Originally Posted by wingisland
Also I was recently talking to a Twotter pilot who said they leave the right engine running on short stops to help...
I think the key words in the original question are "short stops". If short stops imply short sectors - and 5 to 10 minute sectors are not uncommon in Twin Otter operations - then the concern of the crew might be that the ship's battery has not been fully recharged since the previous start cycle. In this case, the crew would use the generator of the operating engine to assist starting. The AFM for the DHC-6 describes this as a 'generator assisted start' and states that this is accomplished by initially spooling up the engine to be started using the battery alone, then bringing the generator of the operating engine online to further increase the Ng of the engine being started. It is not desirable to have the generator of the operating engine online right from the get-go because the relay that connects the operating generator to its bus is not designed to cope with very high amperage going through it in the wrong direction (wrong direction being from the generator to the bus).
If that is not the case, then the crew are simply using old, out of date operating procedures that were handed down from someone's grandfather. A contemporary battery (lead acid or ni-cad) should be able to accelerate a small PT6 such as the -20, -27 or -34 used on Twin Otters up to between 16 and 18% Ng. This is more than sufficient Ng to provide a start that is comfortably within limitations, even at hot and high aerodromes.
Another possibility - this being a very common error - is that the crew are mistaking the colour markings on the Twin Otter T5 gauge for the temperature limitations applicable for starting conditions. For a normal Series 300 Twin Otter equipped with -27 engines, the top of the green arc is at 695°, this corresponds with the maximum climb and cruise temperature. The top of the yellow arc (equal to the bottom of the red arc) is at 725°, this corresponds with the maximum takeoff and maximum continuous power limitation.
Starting temperature limitations are not marked on the face of the T5 gauge, save for one tick at 1090°. The actual starting temperature limitations for a -27 engine installed in a Series 300 Twin Otter are as follows:
Up to 925° for the entire duration of the start (equal to what would be green)
Up to 980°, as long as temperature declines below 925° within 10 seconds (equal to what would be yellow)
Up to 1090° as long as temperature declines below 980° within 2 seconds (equal to what would be red).
Because these engines almost always have peak starting temperatures below 725 (the painted red arc corresponding with the maximum takeoff and maximum continuous power limitation), it is not uncommon for the crews to mistakenly assume that the coloured arcs also apply to starting conditions. They do not.