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We have had more... than... one question about jet aircraft taxiing about the airport without their jet engines running. Those address several issues such as cost vs savings and regulatory approval.

My question assumes that an in-wheel electric motor, tug or even pull-chain based solution has been developed, certified, deemed cost-effective and installed.

Let's say that I am now sitting on the taxi-way and am 5th in line for take off. It's time to fire up those jets so they can warm up and we can get the beast into the air. According to this question about how turbine engines are started, it appears that I'm going to need some sort of ground-based support to get the N2 stage spinning. That question indicates that a couple of early, WWII era, jet engines had a built in motor that could get the turbines spinning fast enough for a start, but it seems to me that any modern commercial air-carrier grade engine will need some sort of external help to fire up. I suppose if there was a tug that had pulled me to this point, it could have the support infrastructure built in to get the turbines spinning, but any sort of self- (or ground-) propelled system would seem to lack that.

Now that we're nowhere near the terminal, how the heck are we going to start the engines?

I realize that this question is somewhat speculative in nature, but I believe it could be answered based on some existing or in-development ability (that I'm not aware of) to start an engine without ground support. It could also be my misunderstanding that ground support is not required to start a modern commercial aircraft engine (GE90, RR Trent series, LEAP etc.)

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    $\begingroup$ I think this premise: "but it seems to me that any modern commercial air-carrier grade engine will need some sort of external help to fire up" is false. While its not unheard of for an aircraft to have an inop APU, its unlikely that it would leave the gate without a started engine for cross-bleed air if thats the case. I've flown many times where the engines weren't started until after push-back without any ground assist... $\endgroup$ – Ron Beyer Jan 20 '16 at 21:18
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    $\begingroup$ There are numerous ways to start turbo-engines. It is possible to start them without ground support. $\endgroup$ – user7241 Dec 23 '17 at 18:48
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    $\begingroup$ See Auxiliary Power Unit $\endgroup$ – Steve Kuo Dec 24 '17 at 4:50
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Modern jet transport category aircraft have APUs and they will be running if no engines are. Batteries on jets do not work very long with all electrical buses energized and the first thing you do after turning the batteries on is seeing that they are working correctly and then starting the APU (assuming no GPU is hooked up, if there is a GPU then starting the APU will be deferred until just before the GPU is disconnected).

Thus, any aircraft on a theoretical no-engine taxi to the runway will have a running APU providing electricity to run the airplane and bleed air for an attempt at air conditioning (some aircraft are better at this then others). When it is time to start the engines they can be started using the APU as the bleed air source and this is a normal method of starting and no different than the method used currently when the plane leaves the gate (and for some airplanes they only start one engine and taxi on that as long as possible before starting the other engine).

As long as the plane has a running APU, it can complete a normal engine start with no ground assistance. The only limitation will be turbine warm up time, which just means you'll need to make sure the engines are started a few minutes before takeoff thrust is applied.

Just an aside, but currently the only ground support strictly required is a tug to push the airplane away from the gate and someone to operate the jetbridge. Aircraft and engine startup is self-contained unless the APU is inoperative.

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    $\begingroup$ One thing that you don't cover is that some jet engines (particularly smaller ones) use electric starters and therefore don't need air to start at all. In fact, they typically are started from the batteries. $\endgroup$ – Lnafziger Apr 13 '16 at 22:36
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This depends on the aircraft.

Most large jet aircraft are equipped with an APU (Auxiliary Power Unit). This is a small turbine power source usually in the back of the plane that can be started using only the onboard batteries. Once started the APU can provide electrical power as well as bleed air to start the engines.

Not all planes (but most modern ones) have an APU to provide bleed air and some require a ground based source of air pressure to start up, generally the airport has mobile units for this.

Piston planes (although not many are flown commercially any more), can generally be started by their internal batteries. If you are lucky enough to be in a piston plane form the 30's or 40's you may encounter a Coffman Starter which basically uses a blank shotgun shell to generate pressure in the cylinder and turn the engine over.

Small turbo props often have turbines small enough to be started by an electric starter. This extends to some small jets as well.

If a plane is out on the ramp and for what ever reason the batteries are dead or they die trying to start the APU, most airports have portable Ground Power Units that they can bring out to the plane for starting and providing power.

