Did they have to jump-start that CRJ-200 I was on last week ?

[LoneRanger]

Okay, recent honeymoon we flew out on a Delta Connection CRJ-200. Flight attendant secures main door and shortly thereafter the push back tug backs us up. After the tug is done, maybe a minute or two, we're told by the flight attendant that it's going to be about 15 minutes we'll have to sit there until "they start our engines." I got the distinct hunch that the APU or whatever on the CRJ-200 wouldn't start the engines and they had to send out to the local FBO (which is a jet service shop and has a few charter business jets) for a mobile APU to start our engines.

I may be completely wrong and not know WTH I'm talking about either, but after the approx 15 minute delay sitting still after the pushback with no engine start it makes me wonder if they had to jump start the fckr.

 

[Donald]

At least they did not tow it real fast to jump start it like you jump start a car?

 

[Astro14]

Disclaimer: I've sat on the jumpseat, but not flown this airplane, so what I say is generic, and applies to all the airplanes that I've flown, and, I suspect, applies here.

Your CRJ-200 had no APU. That's not uncommon. You can fly the airplane without an APU - not on an ETOPS flight, and not to places where the weather's really bad, and a few other circumstances, but on a normal day, to a regular airport, you can operate the airplane in passenger service without an APU. The APU is usually started by the airplane battery. On a cold day, you might need ground power to help get that APU started.

Now, jet engines are usually started by an air turbine starter motor that turns the engine. Very compact. Very light. Needs high pressure air, and LOTS of it to operate. APU usually provides that air, on a normal day, but the other engine can provide that air as well. That latter case is known as a "cross-bleed start" and often requires running up the RPM on the operating engine to supply sufficient air. During the start, you need electrical power for control systems, cockpit instruments, etc. That comes from the airplane battery, or from the generator on an operating engine.

A typical engine start takes about 90 (+/- 30) seconds. APU start is about the same. Depends on that model of engine.

Cross-bleed start can't be done near other airplanes, or ramp personnel (jet blast hazard from the increased RPM on the operating engine) so, it's typically done away from the gate.

Even a normal engine start is often not able to be accomplished at the gate due to jet blast hazard, airline SOP, or the tug itself not able to push an airplane against the idle thrust of the operating engine.

Usual sequence for no APU: Push back (which requires door closed and passengers seated). Connect air. Start the engine. Disconnect air. Taxi away from the ramp. Cross-bleed start.

So, is your question: was the airplane operating without an APU that day? Yeah, it was.

Is your question, why so long to get the engines fired up? Well, step 2 there, connect air, requires a running start unit (a jet turbine on a cart).

When your crew pushed back, the person bringing that cart might not have been ready, and the start cart itself has to be started, which takes a couple minutes, too. Like the APU, it starts using a battery.

Add a couple of minutes to each step as the radio call is made, they find the guy who was supposed to be standing by (because he works for a ground service company, not the airline itself), he hooks up the cart, then drives over, plugs in the intercom, starts the cart, and hooks up the air. Now, the crew wants to start both on the cart, because it saves the hassle of a cross-bleed, and yeah, you're at 15 minutes, easy.

It's possible, too, that the cart didn't start the first time, or the airplane battery went flat while waiting (in which case, 28VDC and 115V/400HZ AC has to be plugged in from the cart), or that Bill-Joe-Jim-Bob was in the bathroom when the crew made the radio call...but your experience isn't that unusual at a small airport.

At a big airport, where the airline itself is big (say, SFO for UAL), that cart is next to my 757 and running when I am ready to push back, and I've already talked with the ground crew on intercom. So, you likely wouldn't even know that we were dispatched without an APU, as the whole push/start sequence happens quickly and efficiently.

Now, I don't want to impugn your crew on that day, but they might not have known how long it would take Bill-Joe-Jim-Bob to get them a start cart, because they might not have thought to ask, or because between them asking, and the request, BJJB was asked to help start another airplane... it's hard to know exactly, but yeah, sometimes, when you rely on other folks, who work for another company, they just don't share your sense of urgency...

 

[Astro14]

Originally Posted by Donald
At least they did not tow it real fast to jump start it like you jump start a car?


For some airplanes, the minimum airspeed to get what's known as a "windmill start" where the air flowing through it is enough to get it started, is 300 KIAS. For most airliners, it's closer to 180 KIAS.

Either way, that would have to be one impressive tow tractor...

 

[andyd]

Could an engine start in a plane launched off a carrier ?

 

[Wolf359]

Originally Posted by andyd
Could an engine start in a plane launched off a carrier ?


You mean before it hit the water or after?

 

[FowVay]

The N2 (high pressure core) is what is driven by the starter. The starter is driven by either auxiliary air from the APU, air from the other engine or air from a start cart.

