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willywonka

Starting GE engines simultaneously

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This can be done in the sim if you've tried it. Page 24 of this FAA document (pdf) says, in regards to the GE90-115B engines on the 777-300ER: "cannot be started simultaneously".

 

Searches also show that this seems to be correct, including from PMDG's own tech team. PW engines can be started both at once, but thats not modeled nor available on the 77L/W

 

So, how about it? Can the GE90-115B be really started simultaneously or not? How come we can in the sim?

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So, how about it? Can the GE90-115B be really started simultaneously or not? How come we can in the sim?

 

If you have the required bleed air, the engines will start to turn. From everything I've seen, it's possible, but you'd be hard pressed to find any operator who actually allows it because it would probably murder the APU after a few times.

 

 

 

Page 24 of this FAA document (pdf) says, in regards to the GE90-115B engines on the 777-300ER: "cannot be started simultaneously".

 

This is not referring to a physical incapability. It's referring to a standardization for differences training specifying that operators are required to make a remark in their differences training that starting both GEs simultaneously is prohibited. Not physically impossible.

 

 

 

Searches also show that this seems to be correct, including from PMDG's own tech team. PW engines can be started both at once, but thats not modeled nor available on the 77L/W

 

That post is referring to SOP more than physical ability.

 

 

 

Using a comparative topic:

You should never exceed Vne, but the plane is certainly capable and will do it if you put it in the right situation. One can also do a barrel roll in an airliner. I wouldn't recommend it and the FAA also states that acrobatics cannot be performed in transport category aircraft (in a regulatory sense), but it's still possible that someone could do it.

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That was indeed my first thought, Kyle. At first I learned that it was an operator's limitation to save wear on the APU. It was the FAA's language that made me curious. They say "cannot be", which is a fact, rather than "should not be", which is an advisory. So I had to question if it was factually possible or not.

 

Searches on other parts of the interweb says that the GE's air demand is just too much to be possible at all. Who knows... 

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They say "cannot be", which is a fact, rather than "should not be", which is an advisory. So I had to question if it was factually possible or not.

 

Well the FAA also says VFR "cannot" fly through clouds, rather than "should not." The FAA has the ability to restrict operators from doing so on a regulatory level. So far, much to their own dismay, they haven't been able to regulate physics.  :wink:

 

 

 


Searches on other parts of the interweb says that the GE's air demand is just too much to be possible at all. Who knows... 

 

Most people are probably just regurgitating the same stuff they were fed in their training classes. Their operator specifically prohibits it because of the strain on the APU, and not that it's impossible (though, in certain conditions, it very well may be - try starting an engine with the packs on in the NGX and sometimes it'll light off, and other times it won't - it's all about the availability of bleed air).

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Hearing an SR-71 start is truly something crazy!!  If you go to you tube - you can reference what Kyle is speaking on.

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Well the FAA also says VFR "cannot" fly through clouds, rather than "should not."

Haha, really? I guess all of this falls under "covering their backs". It's easier and safer to say "cannot" rather than "should not". Maybe it's because the engines/APU were never designed to start simultaneously, so perhaps it's possible although it's never meant to be used "in that way."

 

I'm wondering if this should be part of service based failures? You can start simul once or twice, and then the APU would under go a failure?

 

I think even FS2Crew 777 allows starting of both engines? There's a command "START BOTH ENGINES" available but I haven't used it.

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Haha, really? I guess all of this falls under "covering their backs". It's easier and safer to say "cannot" rather than "should not". Maybe it's because the engines/APU were never designed to start simultaneously, so perhaps it's possible although it's never meant to be used "in that way."

 

I'm wondering if this should be part of service based failures? You can start simul once or twice, and then the APU would under go a failure?

 

I think even FS2Crew 777 allows starting of both engines? There's a command "START BOTH ENGINES" available but I haven't used it.

I don't think there's a risk of damaging the APU. It's not like an electric starter motor that might be damaged by excessive torque. The APU is automatically controlled to remain within limits. The problem would be that insufficient air flow could cause a slow and hot start, which would damage the engine.

 

I think the problem arises because simultaneous engine starts are possible with some engine types and not others. If it's approved with the most GE90s but not with the bigger fanned GE90-115B then a special mention needs to be made to avoid operators and pilots assuming it's possible with all GE90s. Presumably FS2Crew have included the option to cater for versions where simultaneous starts are approved.

 

There is a similar situation on the 744 where dual engine starts are possible with some engine types but not others. Some operators use the capability others don't. If the APU is powerful enough and autostart is in use then it's perfectly safe. It should be remembered that just because a procedure is not approved does not mean it isn't possible.

