Hi Everyone,I was wondering if someone could explain to me the typical start-up procedure for engines on a modern jet aircraft.for example:Under what circumstances is EXTERNAL power used?And when is APU power used to start engines?Also can a modern jet aircraft start-up its engines, directly from its own internal battery power supply?Any info would be greatly appreciatedThanks in advanceJohn

Donny AKA ShalomarFly 2 ROCKS!!!The Cessna Citation can start its engines from its own battery. The use of external power in most *small* turboprops and *small* jets is to provide extra amps to the battery so that the engine will not be damaged by excessive heat caused by a fire with not much air going thru to provide cooling. Even with it available, there is still a delay as the plane's operating engine generator is brought on line to help the charge after the first engine starts, but a much shorter one with a power cart.On larger jets/turboprops, the usual use is to provide lighting, and allow preparations in the cockpit (FMC etc) to procede without fuel comsumption. The APU is started to provide environmental controls (A/C heat) and on many larger planes to provide high-pressure air that actually starts the engines. Even when no environmental control is needed, or external environmental is available, all of the large passenger jets and even Dash 8's I have seen as a caterer at KABE for a few years started their APU every time, even when plugged in. And there were a signifigant percentage of delays due to dead batteries, surprising when the plane was plugged in overnite. Many times, when I would peek at the cockpit, I would be surprised by how much was left on. A few times every MFD and all the avionics were displayed in all their glory, other times just a few lights here and there and everything in between. (We arrived before the crews on the morning flights)Best Regards, Donny

Hi Donny,Thanks for that wonderful explanation. A couple more things........though!!Q1.) Once the engines come on-line, when does the APU get switched off. ..........or is this done automatically.?Q2.) Which external lights are displayed to indicate to the ground crew that the engines are running.?Thanks for your invaluable help and knowledge.RegardsJohn

Hi John,Q1 - APU is switched off during taxi out. Some APU's are able to run in the air like the 747 but is used for emergency situations, ie. all engine flame outs in the air. This happend once on a KLM 747-400 after flying through valcano ash. He was able to start all 4 engines again with the APU in mid air.Q2 - The beacon light must be switched on prior engine start.Jarno

Donny AKA ShalomarFly 2 ROCKS!!!Concur with Q1 and Q2. I Should have mentioned that the state of the battery in a small turboprop is the deciding factor in use of external power. If they have more than 24 volts, most pilots up to a few years ago prefered to burn a little more fuel (price may have changed *some* minds) by starting the first engine from the batteries and waiting for the battery to be recharged by the generator, rather than waiting for, and paying for, a power cart. In this procedure, the generator must be shut off before engine two is started, or some expensive stuff can happen due to the massive load when the engine is just starting to spool up from zero/very low RPM.There is even more detail to be found in "Aurora Documentation" (about turboprops) and "Peregrine" (small jets) docs in the "Fly II aircraft category" of Avsim. If you have the complete manual, it's included, if you don't, it's in "miscellaneous Files" in Avsim. Yes, confusing...Best Regards, Donny

Thanks Guys,You really know your stuff!!Kind RegardsJohn