If a start cart is required it's usually because the APU won't start due to either an inop APU or a low battery. This isn't an uncommon occurrence.

 

[Astro14]

Originally Posted by andyd
Could an engine start in a plane launched off a carrier ?


f I understand your question correctly, no.

First, an airplane leaves the end of a carrier at about 160 KIAS. A bit more if it's heavy. And it's got an altitude of about 60 feet. So, without engine power, it's got about 2 seconds before it hits the water.

Turbines aren't like piston engines. Not at all. Like I said above, it takes them 90 seconds (give or take) to get started when on the ground.

For simplicity's sake, we will consider only the N1 turbine and shaft in this discussion, knowing that most commercial and fighter engines are double or even triple spool engines. All speeds are referenced to percent of maximum RPM. So, the maximum RPM might be 4,000, but the gauge in the cockpit reads from 0 to 100% (well, perhaps as high as 110% to capture an overspeed condition).

Let's talk about getting it started. When you engage the starter, you're opening a valve to send air to that air starter, and it begins to move the turbine. Lot of mass in a turbine, so the N1 increases at roughly two percent every second, and about 25% or so (again, varies by engine, but let's say 10-20 seconds into the start) the pilot, or the fuel control, will introduce fuel. Fuel is injected into the combustion section, and igniters (which are like continuously firing spark plugs) are energized.

Only a very small amount of fuel is injected initially, because not much air is moving through the engine. To get the engine much above 25% is going to take combustion, but you can't just dump the fuel in, there has to be enough airflow to keep the temperatures within an acceptable range. By 50% RPM (about 30 seconds into our start sequence) there is no need for the starter motor any more. But the turbine can't accelerate too quickly, or again, it will overtemp, so the fuel control gradually increases the fuel flow as the RMP increases.

Stabilized idle, which is roughly 65-75%, is reached in about 60-90 seconds, in most engines. You're allowed up to two minutes to reach a stabilized idle as lots of things can affect the rate of start: air pressure going to the starter, oil temperature (drag from colder oil affects acceleration rate), air temperature, fuel volatility, tail wind or head wind, igniter efficiency. There is a normal range, but it takes a while to get a jet engine up to idle speed.

Starting the engine in the air is much the same. If you're windmilling it, that is, using the airplane's airspeed to accelerate the turbine to a speed that fuel can be introduced, then it can get up to idle in as little as 20-30 seconds, because the mass of air going through it is better for acceleration than that tiny little starter, but it still takes a long time to reach idle.

Once a jet engine is at idle, it takes a while to get it up to 100% RPM, and making full thrust. Several seconds, usually. Eight seconds was the F-14 number, from idle to full power, and then a few seconds more to get max AB.

So, to get from an engine that is off through a start, to making power enough to climb, is going to take at least 45 seconds.

But your airplane thrown off the front of the carrier has two seconds before water impact.

So...no...

 

[eljefino]

Today I learned something. Thanks.

 

[andyd]

Me too, thanks Astro 14

 

[Linctex]

Originally Posted by Astro14

So, is your question: was the airplane operating without an APU that day? Yeah, it was.
Is your question, why so long to get the engines fired up? Well, step 2 there, connect air, requires a running start unit (a jet turbine on a cart).
When your crew pushed back, the person bringing that cart might not have been ready, and the start cart itself has to be started, which takes a couple minutes, too.


The ones I used had V6-92 or V8-92 Detroit Diesels in them.

If you think a Detroit in a truck is "an amazing device for converting fuel into smoke and noise"...............

Then you've never seen one perform "huffer" duty!!

That's like the epitome of a "smoke and noise" ground show! (vs. air show)

 

[Astro14]

Originally Posted by Linctex
Originally Posted by Astro14

So, is your question: was the airplane operating without an APU that day? Yeah, it was.
Is your question, why so long to get the engines fired up? Well, step 2 there, connect air, requires a running start unit (a jet turbine on a cart).
When your crew pushed back, the person bringing that cart might not have been ready, and the start cart itself has to be started, which takes a couple minutes, too.


The ones I used had V6-92 or V8-92 Detroit Diesels in them.

If you think a Detroit in a truck is "an amazing device for converting fuel into smoke and noise"...............

Then you've never seen one perform "huffer" duty!!

That's like the epitome of a "smoke and noise" ground show! (vs. air show)


I'll bet it is!!

I've only seen turbines provide the compressed air (though I have a story about that, in Manchester New Hampshire), but a big engine turning a compressor would work.

I should add that I've seen two jet starter turbines blow up.

They're under a lot of stress. Tiny unit, makes quite a bit of horsepower.

Once on the deck of an aircraft carrier, the starter blew up on the airplane next to me with a "BANG". It sounded like a shotgun had gone off in the cockpit. It damaged the engine access door of the airplane as it threw shrapnel through the side.