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    $\begingroup$ If you noticed, he just asked about jet aircraft on the taxiway waiting to take-off. $\endgroup$ – SMS von der Tann Jan 20 '16 at 22:02
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    $\begingroup$ I was answering in the spirit of the title question but you are correct there was no need to cover non jet aircraft, i was simply offering it for comparison. $\endgroup$ – Dave Jan 21 '16 at 0:06
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    $\begingroup$ If the plane is a turboprop, it's possible to start the engines without using the APU. It's called a "buddy start". Two planes are parked nose to nose, one with engines running, the props are reversed and engine power is increased. On the non running craft, the props are set to windmill as the airblast hits them. Probably in a multi engine plane, only I engine will be started, and the others feathered until the first engine is running. This is done on occasion in the military, when the APU of the non running aircraft is down. It probably isn't allowed on a commercial plane. $\endgroup$ – Howard Miller Jan 21 '16 at 5:36
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While not used for commercial aircraft, many military aircraft used started cartridges. These were slow burning explosives much like black powder packed into a canister and installed in the side of the engine. When ignited they produced a relatively high speed exhaust that rotate the turbines to reach starting speeds.

They were used regularly on alert B-52 and KC135 aircraft during the cold war.

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Others have noted that the APU is the normal means of engine start. I would like to note the extreme undesirability of your proposed scheme. Airlines live and die on despatch reliability and OTP, On Time Performance. Instead of performing engine start at a place easily accessible to maintainers, you want to do it shortly before take-off. This not only leaves no slack time to deal with MEL items that may only be encountered after engine start, or even a failure to start, it leaves the aircraft far away from where these matters may be dealt with. Worse still, it greatly increases the time pressure on pilots.

Most undesirable.

It would seem a very good way to turn a minor MEL item from no delay into an hour or more of delay.

It turns many common minor squawks into ground turn-backs instead of an on-time departure. I'm sorry, but it is a bad idea, from safety and passenger satisfaction perspectives too. It means that you have loaded pax and taken them around the airport and guaranteed significant delays to their travel if a problem arises during engine start and post-start checks - with a normal departure sequence, if problems arise before push back, you may be able to transfer loads to a spare aircraft and still achieve an on time departure (defined as within 15 minutes of scheduled departure) even if you can't do that, you may still get them to their destination on time by electing to increase fuel burn to increase speed. This is quickest on services without meals.

Whilst all airlines are very conscious of fuel usage, this scheme has little potential to save fuel, whilst imposing tremendous operational difficulties on the operator that do no one any good.

To get back to your question, as well as cartridge starters, there were in the past a number of turbine based starters using such propellants as 'AVPIN' for military aircraft, whilst today, there are still ground gas generator units (turbines with oversized compressor sections) that may be used to start aircraft with a U/S APU and buddy start hoses which may be used to rescue aircraft stranded due to a U/S APU at a port without ground support equipment. Also, some military aircraft use small gas turbines such as the Solar T-65 as starter engines (rather than using bleed air from a small turbine).

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  • $\begingroup$ Single engine taxi is a common practice. Isn't that in conflict with your first paragraph? $\endgroup$ – bogl May 7 at 8:38
  • $\begingroup$ Well, so someone said. It isn't very common on departure in my experience. I have in the past asked Flight Ops to encourage an engine shutdown on post-flight taxi, in preference to operating with one reverser deployed - mainly because doing that sets 'dolls eyes' for a reverser fault that then requires expensive investigation when nothing is actually wrong. $\endgroup$ – Paul Saccani May 7 at 8:44
  • $\begingroup$ In most cases, single engine taxi after push back would be a violation of the FCOM procedures, but different countries and airlines may operate differently. It's a dumb thing to do. The check list says 'start engines', after all... $\endgroup$ – Paul Saccani May 7 at 9:00
  • $\begingroup$ Ok, I understand that single engine taxi out is controversial. EZY seems to do it. Some recommend it for taxi out not shorter than 7'. Others don't do it for the exact reasons that you mentioned. I am not an airline pilot - just curious. $\endgroup$ – bogl May 7 at 9:08
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    $\begingroup$ Okay, I know what an APU is, but what is an U/S APU? $\endgroup$ – a CVn May 7 at 11:02
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Most of the time, the APU of the aircraft is kept running when the airplane is on the ground to provide the bleed air and electrical power to start up the main engines. As it happens, the APU does not need bleed air to start up so if you are starting the airplane up after leaving it, say overnight, you can start the APU up first and then start up the rest of the engines and voilà, you can take-off now. That is how it works on jet aircraft so no one gets stuck in an airport in the middle of nowhere.

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