Starting GE engines simultaneously

[Houghton11 12]

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:

 

[duggy35 4]

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

[scandinavian13 5,192]

 

 

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?

[duggy35 4]

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

[willywonka 36]

 

 

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.

[downscc 1,327]

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.

[vololiberista 205]

 

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.

[kevinh 409]

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.

[777simmer 117]

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.

[Ocean2b 0]

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

[77west 139]

 

 

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.

Give them time... they will find a way!

[G550flyer 1,014]

In the DC10-30s I flew we could. They had CF-6-50s on them. We could start all 3 simultaneously. We did it in the KC-10A also. If in a hurry, you turned them all at once. The only issue was that you had to stagger them just a little to prevent all 3 generators from coming online at the same time. If this happened, the power swap could potentially interrupt power to the starters causing a crash engage that could shear the starter shaft. We separated starter buttons by a second.   

[Ken.Pascoe 1]

Howdy All

 

Years ago I was an FO in Emirates and we had 200/200ER's - all with Rolls Royce Engines. We did simultaneous dual engine starts all the time. Needless to say when the GE's came along one of the changes we had to adapt to was being restricted to one at a time by the FCOM.

 

However human beings being what we are - I can personally attest (based on several instances) that you can start both GE's at the same time, even on pretty warm days in Dubai when the density altitude is pretty high. But as as been described here - it's probably not good for engine life. The APU may be able to handle it ok, but the lower the airflow the more critical the engine start (EGT vs Rotation Speed).

 

I haven't checked to see if PMDG models it well - but the Rollers (with it's 3 spool engines) was very much a more sedate start, feeding fuel in with increasing flow at a fairly regular rate of increase - when the GE's came along the FF went from 0 to about 500 almost instantly, with an associated bump in rotation. The GE's also generate higher vibs during the earlier part of the start cycle. During Hot/Hung starts the Rollers also required a more restricted cooldown/re-engage the start point; whereas the GE's spin for 30 seconds and the gearing kicks straight back in. We were told this was normal and expected, and subsequent operations have certainly borne that out.

 

Ken

[777simmer 117]

Interesting......I did not know that about the Rollers and also did not know other airlines start both (Roller) engines simultanuously.

 

I think I would still prefere one at the time though.

[scandinavian13 5,192]

 

 

I think I would still prefere one at the time though.

 

Was talking to a former BUFF (B-52) driver up at the airport on Saturday about the video at the top of the page (8 engine starts) and he said it was pretty stressful to have to monitor all of that at once.

 

He went on to say that, in the B-2 on the other hand, it was pretty much reaching up top and hitting "START." Done. The rest was all monitored for you.