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Guest icebrain

APU question

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In order to start a real jet, like the lear, do you really have to have the APU running? I understand about bleed air off the APU but there is no bleed air valves in the default lear.

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Maybe the Lear has small enough engines that it can self-start without an APU?RhettAMD 3700+ (@2530 mhz), eVGA 7800GT 256 (Guru3D 93.71), ASUS A8N-E, PC Power 510 SLI, 2 GB Corsair XMS 3-3-3-8, WD 250 gig 7200 rpm SATA2, CoolerMaster Praetorian

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>In order to start a real jet, like the lear, do you really>have to have the APU running? I understand about bleed air>off the APU but there is no bleed air valves in the default>lear.>> I'm not totally sure, as I don't fly the things in real life.But...A friend of mine used to work on them over at Hobby.In the hangar, they are usually run off an external APU if I remember right. Rolls around on a cart, or whatever..I seem to remember him beginning to start the engines one time when I was in the cockpit, and I'm sure the external APU was on, aswe had lights, radios, etc going. He didn't fully start them, butlet me hear the beginning of the start process. Seems to me, he needed the external APU to do that. But like I say, it's been many years, and I'm kinda foggy. So it wouldn't surprise me if youneeded the internal APU to start if out away from the airport.Maybe someone who really flies em can tell you better.BTW, I can ask my A & P friend later.. He'll know fer sure.Or at least as far as the older models.. I think the newestlear they had in their fleet was a 35a.. Some were 25's , etc..MK

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I did a lot of training on the Lear simulator and the engine starters were electric. No APU fitted to the simulator. I'm not certain but I think it was a Lear 35.edit: Battery condition check was critical as attempting to start on depleted batteries would resultin a "hung" start. External ground power (if available) was the preferred electrical power source for starting.Roger

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It depends on the aircraft/engine combo,Three things are needed to start engines:Air- most jet engines use an air driven starter. Some smaller ones use accumulated hydraulic pressure or an electric starter.Fuel- it burns. 'nuff said.Electricity- electronic engine controls and for the igniters.Some aircraft-especially those with smaller jet engines can do electric starts. Most airliners need a bleed source to spool up the N1 shaft to support combustion.I was on a 733 a few years back that had a bad APU (CLT-AVL right before USA dropped mainline jets there. It's a 30 minute flight) I still don't know how they started the engines for the return trip. Engine starts really tax the batteries when running for 30 seconds at high amperage draw.We used a start cart and external power to start one engine, taxied out to a quiet place on the ramp and did a cross-bleed start of the other engine. The engine supplying the bleed air needed to be run at quite a high power level to supply the proper pressure to the ducts. APU's exist to supply bleed and electricity.Copied from a 737 AOM:ENGINE START - AIR START (BOTTLE PRESSURE)When the B737 APU is inoperative, a low pressure ground air source may be provided as an alternate, but because of the high bypass jet engines, both compressor-type and bottle-type ground air units must meet certain specifications and capabilities which are in some cases greater than other narrow-body two engine aircraft. Units meeting the criteria listed below will normally meet the B737 starting requirement.Compressor-Type UnitMust be capable of producing 125 - 135 PPM (pounds-per-minute) and 30 to 40 output PSIG (pounds-per-square-inch).Bottle Type UnitMust be capable of 450 - 500 PSIG operating pressure. At these pressures, the amount of air should be between 4,000 and 5,000 standard cubic feet.* * * *ENGINE START - CROSSBLEED STARTIf difficulties are experienced with an external air source with one engine running, a crossbleed start can be made. Prior to using this procedure, notify tower and ensure that the area to the rear is clear. Increase thrust on the operating engine until there is a minimum of 30 PSI duct pressure and use this air source to start the remaining engine.

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Lear and other bizjets use a 40 AMPHour NICAD battery for engine starting, and to provide backup power for the electrical systems.Manual "Load Shedding" is required in case of emergency, otherwise the expected battery life (at full load) is around 10 minutes. With proper Load Shedding, expected battery life is extended to around 30 minutes.Because of the limited battery amperage available, engine starting is limited to twice per flight hour on average. After using the battery to start the starboard engine, the generator for that engine brought on line and is used to start the port engine.Most bizjets carry a small inverter that can be plugged into an available ground power source (110/220 volts AC), which supplies 28.5 VDC via the external ground power connector. These are used wherever no flightline GPUs are not available.

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When I used to deal with SF34's, they had serious issues with battery starts.At Bos, we almost always used a GPU. I saw crews attempt to start on batteries alone, but it rarely worked no matter the OAT. They were always too hot. You could tell the sound of a start that was not going to work. Even though the engines were free turbine engines, the starters used a lot of juice.I wonder what they did at outstations.The coolest system I've seen is on a Wilga- pneumatic start. The engine pressurized an accumulator and it spun the engine (really fast!) for starts. All you needed was fuel and spark. Then the bottle would automatically repress within 10 minutes at idle.

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>I wonder what they did at outstations.Most bizjet pilots will leave the starboard engine to idle if they're going to be at a remote field with no GPU (or electric outlet!) available for less than an hour...A 40 amp/hour NICAD battery will only allow two attempts to start an engine, so battery starts are very rarely attempted.

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Gulfstreams (at least the "real" ones made in Savannah) use an air start system (powered by APU)... I'm pretty sure there's some kind of external air hookup too, I just don't know where it is.

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