c5load
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3-Stage Engine Spool-Ups

Fri Jan 01, 2010 10:22 pm

I wasn't sure which forum this belonged to, but I was watching a few take-off vids and noticed that most airplanes didn't just spool up to takeoff power right away. They went to about 50%, then 75%, then takeoff power. Why the increments to spool-up?
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ANITIX87
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RE: 3-Stage Engine Spool-Ups

Sat Jan 02, 2010 12:03 am

As far as I know, it's one spool to around 70% N1, check EPR and some other engine parameters, and if everything checks out, release brakes and continue to around N1 for takeoff. Then, once rolling, switch to pre-set TO/GA power.

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tdscanuck
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RE: 3-Stage Engine Spool-Ups

Sat Jan 02, 2010 12:06 am



Quoting C5LOAD (Thread starter):
They went to about 50%, then 75%, then takeoff power. Why the increments to spool-up?

I'm not sure about having two intermediate steps, but you'll see at least one really often because jet engines have a really non-linear spool up curve. They accelerate much faster at higher power settings than at low. As a result, if you go straight from idle to takeoff thrust, you can get big thrust asymmetries since the spool up times can be quite different at the low speed ranges (this is less of an issue with FADEC, but it can still happen). Once they're up to a fairly high power setting, the response is much faster and that's usually when you kick in the autothrottle to take it up to takeoff power (if you have an autothrottle).

You also want to make sure the engines are coming up and behaving well before you commit to the takeoff roll and go to full power.

The second pause (around 75% that you saw) may be for a rolling takeoff...the first would be to do the spool up check, the second would be to get rolling before going to full throttle. With aft CG, you may not be able to go to full throttle until you've got some airspeed to provide pitch authority.

Tom.
 
Fly2HMO
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RE: 3-Stage Engine Spool-Ups

Sat Jan 02, 2010 4:31 am

In my ex-airline it was 3 steps for the 737 fleet.

Power up to 40%, look for normal,stable engine indications, then up to 75% and then finally push the TOGA buttons and let the auto throttles do the rest.

Way back in the day before the advent of FADEC just jamming the power on full could create all sorts of problems for the engine. But with FADEC you *could* whip the throttles back and forth all you want and the computer will *hopefully* make sure nothing breaks.

[Edited 2010-01-01 20:33:31]
 
Flighty
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RE: 3-Stage Engine Spool-Ups

Sat Jan 02, 2010 4:31 am

Starting more gently also prevents a giant fuel slosh from happening. Just a thought.
 
Mender
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RE: 3-Stage Engine Spool-Ups

Sat Jan 02, 2010 11:01 am

Although it really depends on aircraft type, you usually cannot safely achieve take off power without causing a surge if the wind direction is not virtually head on. The down wind engine is shielded by the fuselage, thus is kind of starved of air.

I guess it would be standard practice to use an incremental increase in thrust with the brakes released so that the ram air effect of moving forward will reduce the chance of an engine surge. Basically the pilot has one less thing to worry about.
 
Fly2HMO
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RE: 3-Stage Engine Spool-Ups

Sat Jan 02, 2010 9:26 pm



Quoting Flighty (Reply 4):
Starting more gently also prevents a giant fuel slosh from happening. Just a thought.

I fail to see how that's an issue. It's certainly not an issue under severe turbulence or during an RTO, and your average takeoff has much less acceleration forces than either of those events.
 
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Jetlagged
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RE: 3-Stage Engine Spool-Ups

Sat Jan 02, 2010 9:42 pm



Quoting Flighty (Reply 4):
Starting more gently also prevents a giant fuel slosh from happening. Just a thought.

Sooner or later you reach takeoff thrust at which point the slosh effect is the same whether you got there rapidly or not. Also if the tanks are full (or nearly full) there is no fuel slosh. Finally, fuel tanks have baffles to limit fuel slosh effects.
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jetlife2
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RE: 3-Stage Engine Spool-Ups

Sun Jan 03, 2010 6:27 pm



Quoting Mender (Reply 5):
Although it really depends on aircraft type, you usually cannot safely achieve take off power without causing a surge if the wind direction is not virtually head on. The down wind engine is shielded by the fuselage, thus is kind of starved of air.