Did they use a huffer for that, or was that driven off of an internal APU? Our SR-71 originally used huffers that ran off of two Buick V-8 401 cubic inch engines: http://www.wvi.com/~sr71webmaster/ag330_sr.htm

I think they used internal high pressure air bottles for that.

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Did they use a huffer for that, or was that driven off of an internal APU? Our SR-71 originally used huffers that ran off of two Buick V-8 401 cubic inch engines: http://www.wvi.com/~sr71webmaster/ag330_sr.htm

I'm not sure of the exact method. I'll have to ask some people I know. But if the Vulcan was at dispersal and they got a call they could put all four switches to start at once. These were the same engines that Concorde used. Just imagine the Vulcan or Concorde taking-off from St. Maarten. They would blow the holiday makers back to their origins!

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Except the Vulcan's Olympus engines didn't have reheat. Still very powerful though.

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There is a similar situation on the 744 where dual engine starts are possible with some engine types but not others. Some operators use the capability others don't. If the APU is powerful enough and autostart is in use then it's perfectly safe. It should be remembered that just because a procedure is not approved does not mean it isn't possible.

 

That's a different scenario though. The 744 was designed to be able to dual start. Whether an operator decides to is up to them. In this case though, it appears that the GE90-115B was never designed to dual start. Perhaps it's possible per the sim, but it's not designed that way and can incur damages to the aircraft. It is not at the operator's option whether to dual start or not, unlike the 744.

 

I think what we've covered so far (just to summarize to myself) is that the GE90-115B must not be started simultaneously. It's probable though that the APU can supply enough bleeds to dual start as "proved" in the sim.

 

 

 


Presumably FS2Crew have included the option to cater for versions where simultaneous starts are approved.

 

I thought that too since some operators and engine combos do perform dual starts. But since the FS2Crew is meant for PMDG's version of the 777 which only includes the GE90-115 engine, dual start should not be an option. Having said that, I suppose some people fly their 77L like a 772ER anyway since we don't have one, and would like to simulate dual engine starts even if it's on the wrong type.

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That's a different scenario though. The 744 was designed to be able to dual start. Whether an operator decides to is up to them. In this case though, it appears that the GE90-115B was never designed to dual start. Perhaps it's possible per the sim, but it's not designed that way and can incur damages to the aircraft. It is not at the operator's option whether to dual start or not, unlike the 744.

The engines in the 777 were designed for simultaneous autostarts too. Also some 744 engine types can't be dual started. So it is exactly the same thing.

 

The likely reason the -115B shouldn't be simultaneously started is the larger fan, which means more torque is required to get the engine turning. That means the APU must provide more air flow and it may be on the edge of acceptable limits. If you tried it in the real aircraft it might well work. Possibly too close to the limits for acceptable operation. So it isn't permitted.

 

For a good simulation, if something can work in the aircraft it should work in the sim, even if the procedure is not approved. Therefore to prevent it working in some way would be bad simulation.

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The Avro Vulcan Bomber could start its four Olympus turbojets simultaneously!

 

I believe they utilized cartridge start for that. Basically starter would be driven by gases escaping from a gunpowder cartridge.

 

 

Here's the B-52 starting all 8 (!) simultaneously:

 

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You won't damage the APU by multiple simultaneous engine starts. The RPM for high bleed air demand (ie. engine start) is fixed, typically 100%. Therefore, bleed air pressure is fixed. What the operator chooses to with that available pressure downstream has no effect on the source. The first bleed air pressure regualtor after the bleed stage will modulate to supply the required flow to maintain downstream pressure to the point where it is full open (maximum demand). At that point, further demand downstream will result in reduced pressure at the start valves, while the load on the APU remains the same.

 

Typically, engine start valves can only be opened individually because the available pressure would only be enough to crank one engine to a sufficient RPM to then be able to safely add fuel.

 

Steve McLaren

Steve McLaren

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You won't damage the APU by multiple simultaneous engine starts. The RPM for high bleed air demand (ie. engine start) is fixed, typically 100%. Therefore, bleed air pressure is fixed. What the operator chooses to with that available pressure downstream has no effect on the source. The first bleed air pressure regualtor after the bleed stage will modulate to supply the required flow to maintain downstream pressure to the point where it is full open (maximum demand). At that point, further demand downstream will result in reduced pressure at the start valves, while the load on the APU remains the same.

 

Good to know. So, is it an increased risk of a hung/hot start that you shouldn't crank both GE90s at once?

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I don't work on the 777 so can't speak for it.

