of course it would depend on the circumstance..
For me, I would ensure the following was confirmed
- Hand signal given to indicate parking brake set.....
-Approach aircraft and Headset operator confirm engines are set to ground idle...
-All of the above acceptable, insert bypass pin and connect pushback tractor
YouTube - speedbird7201's Channel
The above link is the company I work for, and as you can see the tractor is pushing with the engines running.
If say it encountered a technical fault for whatever reason, the connection while engine run shouldn't be a problem "but" the engines must be at ground idle.
If you where to talk about from Runway to stand towing, maybe think about inbuilt GPU's on-board the tractors, so once connected a GPU lead can power the aircraft resulting in the engines being shut down....
TBL's are not recommended from Runway to stand tow, as the scoop* or cradle can rip the landing lights/taxi-way lights resulting in expensive problems....
Exaviator, those stands where the aircraft are parked backwards are to ensure the tail does not hang over the RZ and taxi-way, and are usually empty in terms of load on entering the stands, unlike the normal cargo stands like 609 where the CX freighter parks, where conventional parking happens.
LCY has push onto option but usually the APU is used to compensate for Loss of power or 1 prop is left on feathered (last time i flew into City)
BAA encourage TBL's not to be used for the reasons as a above when collection stricken aircraft from the runway, however I herd once, that in Arab Emirates there was talk of an electric belt as such similar to that of an aircraft carrier which latched onto the nose gear on taxi off from the runway..... This was then automated and parked the aircraft without any power input from the aircraft
If this actually happened like the idea that Richard Branson wanted to save money and the e-zone by towing to the hold points, and starting up by the runway threshold prior to takeoff, then we may be onto a winner...