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teopereira

Engine start with packs on?

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Some days ago, I flew in a brand new 737-800... I noticed that, during the pushback, the engines were started while the air conditioning was on. How is that possible?

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How did you know that packs were on?

Flow of air through the outlets can be caused by residual pressure.

As far as I know it wouldn't be enough pressure to keep packs running when starting engines on 737. And I don't think anyone will pushback aircraft together with ground air cart and starting engines at the same time.

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We always notice when the packs are turned off for engine start... that's the normal thing. But this last time the air conditioning was pumping cool air while the engines were being started...

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1. The packs were probably off since that is the normal ops. Since you were pushing back, that means the APU was supplying bleed air to start the engines (no ground air) and the APU cannot output enough bleed air to run a pack AND start an engine.

 

The recir fan was probably on and pushing air to the passengers is my guess.

 

While you cannot start an engine AND run a pack with APU bleed air, you can do some clever things with the pneumatic system to make it look like that is what you are doing (to the passengers anyway). Maybe that is what happened.

 

Here is what the pilots may have done:

 

1. Start the right engine first on APU bleed air.

2. When the right engine is up and running:

   --close the Bleed Isolation Valve.

   --Turn on the right pack and run it with bleed air from the operating right engine.

3. Now with the APU bleed air feeding the left pneumatic manifold only, start the left engine on APU bleed air as you normally would.

 

That's the only way you can pull this off to make it appear like you are "starting engines while the packs are on".

 

To repeat: the APU does not have enough bleed air output capacity to start an engine AND run a pack at the same time.

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1. Start the right engine first on APU bleed air.

2. When the right engine is up and running:

   --close the Bleed Isolation Valve.

   --Turn on the right pack and run it with bleed air from the operating right engine.

3. Now with the APU bleed air feeding the left pneumatic manifold only, start the left engine on APU bleed air as you normally would.

 

 

That is correct. In fact it's a normal procedure called isolated pack operation during engine start and can be found in SP2.2 within FCOM Vol 1. Might be useful whenever outside air temperature is high and you want some more passenger comfort. As stated before during start-up of the first engine air most likely came from recirc fans.

 

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As a matter of fact that is Gol airline´s standard operating procedure here in Brazil where in the summer even two minutes without pack air on a hot/humid day can beocme unbearable for a crowded 737. 

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To repeat: the APU does not have enough bleed air output capacity to start an engine AND run a pack at the same time.

That's putting it a bit strong. It can supply enough air to do both. However it's not normal procedure as the start might be slower and hotter.

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Is there a Boeing procedure that allows you to start an engine with the APU bleed air and also run one pack?

 

If there is, I have not seen it and I have never trained crews to start engines that way. We train normal starts (of course) and cross-bleed starts from the opposite, running engine. Packs are always both OFF. (Note: this was not the 737).

 

And if there is such a procedure somewhere, it would only be for the Garrett powered APU -300/400/500 models and any NG as those APUs are rated to operate 2 packs (means the classic 737 APUs could only power one pack).

 

If there is no such Boeing procedure, how do you know the APU can do that? You must be saying that based on experience and/or something you read?

 

Thanks...

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That's putting it a bit strong. It can supply enough air to do both. However it's not normal procedure as the start might be slower and hotter.

He's right. With the packs on and trying to start an engine you will only get about 17% N1 or so

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@Jack

 

Well, that's kind of what I was thinking but Kevin is very sharp and knowledgeable, so I'm sure he made his statements for a good reason.

 

There might not be a Boeing procedure (I'm guessing here until we hear from Kevin) for starting an engine with one pack on, but as someone else has stated, many operators do things that are not Boeing standard. So, there may in fact be several operators who have this procedure in their Ops Manual.

 

That being said, I was also thinking that with these engines costing upwards of $5M+ each, I don't know why a carrier would take the chance of increasing their hot starts by allowing such a procedure. A hot start on any turbine engine is really bad news for the turbine blades. Just thinking out loud.

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I just know from first hand experience. I never heard of any company starting with packs on. The EECs are now programmed to not schedule any fuel until 20% if I remember correctly so you shouldn't even get a start in theory with the packs on from what I've seen

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He's right. With the packs on and trying to start an engine you will only get about 17% N1 or so

That may be the case for cranking with both packs on. Ralph talked about starting with a pack running. That's much less pneumatic load, so a start might be possible.

 

The APU can supply a certain amount of air flow before it reaches its EGT limit. Call that z lb/sec. A pack requires x lb/sec air flow. Engine starting requires y lb/sec (note, y reduces as N2 increases). If x plus y is less than z then a start with a pack on should be ok. Adding a second pack could be more than the limit and so reduce flow to the starter and prevent N2 rising high enough.

