Why must some aircraft get permission to start their engines? Some larger airliners must get ground control permission before starting their engines.


Is this based on size? Does a Learjet need permission?
Is this just turbines? For example does a DC3 or DC6 need permission?
How would a pilot find out? Does the airport supplement or some other document show when it is required?
Are there exceptions such as jets parked on remote ramps, not at gates, non-commercial?


I know from flying small recips that I never needed permission anywhere in the US or Canada - but certain airliners did.

This airport in Australia says they enforces their right for the engine start. Part of the authorization comes from environmental noise control.  townsvilleairport (Australia).

Van Nuy - US, "All  aircraft  with  engines  capable  of  producing 
more  than 14,750  lbs.  of  thrust must  be  pushed  back  or  towed  onto  the  taxiway  prior  to  engine  start." (Airport ops) (Of course you need ground approval prior to entering a taxi way...)

When working at Anchorage International, Alaska with Continental 737-757, we had to contact ground control. We could start the engines at the gate while doing maintenance; there were no ground crew around. We started the onboard APU, turned on the red strobes, got permission from ground, then started the engines. I never read a document, this info was relayed to me by my supervisors.
Airliners usually push back from the gate and then start their engines when well clear. 

If the aircraft has an unserviceable APU, a ground based power supply must be used at the gate for the start. Permission(coordination) is required so that their are no hazards for ground crew and vehicles in the gate area.

After one engine is started at the gate, the aircraft is pushed back. The next start, still needs permission(coordination) because the operating engine usually has to increase it's thrust above idle in order to have enough bleed air pressure for the "Cross Bleed Start".

EDIT:  "Permission" may be not be mandatory at many airports, so "Coordination" with Apron Control, Ramp Control, or Ground Control may be a better description.