Starting GE engines simultaneously

[willywonka 36]

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?

[James Bennett 52]

I can't. Did it by accident ages ago ando got loads of EICAS alerts.

[scandinavian13 5,192]

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.

[willywonka 36]

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... 

[scandinavian13 5,192]

 

 

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).

[vololiberista 205]

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

[scandinavian13 5,192]

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

 

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

[AirCanada235 103]

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

[willywonka 36]

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.

[kevinh 409]

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.

[vololiberista 205]

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!

[kevinh 409]

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

[dgraham1284 92]

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

 

https://www.youtube.com/watch?v=JjdyQpEUYzI

 

Nothing like american muscle.

[willywonka 36]

 

 

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.

[kevinh 409]

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.