Cross bleed starts

jrh

Well-Known Member
Occasionally when I'm taxiing out, I'll hear an aircraft talking to ATC about their need to do a "cross bleed start" and making sure the area behind them is clear. I think it's usually been 737s or RJs I've heard.

So, not being a jet driver myself, anyone care to explain what's going on? I'm assuming the jet is powering up one engine, then using bleed air to get the other side going.

However, I'm curious about the specifics. Why is this required in some cases versus traditional (electrical?) means of starting?

I'm familiar with PT6 turboprops from flying the Meridian, and have watched crews start any number of jets while jumpseating, but otherwise don't know much about jet operations. Just curious. Thanks!
 

EIR

Erik10
Im familiar with cross gen starts, however, I've nver heard of cross bleed starts.

Adler, enlighten us.
 

dasleben

That's just, like, your opinion, man
So, not being a jet driver myself, anyone care to explain what's going on? I'm assuming the jet is powering up one engine, then using bleed air to get the other side going.
That's the gist of it. The one motor is used to turn the air turbine starter for the other motor. Most larger jets don't have electrical starting capability (787 excluded!), so everything is air start (mostly off the APU, but can be a crossbleed start, or use of a huffer cart).

In our case, we have to power up to 70% N2 (~30 psi of duct pressure) on the operating engine. It's not a whole heck of a lot off idle, but it's enough to require us to be clear behind.
 

Dugie8

Well-Known Member
Some airplanes use bleed air to spin the starter. Can be bleed Ir from the apu or bleed air from a buffer cart or bleed air from the other engine(s). CFM56 engines took about 50 psi of air to start.

I believe the CRJ has both an electric starter and a bleed air starter.
 

ackeight

Well-Known Member
Usually larger aircraft uses bleed air to start the engines. They are probably starting #1 and either isolating or completely shutting down the apu. Then they are using the bleed air from #1 to start #2. This requires a higher than idle power setting on #1 which is why they are asking if the jet blast area is clear.
 

PositionAndHold

Well-Known Member
Jet wise I've only flown the erj and currently a Lear 45. The Lear is purely electric start. In the erj you need an air source to start either main engine, it usually comes from the apu, the other engine or a ground cart. The apu is started on batts. and then is available as a source of air to start either engine. You would use the apu to start both. If the apu is inop, you use a "huffer cart" to start an engine, once you have one started you now have a source of air to start the other, or a "cross bleed" start. It's important to have a clear area since you have to increase the rpm on the running engine in order to provide enough bleed air to start the other one. I'm sure ill get called out for probably forgetting some thing, it's been about four years since I've delt with using bleed air to start. :)

Edit: ackeight beat me too it in much simpler terms.
 

Alchemy

Partner, Ally, Friend
It's usually done because the APU on the jet is inoperative. Normally, in what I've flown, the APU is used to start the engines via a bleed air driven starter. However, if the APU is deferred, then one engine is started at the gate from a "huffer cart" (ground pneumatic source) and the remaining engines are started using air from that engine, which usually requires more than normal taxi thrust.
 

jrh

Well-Known Member
So when would this procedure be used? When the APU is inop for some reason?

Edit: Thanks for reading my mind, Alchemy.
 

N519AT

Ahh! This is how I change this!
Bigger jet engines use bleed air to spin the engine because an electric starter would draw so much current it would basically be impossible to do. So they don't even have electric starters.

Usually cross bleed starts aren't necessary as the APU is functional and provides the bleed air for engine start. Usually only if the APU is INOP or the bleed air function of the APU is INOP do you have to do a cross bleed. To get the first engine started you use something like this:



Have the ground crew hook it up, turn the air when you're ready to start, engine spins, introduce fuel and PFM from there.

edit: I look like a tard. when I opened it and started typing there was no replies. I should probably add Im just a little turboprop guy with electric starters and have never flown a jet with pneumatic starts.
 

dasleben

That's just, like, your opinion, man
"You know on the 777…" the APU is hoss-enough that they can do dual starts.
I watched that go down while I was sitting in the jumpseat on a company 777 once. Dual engine autostart... Engine start selectors both to START, fuel control switches both to RUN. If the engine has an abnormal start, it'll shut itself down and automatically pop up an electronic checklist. All I could say was "Oh come on." :) Meanwhile, I feel like I'm starting a steamboat in 1904 with all the workarounds for APU backpressure and center pump interruptors.

I think the 747 bubbas can start all 4 at the same time, though I don't think they do... Polar742 Alchemy
 

Alchemy

Partner, Ally, Friend
I watched that go down while I was sitting in the jumpseat on a company 777 once. Dual engine autostart... Engine start selectors both to START, fuel control switches both to RUN. If the engine has an abnormal start, it'll shut itself down and automatically pop up an electronic checklist. All I could say was "Oh come on." :) Meanwhile, I feel like I'm starting a steamboat in 1904 with all the workarounds for APU backpressure and center pump interruptors.

I think the 747 bubbas can start all 4 at the same time, though I don't think they do... Polar742 Alchemy
From what I've been told starting all 4 simultaneously used to be SOP at Northwest on the -400. You're only supposed to start 2 at a time now though. Maybe it shortened the life of the APU to put that much strain on it.
 

PositionAndHold

Well-Known Member
"You know on the 777…" the APU is hoss-enough that they can do dual starts.
I started life as the Garret TPE331 turbo-prop. Then became a series of apu's up to the one that's in the 777, the GTCP331-500. If I remember correct, the variants of the TFE731 that powers a ton of small to med sized corp jets started as an apu for the DC-10. Yeah. I'm. Bored. :)
 
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