Transfering Electrical System from APU to Generator

[Guest]

Hi! I've been going through the whole checklist until it's time to switch from the APU power to the engine generator power. When the engines are on and everything is fine running, the generator buttons for the engines have the OFF sign on then, even if I click on them to activate them.How can I solve this problem?Teo :-(

[Guest]

Oh by the way it's the 757.Teo

[Jet Tech 31]

Teo,Could you possibly post a sceenshot to help clarify?As long as the engines are up and running and the Bus Tie's are closed, Generators should be on line. RegardsPaul:-cool

[Guest andrewluck]

PaulWhilst on the subject of the electrical system I'm rather puzzled by the APU GEN switch and I guess you're the guy to ask :-)When I first turn the battery on, the APU GEN is shown as on, even though the APU isn't actually running, i.e. no OFF flag. Also, once the APU is running, turning off the GEN switch puts the OFF flag into view. My understanding of the switch logic was that it should just turn off the ON indication as the underlying system (the APU) is still available.Any insights? My avionics experience is now somewhat out of date, based as it was, around the F4. :-)Andrew Luck18 miles SW EGSH

[Jet Tech 31]

Andrew,Here's the basic scoop on the how these Korry Switches work.Anytime the switch is selected on, the "ON" indication, or flowbar in some cases, will show. This is simply a flip legend inside the switch and doesn't reflect whether or not the system is operating. The "OFF", "PRESS" or other fault indications reflect the individual system status in most cases. The Hyd pumps and Eng Generators are good examples of this showing real time status of the system.The APU Generator and Center Tank Fuel Pump switches are a little different than most switches on the overhead. The APU Generator Switch for example will only show "OFF" when the APU is operating and the APU generator is either not selected on or has a fault condition when it is selected "ON". The Center Tank Fuel Pumps are another oddball system in that they do not indicate low pressure when selected off when center tank Fuel QTY is below 500 lbs. The theory behind this is a dark cockpit when everything is working properly. Hope this helpsRegardsPaul:-cool

[Guest andrewluck]

PaulThanks for the info. From what you've said I deduce that there is a problem with the APU GEN switch as it will always show the OFF flag anytime the APU generator is turned off, even when the APU is not running. In flight I'm always showing one amber OFF flag on the overhead and it's the APU GEN.Andrew Luck18 miles SW EGSH

[Jet Tech 31]

Andrew,Actually, the normal procedure is to leave the APU Generator switch and APU Bleed valve switches turned on even with the APU shutdown. In this configuation, the lights will be extinguished giving you a dark Flight Deck and that way when you fire up the APU on taxi-in, the APU is ready to provide Electrical and Pnuematics automatically on engine shutdown. But you are correct in that the 'OFF' light should be out with the switch off and the APU shutdown. RegardsPaul :-cool

[Guest andrewluck]

PaulAPU's have always been problematic for Fly!. I'm guessing this is a problem with the underlying Fly! code.I'm interested that normal procedure is to leave the APU GEN and Bleed switches ON. I was under the impression that it was a requirement to run the APU without load for a few minutes before shutting it down. Unless you put the switches back on after APU shutdown.Thanks for your patience in answering all this. Andrew Luck18 miles SW EGSH

[Jet Tech 31]

Andrew,Thats the great thing about electric AirplanesThe 757 APU automatically closes the Bleed valve after the second engine is on line and supplying Bleed Air (however you won't see any change in the APU bleed indication). If the pilot reaches up adn turns iff the APU before 90 seconds of cool down without Pnuematic load, the APU automatically delays sutdown until the 90 second time period has been reached, then justs turns itself off.God Technology is great.Paul:-cool

[Guest andrewluck]

PaulNow all we need is a button that when pressed lights up with "Do not press this button again" :-)Thanks for the explanation. It's now crystal clear.Andrew Luck18 miles SW EGSH