Sorry but that is not accurate. Modern turbofans have to demonstrate stall-free operation in significant crosswinds. It is a certification requirement and part of FAR33. Imagine the operational consequences if that were true.
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Tristarsteve
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RE: 3-Stage Engine Spool-Ups

Sun Jan 03, 2010 7:43 pm



Quoting Jetlife2 (Reply 8):
Quoting Mender (Reply 5):
Although it really depends on aircraft type, you usually cannot safely achieve take off power without causing a surge if the wind direction is not virtually head on. The down wind engine is shielded by the fuselage, thus is kind of starved of air.

Sorry but that is not accurate. Modern turbofans have to demonstrate stall-free operation in significant crosswinds. It is a certification requirement and part of FAR33. Imagine the operational consequences if that were t

Ever tried running a large engine at take off when static. It is very very susceptible to surge. You must point the aircraft into wind for a high power run.
I surged at PW4000 on a B744 recently with a 4 kt tail wind.

Now on the TriStar with the RB211-524 engine, it was impossible to reach take off power on nbr 2 engine when standing still. You needed about 50kts headwind before you could get take off. When we did installation ground runs we never went to take off. Max Continuos was the max allowed. The S duct would not funnel enough air into the engine without some ram effect!
 
tdscanuck
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RE: 3-Stage Engine Spool-Ups

Sun Jan 03, 2010 8:18 pm



Quoting Jetlife2 (Reply 8):
Sorry but that is not accurate. Modern turbofans have to demonstrate stall-free operation in significant crosswinds. It is a certification requirement and part of FAR33. Imagine the operational consequences if that were true.

I believe all the requirements are in flight...which means, minimum, about 100+ knot headwind. Aircraft inlets aren't particularly good at static performance...that's why you see bell-mouth inlets on static test stands.

Tom.
 
jetlife2
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RE: 3-Stage Engine Spool-Ups

Tue Jan 05, 2010 1:48 am



Quoting TristarSteve (Reply 9):
Ever tried running a large engine at take off when static. It is very very susceptible to surge. You must point the aircraft into wind for a high power run.

If this were true you could not take off in a cross wind. Large engines are tested with inlet distortion specifically to demonstrate that they operate successfully. Aircraft are certified for certain crosswind limits, for example 40kt. Those requirements in turn are placed on the engines. I quote from GE public domain material:

"The tests now going on at Peebles subject them to 90-degree crosswinds of up to about 70 miles per hour. That compares to a certification requirement of about 40 miles per hour and well beyond anything they’re likely to encounter in flight. The crosswinds are generated by a bank of 19 fans, each eight feet in diameter. The fans are arranged in a 50-foot-high, hexagon-shaped wind tunnel that’s mounted on tracks. It can be rotated 180 degrees, from directly in front of the engine – mounted on a stand 25 feet above the ground – to directly behind."

Quoting Tdscanuck (Reply 10):

Quoting Jetlife2 (Reply 8):
Sorry but that is not accurate. Modern turbofans have to demonstrate stall-free operation in significant crosswinds. It is a certification requirement and part of FAR33. Imagine the operational consequences if that were true.

I believe all the requirements are in flight...which means, minimum, about 100+ knot headwind.

Here are the requirements. The specific phrase "limiting inlet air distortion" can be approximately translated as "the certified crosswind limit'

§ 33.65 Surge and stall characteristics.