Generally speaking if the pressure drop is excessive then risk of a hot start is higher if not likely. I'm not aware of any APU that can crank 2 large mass turbofans simultaneously to sufficient RPM to support self sustain. Someone out there could probably correct or confirm this.

 

Steve McLaren

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For a good simulation, if something can work in the aircraft it should work in the sim, even if the procedure is not approved. Therefore to prevent it working in some way would be bad simulation.
Thanks. That clears up the suspicion I had earlier. It should work just like what is shown in PMDG's sim, but it isn't permitted.

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My poor memory is reaching for a comment I read during early beta of the 777, where under certain conditions the bleed air would be sufficient to simultaneously start both engines but in practice it is generally not done.  I like having the same repeatable work flow, and stick to #2 first then #1.

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For a good simulation, if something can work in the aircraft it should work in the sim, even if the procedure is not approved. Therefore to prevent it working in some way would be bad simulation.

If it is "technically" posible in real life it should also be present in the sim. However, I'm sure that in real life it would form part of the engine limitations schedule which should also be part of the sim. So therefore, if you do it too often some form of damage could occur such as increased risk of vibration.

Another point to mention I doubt very much that any form of simultaneous start would be performed in ISA+10 or more conditions.One is risking a hot start anyway with just one engine! Also even though the APU bleed valves would supply a fixed pressure, again the demand would make the APU overwork and get too hot so it could shut down at the critical moment.

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If it is "technically" posible in real life it should also be present in the sim. However, I'm sure that in real life it would form part of the engine limitations schedule which should also be part of the sim. So therefore, if you do it too often some form of damage could occur such as increased risk of vibration.

Another point to mention I doubt very much that any form of simultaneous start would be performed in ISA+10 or more conditions.One is risking a hot start anyway with just one engine! Also even though the APU bleed valves would supply a fixed pressure, again the demand would make the APU overwork and get too hot so it could shut down at the critical moment.

The APU would reduce air output if demand is too high. It can't be over stressed due to excess demand. So any damage would be to the engine not the APU. Not sure why vibration would be an issue. Most likely high EGT due to slow acceleration. FADEC autostart would cut fuel before any damage would be caused.

 

Dual starts on the 744 aren't performed at higher altitude airports or at high OAT (30 C I think). No doubt limits like that apply to the 777 too.

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As said.....if the engines turn fast enough with whatever source making that possible then sure you could start them both simultanuously.

 

During a dual engine fail (inflight) this is attempted for example.

Memory item: Fuel control switches - both cutoff and run

It does not say first one and then the other.

Sure....windmilling is your source in that case, but it makes no difference.

 

I agree with those who say that the APU will provide what it can. No more no less and will not get too hot.

 

But think about it.

Why would you even want to start 2 engines at the same time on ground?

For the 2 minutes that you save that way?

We usually start one engine during pushback (unless restricted by the airport like JFK where we are not allowed to start up untill after the pushback is complete) and the other one once push is complete.

In FSX PMDG777 I start both during pushback (one at the time) so that the phrase "ground we got two normal starts, you can disconnect, see you next time through...." comes AFTER both engines are actually running. (not that it matters much in FSX if the ground guy is gone while you start the second engine)

 

But in real life you got only one technician outside who is monitoring the engine while it starts up.

One is easier to monitor than two!

Same inside.....if you get start up problems then things are easier to handle and more easily recognised with one engine starting at the time.

There is already enough distraction as it is while monitoring the engine start (ATC wants something like "please extend your pushback for incoming traffic"/a new ACARS load sheet comes in/Purser comes in and wants something like "the headcount does not add up with the load sheet, what do we do?"/ground guy wants something like "say again...where do we need to puch back to?"/the airline frequency wants something from ya "did you receive loadsheet number 2?"/etc).

I know they are all little things.....but you really want to look at that engine start.

In the cockpit you dont feel/see much of what is going on back there, but those babys are really really powerfull and if something goes wrong......... (no oil pressure rise) you want to shut them down quick before a lot of damage is done.

So no need for more distractions if you ask me, even though the 777 engine start is highly automated.

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Starting both engines simultaneously would be very similar to completing a ground air start with a very poor start cart. I've seen this happen before, the procedures calls for at least 25 psi duct pressure for the start and our psi was 18. The start was still successful however the EECs did their job and very slowly added the fuel and the engines started, albeit after a longer than normal time. Two engines at once would likely do the same, however there would probably be more chance of a failed start (hung). FYI have a look at the duct pressure with the APU running, it's usually between 30 to 40.

 

I'm glad it's difficult to start these massive engines.... The guys in the flight deck may have had a laugh during the taking of my profile pic!

 

Alex Dority

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