Once in Seattle, while doing a walkaround at night. A 757 had pushed back and was starting up, I heard a grinding sound and saw a SHOWER of sparks spewing from the bottom of the P&W 2037 engine. Like the output of several big angle grinders hitting steel...which is basically what was happening...rotating parts hitting stationary parts at high speed and with tremendous horsepower. It was spectacular...

 

[Astro14]

I should also add that engine light-off (combustion) takes anywhere from a few seconds to 15 seconds after fuel is introduced to the engine.

So, you go fuel on, and see a few hundred lb/hr on the fuel flow gauge, and a little while later, the EGT (Exhaust Gas Temperature) begins to rise.

This is the critical part of the start; making certain that you don't overtemp the turbine as combustion begins. Pilot starting the engine (FO in the airlines) is watching that EGT like a hawk, along with RPM increase, to ensure that it continues to accelerate and stay within limits. Ignition is not instant, and no light-off after 20 seconds and you have to cut the fuel off and continue to crank the engine to blow out the unburned fuel. You'll have a pretty big puddle of unburned fuel after 20 seconds of injection with no combustion, which needs to be cleared, or the next start WILL get too hot...

Again, modern engine controls automate all this, but I fly older airplanes, so it's still a manual process. In the 757, for example, to start an engine, with APU running, you have to:

1. Turn off the APU bleed supply switch
2. Wait for it to completely close (light out)
3. Turn off the packs (this sequence prevents an APU auto-protection shutdown from a pressure spike when the packs are shut off)
4. Turn the APU bleed back on
5. Place the selected engine (usually left, but could be right) switch into GND, which opens the starter valve
6. When RPM reaches 25% or max motoring (which is defined as less than 1% RPM increase over 5 seconds) - Fuel control switch to on. That opens both the low pressure fuel valve in the engine pylon, called a spar valve, and the high pressure valve on the engine itself, and allows fuel to go into the engine.
7. Then, as the RPM increases, at 50%, the starter switch will return to "auto" and the starter itself disengages.
8. Stabilized idle (roughly 65-75%) has to be reached within 120 seconds. RPM, EGT, and other parameters define that. If not, and that's happened to me, then you shut down the engine using the engine control switch, and depending on the failure type, you might be able to re-attempt the start, or it might need maintenance to take a look before trying again.

A couple of notes: the engine ignition selector switch, right above the two engine starter switches, can be in "1", "both" or "2" depending on which igniter system you want used. So, we typically alternate use of systems 1 and 2, but if we have an abnormal start, we'll abandon the start (fuel control switch to off, which closes both fuel valves) and then put that switch in "both" and fire both sets of igniters for a subsequent start.

There are limits on that little starter, too. All that stress on it means that it can't be engaged for longer than 120 seconds without some time off to cool down.

For the second engine start, it looks like the above.

If you're crossbleeding engines (using high pressure air from one engine to power the starter on the other), you've got to run up the power on the supply engine until you get 30PSI in that high-pressure duct with the starter valve open, or there won't be enough energy from the starter motor to keep the start within EGT limits.

I was trying to keep this simple, but looking back at all I typed, I'm not certain I've achieved that...I hope this helps. It's definitely not like a car. It takes a bit to get a jet engine going.

 

[LoneRanger]

Another thanks to Astro14 for the very detailed info.

 

[Astro14]

Originally Posted by FowVay
The N2 (high pressure core) is what is driven by the starter. The starter is driven by either auxiliary air from the APU, air from the other engine or air from a start cart.

If a start cart is required it's usually because the APU won't start due to either an inop APU or a low battery. This isn't an uncommon occurrence.


Yeah, I deliberately avoided the N1 and N2 distinction (N3 for the RB-211 engines) to keep it simpler to understand.

The RB-211, or the PW 2037, idles at about 22-25% N1, or Fan speed, but the high pressure core (N2, or N3) is much higher, like 62-78% RPM, at idle...but by treating all engines as straight pipe turbojet with one compressor/turbine speed, it makes how they start a simpler discussion, even if I've given up some technical accuracy...hope you'll forgive me on that...


So, to introduce that complexity into the discussion, this is part of why the start process is slow...

The N1 (fan, or low pressure compressor and turbine) isn't mechanically connected to the N2 (high pressure) or, if there is one, N3. They are aerodynamically balanced. So, during start, you're moving one part of the engine, and increasing airflow is moving (accelerating) the other part (which is concentric, by the way) of the engine. You can't get too far ahead of that, there is just a limit to how fast you can accelerate everything, because you're increasing the speed of the other part just by moving air through it.

In the discussion above, I was using the term N1 as if there was only one speed. So, yeah, a turbofan idles the fan at much lower speed than the core, but that core speed determines when you go fuel on (at least, for manually started engines, I'm not sure how the automatically started ones work, I don't fly those airplanes).