When the engine is operated in accordance with operating instructions required by §33.5(b), starting, a change of power or thrust, power or thrust augmentation, limiting inlet air distortion, or inlet air temperature may not cause surge or stall to the extent that flameout, structural failure, overtemperature, or failure of the engine to recover power or thrust will occur at any point in the operating envelope.
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DocLightning
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RE: 3-Stage Engine Spool-Ups

Tue Jan 05, 2010 11:34 pm



Quoting TristarSteve (Reply 9):

Now on the TriStar with the RB211-524 engine, it was impossible to reach take off power on nbr 2 engine when standing still. You needed about 50kts headwind before you could get take off. When we did installation ground runs we never went to take off. Max Continuos was the max allowed. The S duct would not funnel enough air into the engine without some ram effect!

Did the 727 have the same problem?
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Tristarsteve
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RE: 3-Stage Engine Spool-Ups

Wed Jan 06, 2010 4:48 am



Quoting DocLightning (Reply 12):
Did the 727 have the same problem?

The problem with the Tristar was that the original engine was the RB211-22B with 42000lbs. Four years later they fitted the RB211-524B with 52000lbs. The engines were the same size, and the new engine was fitted into the old airframes with the same S duct. The S duct could not funnel in the 25pc greater mass flow of air without some fwd aircraft speed.

Quoting Jetlife2 (Reply 11):
If this were true you could not take off in a cross wind.

Sorry, must have been imagining all those surges and rumbles. When I had my last surge, the MH station manager was on the flight deck with us, and he thought we had destroyed the engine! I managed to get the airport authority to change the ground run rules after that as they heard the surge in the terminal. (We turned the aircraft round and it worked well)
 
musang
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RE: 3-Stage Engine Spool-Ups

Wed Jan 06, 2010 11:18 am

On our 737 classics its effectively a three stage deal.

Stage one - manually set 40% N1,
Stage two - engage autothrust, which winds them up to within a few % of T/O thrust,
Stage three - the A/T fine tunes to T/O N1.

In icing conditions stage one is to set 70% manually in case there is ice on the fan blades etc, but many of the crews feel the need to set 40% first and when that's stable, set 70%, although its not required according to the SOPs.

Regards - musang
 
JETPILOT
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RE: 3-Stage Engine Spool-Ups

Wed Jan 06, 2010 6:14 pm

Jet engines when idling usually ingest more air through the low pressure compressor than the high pressure compressor can use. So they put in what is called interstage bleed valves. When the engine is spooled up the first time you are looking for indication on the EPER gauge that the IBV's have closed. If they have not closed and you go to full power the HP compressor will surge and scare the sh*t out of you. The second spool up, as already mentioned, is after manually pushing the throttle foward you press the TOGA switches on the auto throttle and the autothrottle spools them up to TO power.
 
jetlife2
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RE: 3-Stage Engine Spool-Ups

Fri Jan 08, 2010 4:18 am



Quoting JETPILOT (Reply 15):
So they put in what is called interstage bleed valves.

RR engines only (unless there is a PW case I am not aware of). Not GE (we use VSVs not IBVs)
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Tristarsteve
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RE: 3-Stage Engine Spool-Ups

Fri Jan 08, 2010 4:43 am



Quoting Jetlife2 (Reply 16):
Not GE (we use VSVs not IBVs)

So what do the VBV do on the Cf6 and CFM56?
 
tdscanuck
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RE: 3-Stage Engine Spool-Ups

Fri Jan 08, 2010 5:18 am



Quoting TristarSteve (Reply 17):
So what do the VBV do on the Cf6 and CFM56?

VBV's change how much air is moving through one of the compressors.

The basic problem is that you need to make sure the flow through the low and intermediate/high compressor are matched. You can either dump excess air overboard (IBV's) or control flow in one compressor to match the other one (VSV's).

Tom.
 
JETPILOT
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RE: 3-Stage Engine Spool-Ups

Fri Jan 08, 2010 8:57 am

I was told in ground school that both the JT3D-3B o nthe DC8 and the JT8D engines o nthe 727 had interstage bleed valves. On the 727 we do a 1.4 EPR check as we push the throttles up to make sure the bleed valve has closed and the needles jumps. On the DC8 we also had a 4 spooled and stable check to make sure the bleeds closed as well. And when it didn't and you went to TO power the engine compressor stalled.

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