 

The fact the start air flow reduces as rpm increases affects the outcome, as do ambient conditions which affect the values of x, y and z. The amount APU output pressure reduces with flow demand is important too. So whether it's possible or not is not a simple yes/no question. That's why I replied as I did to Ralph's post.

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One pack might work. Not sure why anyone would want to start with the packs on in the first place but oh well. If the engine bleeds are on the off position, the remaining pack should revert to high flow, demanding more air from the APU which might lessen the chance of a start though. So leave the engine bleed air switches on. Nice thing about this is you aren't going to wreck a $6m engine.

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>One pack might work. Not sure why anyone would want to start with the packs on in the first place but oh well.

 

Jack, you are too funny. I think this whole thread centers on the one pack being on, not both packs. For sure the APU cannot start an engine if its pumping air into both packs. I'm pretty sure no operators in the 737 world are starting engines with both packs running and only having bleed air coming from the APU. So, I'm pretty sure we're only talking about the possibility of starting an engine with only one pack running.

 

Re-visiting my comment about whether a carrier would authorize flight crews to try and start an engine if the hot start risk was higher than normal (meaning they would turn on one pack and have it running during an engine start).

 

I have a Captain friend who flys 747's for United and he said (understanding that carriers all operate differently) that any time they get a hot start they must record it in the log book. Then maint must take one of three actions: do a visual inspection of the engine, bore scope the turbine section or remove the engine. The action taken depends on the pilots write up as to how high the temp got before they were able to abort the start.

 

Another interesting side note to all of this is the aviation news report I saw over the weekend. A crew of a twin engine jetliner (not sure what aircraft type) was doing a cross bleed start because they wanted to use the APU air to run one of the packs after starting the first engine. The problem was the Captain ran the power up too high to get the needed duct pressure and the aircraft started to move at the gate and the tug bar broke and the ground crew tried to tell the captain to shut down the engine. But that took a little time because the Captain did see his plane was moving but he thought it was the tug driver moving it. It wasn't as the tug driver jumped off the tug as soon as the tow bar broke. What a mess.

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One pack might work. Not sure why anyone would want to start with the packs on in the first place but oh well.

In reality you wouldn't take the risk, naturally. But in a simulator if it's possible to attempt it on the aircraft then it should be simulated that way. So if a pilot in the sim forgets and starts an engine with a pack or packs running then the consequences follow automatically. Every full flight sim I've worked on has been like this. It's analagous to the situation with boost pumps being on before starting engines. Procedurally that's what should be done, but not doing so does not prevent the engine starting.

 

Fortunately PMDG have not done what Level D did with the 767 and effectively disabled a start with the packs running by setting duct pressure to zero if you try it. You can't even start with a pack running on the other engine with the isolation valve closed. At least you can try and start an engine in the NGX with a pack running.

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Another interesting side note to all of this is the aviation news report I saw over the weekend. A crew of a twin engine jetliner (not sure what aircraft type) was doing a cross bleed start because they wanted to use the APU air to run one of the packs after starting the first engine. The problem was the Captain ran the power up too high to get the needed duct pressure and the aircraft started to move at the gate and the tug bar broke and the ground crew tried to tell the captain to shut down the engine. But that took a little time because the Captain did see his plane was moving but he thought it was the tug driver moving it. It wasn't as the tug driver jumped off the tug as soon as the tow bar broke. What a mess.

Amazing they tried to do a crossbleed start with the brakes off and only the tow bar holding them stationary. Of course if the Captain accelerated the engine beyond the HP bleed switchover point then they'd be trying to get 40 psi from LP air, so quite a lot of thrust. The type concerned must be narrowed down by the fact you can have the APU supply a pack and be isolated from the crossfeed manifold.

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what Level D did with the 767 and effectively disabled a start with the packs running by setting duct pressure to zero if you try it. You can't even start with a pack running on the other engine with the isolation valve closed.
Didn't know that! It's been years since I flown the LDS763!

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I think its about 30 PSI to the starter, not sure how much the pack running would take out of that pressure, but i would think that the engine would run hot on startup, if it managed to light!

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I think its about 30 PSI to the starter, not sure how much the pack running would take out of that pressure, but i would think that the engine would run hot on startup, if it managed to light!

I think the APU can produce about 40 psi no load and 30 is the minimum to attempt a start so there's some margin over that could run a pack. I said at the outset the start might be hotter and slower.

 

The way it's simulated in the NGX a start with one pack running is almost no different to no packs. However with two packs on and the start valve open duct pressure goes to zero and prevents the engine cranking at all. So the effect of one pack on a start is perhaps underdone and the effect of two packs rather overdone. Not really important when most users will follow the checklist and have both packs off anyway.

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