Iz,Thanks for the info! I hope you have not "converted" to PMDG forum exclusively now that you are transitioning to the NG ;) I was wondering if you could tell us about how flying the NG differs from the 757 ? Did you request the transition yourself ? Why leave the beautiful 757 ? Tero

Thanks, Iz. I recall from a previous message that someone, perhaps you, had said that APU and External Power behave differently on engine start, so all I really had to do was confirm one scenario.I might have to ask you a few questions on the NG electrical system later ;-) Cheers.Ian.

Hi Iz!I have a follow-up question, if you don't mind:Let's assume the aircraft is on the ground, with external power connected. The left bus tie breaker is in auto, the right one is off. Thus the right AC bus is not being powered.Now we start the right engine, and the right IDG will power the right bus. Would it still kick out the external power and leave the left bus unpowered or would in this case external power remain connected for the left bus and the IDG in the right bus?Thanks,Mark

Mark, that question is easy to answer: You won't be able to start the right engine in that scenario.Tero, no, my company phased out the 757, there are no more in our fleet. So it wasn't a voluntary switch for me. The 757 is a nicer aircraft on almost all fronts except the newer avionics in the 737NG.It's bigger, faster, more powerful, more flexible (range, payload), roomier, more comfortable, easier to fly, has better system handling, more automation, EICAS, more redundancy, better APFD and its systems are more logical.I flew a couple of days with another guy who flew the 757 as well and we sort of reminisced about that beauty of an aircraft.The 752 was a rocket, just amazing performance and it'll do anything you throw at it. The 753 (owned by Condor) was a very comfortable airplane, brand new, also very stable to fly, still outperforming all A319/320/321s.

"Mark, that question is easy to answer: You won't be able to start the right engine in that scenario."Is there a simple explanation for this, Iz?I had a look at what was on the Right Bus. This included the APU Bleed Valve (a ground pnumatic supply would bypass this however).The Right Bleed Isolation Valve is also powered by the Right Bus, but, assuming the valve was already open, this shouldn't present a problem... as far as I can see.The Start Valve is powered by the Battery Bus, so, again, this should not be a problem.All we need now is fuel and ignition. I'd be very surprised if these would be impeded by a lack of a Right Bus (other than tank fuel pumps, but this will not stop suction feed).Have I overlooked something? :-)Thanks.Cheers.Ian.

Ouch, I thought I could get away with it that easily..But anyway, scouring through my manuals, it's hard to find any specifics since they're so low on information these days. I have no diagrams that specify which systems depend on which busses.However, there is no Battery Start procedure available for the 757 (as there is for the 737NG). This means that it is not possible to start the engines even on standby AC from the battery. So regular AC power should be available. And if you isolate one side of the AC, I think it's highly unlikely that the one engine draws main AC power from the oppposite AC side.The respective fuel pumps are AC from their respective side and I'm pretty sure that suction feed does NOT work. It states in the manual that reduced fuel flow (especially at low thrust settings and high altitudes) can cause engine flameout because suction is too low. Now imagine an engine that's not running at all, perhaps N3 is spinning due to the APU bleed, but there's not nearly enough suction to get any fuel in the engines without N3 being at least at 70% already, and there's no way to get that up there, without a lit-off engine.Hope that solves it?

"Hope that solves it?"Nice try, Iz, but... :-)"The respective fuel pumps are AC from their respective side and I'm pretty sure that suction feed does NOT work."I've seen fellow engineers make the mistake of not switching on the fuel pumps before starting a 767 engine and the engine started without any problems, so I know that suction feed works pretty good on the ground :-) There are suction feed bypass valves in 767 tanks for this very purpose. Also, I've been told that it's an FAA requirement that engine pumps are powerful enough to supply fuel for all flight regimes (although there is anecdotal evidence that they don't always live up to expectations... and, as you say, your manuals also warn you about this). By the way, my manuals tell me that the 767 tank fuel pumps are powered this way:Left Fwd = Ground Service BusLeft Aft = Left BusRight Fwd = Left BusRight Aft = Right BusSo, at least some pumps should be working."However, there is no Battery Start procedure available for the 757 (as there is for the 737NG)."I'm thinking that surely an aircraft must be able to make an inflight start (after a dual engine flameout) with windmilling engines and Standby Power alone(?)...otherwise you'd be making a very good impression of a "Gimli Glider" if it happened. So, in our Ground Start scenario, this really only leaves the problem of supplying enough bleed to windmill the engines. Perhaps a Battery Start procedure on a 757 is forbidden on the ground simply because of MEL limitations? (and because the Isolation Valves have to be already open... Luck has to be on our side here)I'm wondering if Wade or Laurent can shed any light on this (as PIC's engines don't start with the Right Bus off ... despite External air and the Isolation valves already configured to allow air to the start valve).Certainly an interesting mental exercise.Cheers.Ian.P.S. I was just looking at the power supplies for the EEC's/PMC's on the 767. I didn't realise that the "Control Alternators" needed the N2's spinning at 50%, whereas on aircraft like the 747-400, the Dedicated Alternators provide power at N2's as low as 8%). In the absense of this power, the Ground Handling Bus can supply the EEC's with power. The Ground Handling Bus should be available if APU or Ext power is available. Even so, if you can start (windmilling) engines in flight where the N2's are below 50%, I'd say that 767 EEC's don't need to be powered for engine start(???).

Hi Iz!I also have another question! (sorry) :-)Let me restate my question from before.Aircraft is once again on the ground, both engines running and both generators selected off. Aircraft is supplied by external power (electricity-wise)Now I press the right Bus Tie Breaker which switches into ISLN and removes power from the right AC Bus. Now I switch on the right gen. What happens?Regards,Mark

AARGHHH Ian, no fair, you have more detailed manuals and resources! Hehe.. :) How about the EECs? How are they powered? Fuel metering etc?Interesting to hear about starting the engines without fuel pumps. The difference with a dual engine flameout without APU in flight is that you totally rely on windmilling engines (unless APU is available, then you can try an APU start). With windmilling engines, you only need to introduce the fuel and add some sparks, there's no need to push that giant engine up to speed from zero.Mark, I can't honestly say what would happen. My guess is that in that case, External power will continue to power the left AC bus. I think it only disconnects when a dual power source situation would exist for the same bus.

"How about the EECs? How are they powered? Fuel metering etc?"I think I mentioned this in my P.S., Iz?Normally they are powered by the spinning N2 rotor (which drives the accessory gearbox which drives a small electrical generator which powers the EEC on the respective engine. When the engines are not spinning at the required speed (50% N2), the alternate source is the Ground Handling Bus. The GHB should be powered whenever APU or Ext Power is available (These power sources don't have to be actually on the Main Busses).I haven't looked into the ops of the HMU, but I don't think this has to be powered electrically (it's basically a hydromechanical device controlled by simple electric trimming signals from the EEC).However, I don't think EEC power is essential during startup, otherwise you couldn't make an inflight (windmilling) start after a dual engine flameout (using Standby Power alone). Windmilling engines can't get the N1's up to 50%. Hope this helps.Cheers.Ian.