Hi,at the moment I am keeping busy with the Start-up sequence of the PMDG737 and the APU...and I have some questions concerning that item:1) Is it possible to start-up the B737 *without* the APU and if yes, how?Here is the situation: I powered the plane *only* with Ground-power while the APU was "off". Then I tried to start-up the engines but nothing happened.2) If it is not possible and APU-produced Bleed-air is strictly mandatory: Ho does a pilot start-up his/her plane when the APU is out-of-order and no a/c-mechanics are there (e.g. at a small airport in the middle of no-where)? :) :)I don`t think that the APU (bleed-air) is the only way to start-up an engine, doesn`t it? 3) How is the procedure at normal large airports like KJFK, EDDF etc.: I know from a captain that when there are more legs to fly and the pause between shutting-off and re- starting the engines for the next leg does not exceed 40(?) minutes the APU stays "on" the whole time the airplane is on-blocks, is that right so far?4)But what happens when the on-block time at the ground exceeds the 40 min border e.g. when the a/p stays for some hours at the ground?Is the plane powered the whole time with the Ground-power unit? And if the answer is "yes" at which point of the pre-flight sequence does the captain command to switch-off the Ground-power? Is that the time the passengers are boarding the a/c (15-25 min. before start-up) or later shortly before sharting-up the engines (1-3 min. prior start-up sequence)?5) "Who" descides to cut-off the power completely so that the a/p is "cold & dark" when the on-block time at an airport is very long: Is that the company, the pilot who flys the next leg or even the head-ramp-agent? Many thanks for your help...:)GreetingsSusan

>1) Is it possible to start-up the B737 *without* the APU and>if yes, how?Yes, if external air is connected. This is not simulated in PMDG's NG however.>2) If it is not possible and APU-produced Bleed-air is>strictly mandatory: Ho does a pilot start-up his/her plane>when the APU is out-of-order and no a/c-mechanics are there>(e.g. at a small airport in the middle of no-where)? :) :)Well, if no air is available and the APU is not functioning it can be started with a helper engine. If all things fail, I guess the co-pilot has to get out and push.>3) How is the procedure at normal large airports like KJFK,>EDDF etc.: I know from a captain that when there are more legs>to fly and the pause between shutting-off and re- starting the>engines for the next leg does not exceed 40(?) minutes the APU>stays "on" the whole time the airplane is on-blocks, is that>right so far?The APU has a cool-down period and needs a pause before restart, so that sounds reasonable.>4)But what happens when the on-block time at the ground>exceeds the 40 min border e.g. when the a/p stays for some>hours at the ground?>>Is the plane powered the whole time with the Ground-power>unit? And if the answer is "yes" at which point of the>pre-flight sequence does the captain command to switch-off the>Ground-power? Is that the time the passengers are boarding the>a/c (15-25 min. before start-up) or later shortly before>sharting-up the engines (1-3 min. prior start-up sequence)?No fixed rule. The external power can stay on just until the very moment before the captain push the throttle forward to taxi. He must, however, remember to turn on the generators first ;-)>5) "Who" descides to cut-off the power completely so that the>a/p is "cold & dark" when the on-block time at an airport is>very long: >>Is that the company, the pilot who flys the next leg or even>the head-ramp-agent?All of the above. If the captain and/or ground agent has no reservations, company procedures prevail./sebastian

Hi Sebastian,thanks for your fast answers! :)>...>No fixed rule. The external power can stay on just until the very >moment before the captain push the throttle forward to taxi.1) But usually the "cable" for the ground power isnt that long for the whole push-back distance, right?2) Generally to the Ground-Power: What kind of power (AC/DC) and how many Volts does such a Ground-Power unit have and how thin (diameter) are that kind of GRound-Power electrical cables which are plugged to the a/c?3) I think that large aircrafts consume much electricity. How does a modern today airport fix that ammount of electricity when e.g. more than 200 aircrafts are plugged to the Ground-Power unit *at the same* time?Is there a special (small) Power-station (nuclear, coal etc.) which produces that large ammount of electricity for so many birds?4) To the APU: Does the cockpit crew can *hear* when the APU is started or is it too quiet?5) I noticed that when the APU is started up and I am in exteriour view I also cannot hear the APU. Isn`t that unrealistic...when *outside* the a/c you can hear the APU starting, right? Thanks again! :)GreetingsSusan

Sue most airports plug the aircraft into ground power as soon as it taxi's in ,if not available they will stay on APU(about a 220PPH fuel burn on an NG). The ground power cable supplies 115VAC 400HZ.The power cable is around 3" in diameter(this may differ).You can hear the APU when started on a 737 with the windows or doors open, otherwise you cannot really hear it.

