examiner. The realistic scenario is one where the failed engine stays failed for the rest of the flight and you actually have to remember to use crossfeed. They put much emphasis upon that during simulator training.

Anyway, my comments were in response to the OPs post:
bq. I am interested in circumstances of engine failure or rough running. Its possible it could be fuel contamination which could potentially transfer to the good engine.

This is why many Piper twins, when operated by the book, shall be taxied with the fuel selectors set to X-Feed so that these problems will occur on the ground and not in the air.

I’ll remember that next time I do an MEP examination.

The test of crossfeed on the ground is there to see that it works, not to transfer any potential contamination.

As I said in my first post, it very much depends upon the instructions given in the AFM. Two excerpts from two different AFM, the second from a biggish Piper:

Check fuel imbalance in the main tanks; use CROSSFEED function to keep the LH and RH main tank imbalance within the permissible limit of 1 US gal
(3.8 liter)

Crossfeed should only be employed when it is necessary to extend range during single engine operation