On the DH8, there are power levers, governing mostly engine torque (but at their extreme positions also assuming control over prop RPM and reverse in various ways), and condition levers, governing the propeller RPM and the fuel on/off selection for startup and shutdown.
This is a rather well working setup. The operating range of the power levers is from 1000hp rated reverse power to Ground Idle (actually a mild reverse), flight idle (the lowest setting allowed in flight, but way too much power for taxiing), the rating detent (putting the power lever into it will result in the engines being demanded power in accordance to a selected maximum rating), and beyond that the overtravel region, in which the engines will deliver maximum transient power and the propellers will be set to max RPM irrespective of condition lever position. The condition levers have 5 positions: Fuel off, Start/feather (the gas generator runs but the propeller is in feather and not producing thrust), and then 850, 900 and 1020rpm. They are only touched for startup, for setting climb and cruise power, once pushed forward during the final approach (but the RPM may keep locked on 850 if desired, it will only automatically increase when the power lever angle passes a certain value) and then for shutdown of the engine.
Sounds complicated, maybe, but itīs usable rather intuitively.
In normal operations, you move both power levers together and they might indeed be connected. But let us look at failure cases. If You have one lever per engine, you can move that lever and see how the individual engine reacts to this. During an inflight shutdown (often performed by memory on my type), itīs not only required to have both pilots verify the correct lever to be moved, itīs also a most wise idea to move the selected lever at a moderate speed to see if the systems reaction is as desired and You are not attacking the good engine.
By contrast, if you only have a button to push in this case, reversing the selection if you find it to be wrong is way less easy.
Or assume situations in which you cannot demand rated power from the engine but like to keep it running if only to provide hydraulics, electricity and/or bleed air. Some overtemperature, be it oil or ITT, or something else. With the normal, proven setup, You just retard the affected engines power lever to the required setting and use the other engine normally. How are You going to do this with only one power lever for both engines without resorting to highly complicated switching?
Taxiing. This is often done with one engine shut down or running in start/feather to save a cent or two for the company. With only one power lever, how will this be accomplished? Also, You remove a LARGE cue for the crews: should they forget to startup or unfeather the other engine for departure, with the current setup this will be easily spotted from the wrong lever settings and also immediately felt as soon as one grabs the throttles: intending to push them to the takeoff setting, but finding only one in ones hand is a slight clue of something being not as it should be. With Your proposed change implemented, expect corresponding incidents.
Mark me highly unconvinced. I strongly prefer the current setup of having separate controls for each engine to Your suggestion.
Last edited by Tu.114; 16th Jan 2017 at 14:16.