I recall seeing a post some time ago by the RW pilot who developed the Geezer Guide saying that his company policy was not to start the APU until five minutes prior to departure to conserve fuel and thereby reduce operating costs.

>1) But usually the "cable" for the ground power isnt that long>for the whole push-back distance, right?Hello again Susan.The GPU's are normally (if not always) mobile./sebastianhttp://forums.avsim.net/user_files/119839.jpg

Hi all,many thanks for your answers, I love this forum really...! :) :) :)@SebastianThanks for the pic...one last question:How is the electricity produced on the *mobile* GPU? Is there a generator which uses fuel, gas etc...the mobile GPU on the pic looks so very small...does it really can supply a large a/p like a B747-400 etc. for some hours without any connection to the airports general electricity...?:)Greetings Susan

>How is the electricity produced on the *mobile* GPU? Is there>a generator which uses fuel, gas etc...the mobile GPU on theThat depends if it is a trailer GPU (the one you see on the picture above) or an automotive GPU. While the latter is powered by a hydraulic pump via genset diesel prime mover, I believe that most trailer GPUs have a two voltage execution. I am not sure however, so you better search around for the specifics.>does it really can supply a large a/p like a B747-400 etc. for some >hours without any connection to the airports general electricity...?Well, if you take a look at the APU in this 737 below it isn't much bigger really. If it's a 747 freighter I wouldn't be surprised if it actually needs LESS power than a commercial 737 jet. Off course, all things being equal, a 747 needs a greater power supply than a 737 to support cabin and utilities. Even so the difference between the two GPU sizes is only noticable, not extreme./sebastianhttp://forums.avsim.net/user_files/119849.jpghttp://forums.avsim.net/user_files/119850.jpg

Hi Sebastian,thanks again! :)One more question concerning your answer:You wrote:"...That depends if it is a trailer GPU (the one you see on the picture above) or an automotive GPU. While the latter is powered by a hydraulic pump via genset diesel prime mover, I believe that most trailer GPUs have a *two voltage execution*..."I don`t understand what you mean with "...most trailer GPUs have a two voltage execution"...because my English is simply too bad at all...:) :)1) What is a *two voltage execution* (how does it function)?2) Does that mean that the the trailer GPUs don`t need fuel, gas at all? If yes how do they then generate their power from?Many thanks again!Have a nice satturday! :)GreetingsSusan

>Hi Sebastian,>>thanks again! :)>>One more question concerning your answer:>>You wrote:>>"...That depends if it is a trailer GPU (the one you see on>the picture above) or an automotive GPU. While the latter is>powered by a hydraulic pump via genset diesel prime mover, I>believe that most trailer GPUs have a *two voltage>execution*...">>I don`t understand what you mean with "...most trailer GPUs>have a two voltage execution"...because my English is simply>too bad at all...:) :)>>1) What is a *two voltage execution* (how does it function)?Basically it means that it is one huge battery, but I am not sure about the details. There is problably a lot more to it. I don't know if the trailers have a built-in generator, so if you want to know more about how they function maybe you should try to contact one of the manufacturers. I don't know what your native language is. There is a german manufacturer here http://www.generatingset.com/ and an english one here www.failsafepower.com.>Many thanks again!No problem. Good luck with your search :-)/sebastian

I notice that when the Hround crew plugs in the EXT PWR. they bang on the aircraft signaling that it is in to the pilots, can the pilots hear that really well?

Ground power units are diesel generator sets i.e. a diesel engine drives the electrical generator to provide 115V 400Hz. So, the power comes from the diesel fuel which is in a tank on the GPU. :-)Iain

YES! I have yelled from the ground at the ground crew while an engine was running and was heard clear as day in the cockpit. As for the GPU's in the US Navy we use some that can power upto 4 aircraft if you parked them close enough. Cords are 50' long 3" in diameter. Everytime we are pushed back we use the APU, never is the GPU plugged in when the aircraft is moving.