 

[Wolf359]

Speaking of engines blowing out, Astro14 have you heard it?

I used to work at a power plant and one of the combustion turbines had a blade break off and that basically took out the entire turbine. They said the whole town a few miles away heard it when it went.

As for the carrier, good description. Aren't there some airports that are at high altitude and maybe in theory if you went over the cliff, you'd have enough altitude to get the engines started before you hit the ground?

 

[Astro14]

Originally Posted by Wolf359
Speaking of engines blowing out, Astro14 have you heard it?

I used to work at a power plant and one of the combustion turbines had a blade break off and that basically took out the entire turbine. They said the whole town a few miles away heard it when it went.

As for the carrier, good description. Aren't there some airports that are at high altitude and maybe in theory if you went over the cliff, you'd have enough altitude to get the engines started before you hit the ground?


So, if by engines blowing out, you mean compressor stalls, sure. Lots of times. Read my F-14 thread. The compressor section in a jet engine is basically a set of little wings that move air. Just like wings, those blades can stall - and the airflow stops moving through the compressor evenly and smoothly. That causes the whole engine to stop running. In fighters, it happens when pilots slam the throttles (for tactical/maneuvering reasons) or are at high AOA, or the engine ingests missile exhaust gas.

To restore normal air flow and get the engine running again, you either have to bring the power back to idle, or shut the engine off. Either method reduces the amount of fuel injected into the combustion section, to reduce pressure there, so that normal air flow can take place.

Compressor stalls happen on commercial airplanes, but are quite uncommon. The engine will "bang" and you may see flames coming out the front end (that's not good) as the pressure in the combustion section escapes out both ends because the compressor isn't forcing air to flow through as it should. Again, idle, or off, usually restores normal flow. Sometimes you'll get a compressor stall that's due to a mechanical failure in the engine, in which case, you won't get a good running engine again.

I'll add that it's not uncommon to see "smoke" from a jet engine during start. It's not "smoke" at all, but unburned kerosene vapor. It can take several seconds between the introduction of fuel into the engine and the ignition of that fuel, so, during that period, you'll see the vapor of unburned jet fuel coming out the back. If it's a really long time between fuel and ignition, you can get an "engine torch" which is a flame coming from the back. Looks scary, but since the engine runs on fire, this is just a fire that's a bit out of its normal area. The risk with a torch is that the start is usually a hot one, that is, the EGT is going to exceed limits, because, well, there's a bunch of extra fuel in there with a slow ignition or with a fuel control malfunction.

However, to a flight attendant, who doesn't remember the words "engine torch" from training, it might look like the airplane is on fire. Might look like that to a passenger, too, so while it's not a big deal (unless you overtemp the engine, in which case, you're going back to the gate) mechanically, the ensuing panic in the back can, and has in the past, caused problems as passengers try to escape the "fire"...opening doors, inflating slides, getting hurt, running around on the ramp with moving airplanes and jet engines capable of sucking them up and killing them...a real mess...

For power plant turbine failures, you should ask Shannow. He posted some pictures up a few years ago. Several tons of high-RPM mass can create spectacular damage. I'm reticent to even discuss it because he's the expert and that last thread was an epic BITOG devolution/spiral into pointless argument as Shannow and I tried to teach physics...

Here it is: https://www.bobistheoilguy.com/forums/ubbthreads.php/topics/2217857/1

The "starting the engines after flying off a cliff" works well in James Bond movies, perhaps, but there are so many variables involved, it's hard to say that you could. A piston engine airplane might get enough windmill RPM to start up, and it takes only a second or so from start to full power in a piston, so, your odds might be better with that than with a jet. Courchevel, France comes to mind as a place where you could roll down the runway, fly off the end, and might have enough altitude to get it going...

 

[john_pifer]

Originally Posted by Astro14


I should add that I've seen two jet starter turbines blow up.



We have one of those at work, but never use it. I've noticed that it has a red line painted on the side (I assume this is where the turbine section is) that advises to stay out of that area.

The thing's sat outside for the 5 years I've been there. It's probably locked up. Shame. Probably cost tens of thousands of dollars.

 

[MolaKule]

I would say 15 minutes for a ground start wait was a rather short time in my experience.

 

[Astro14]

Originally Posted by john_pifer
Originally Posted by Astro14


I should add that I've seen two jet starter turbines blow up.



We have one of those at work, but never use it. I've noticed that it has a red line painted on the side (I assume this is where the turbine section is) that advises to stay out of that area.

The thing's sat outside for the 5 years I've been there. It's probably locked up. Shame. Probably cost tens of thousands of dollars.


I would bet that it cost at least that, if not more...

I should clarify that I meant the starters on the engine. Not the supply cart, but the little turbines on the engine...

Lot of stress on those parts.