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Blackbird
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Ground Idle Vs Flight Idle

Fri May 02, 2008 7:02 pm

Why do aircraft have two idle settings? Why is there a ground idle and a flight-idle?


Andrea Kent
 
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SEPilot
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RE: Ground Idle Vs Flight Idle

Fri May 02, 2008 8:18 pm

Could it be that ground-idle is so low that there is more of a risk of flame-out, and thus is inadvisable to use in flight? As one used to piston engines, I have no experience with jets.
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Blackbird
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RE: Ground Idle Vs Flight Idle

Fri May 02, 2008 8:25 pm

I thought the faster you went the more thrust you produced?

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pilotpip
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RE: Ground Idle Vs Flight Idle

Fri May 02, 2008 8:51 pm

We have three, add one for icing conditions.

Just thought I'd throw that one out there.
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Blackbird
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RE: Ground Idle Vs Flight Idle

Fri May 02, 2008 9:01 pm

Pilotpip,

What is that setting called?
 
113312
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RE: Ground Idle Vs Flight Idle

Fri May 02, 2008 9:22 pm

On many modern turbofan engines, the terms "Flight Idle" and "Ground Idle" are a bit confusing. They really should be Low Idle and High Idle. Most engines are set up to produce Low Idle on the ground and in normal flight. This is termed "Ground Idle" The High Idle setting is set inflight (Flight Idle) so that the engine is spooled up in the event of a missed approach and for a short duration just after touchdown to facilitate rapid reverse thrust.

Logic circuits determine that the aircraft is in the air and in the approach and landing configuration to command the increase to "Flight Idle". The idle setting is returned to the low, or "Ground Idle" value after a predetermined time interval after the landing gear touchdown.

I believe that the terminology is a carryover from the earlier turboprop days that strictly used Flight and Ground Idle.
 
Blackbird
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RE: Ground Idle Vs Flight Idle

Fri May 02, 2008 9:39 pm

How do the logic-circuits determine that the plane is in the air and not in the approach and landing configuration, and in the air in the approach and landing configuration? (I would guess flap position, altitude, and AoA/Airspeed?)


Andrea Kent

[Edited 2008-05-02 14:40:36]
 
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fr8mech
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RE: Ground Idle Vs Flight Idle

Fri May 02, 2008 9:41 pm

Flight idle is designed to allow the engine to go to the higher power settings faster.

As you may, or may not, know, high bypass engines have air management issues. That's the air within the core engine. Between the VSV's (variable stator vanes) and VBV's (variable bleed valves) or whatever the particular manufacture calls these features and other air management components, there is a lot of movement within the engine to move the air through as efficiently as possible.

Now the engine is optimized for high power settings, so at ground idle, the VSV's are mostly closed and VBV's and bleed valves are generally open. At flight idle, they are moving to their high power positions. In fact, the threat of stall/surge is greatly diminished at or above flight idle.

Modern, FADEC engines are great at controlling chapter 75 (engine air). A PW20XX can go from flight idle to 95Z% take-off in 2 seconds. In fact 2 seconds is the max allowed for ground testing after engine change (as I recall). Ground idle to 95% take-off takes 4-6 seconds.

On the JT9D-7A/AH, we would move the throttle very slowly until we passed flight idle (about 75% N2, as I recall). We'd speed up a little until bleedshift (about 1.21 EPR or so) and then as quick as we wanted to take-off power. Two reasons: one was that without the aircraft moving and forcing the air down the inlet, the engine was very susceptible to inlet disturbances. The second was that at the lower power settings, the hydro-mechanical jet fuel control (JFC), the engine vane control (EVC) and various other components would take a little time to catch up to what the throttle wanted the engine to do.

Long story, short: by starting at flight idle, the engine acheives higher power settings, faster.
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Tornado82
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RE: Ground Idle Vs Flight Idle

Fri May 02, 2008 9:49 pm



Quoting Blackbird (Reply 6):
How do the logic-circuits determine that the plane is in the air and not in the approach and landing configuration, and in the air in the approach and landing configuration? (I would guess flap position, altitude, and AoA/Airspeed?)

My guess would be a squat sensor in the gear. Similar to the lockouts for thrust reversers on some models.
 
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RE: Ground Idle Vs Flight Idle

Fri May 02, 2008 9:50 pm



Quoting Fr8mech (Reply 7):
A PW20XX can go from flight idle to 95Z% take-off in 2 seconds. In fact 2 seconds is the max allowed for ground testing after engine change (as I recall).

I don't know the PW2000, but every engine I have worked on the limit is 6 secs. In fact it is a CAA FAA regulation that the engine should go from idle to take off in 6 secs for a missed approach. Idle on the ground is set as low as possible for low noise and fuel saving, so for the missed approach case the idle is raised to High Idle.
On older engines with mechanical systems this was usually a flight idle solenoid that was only energised on the ground for low idle. On FADEC aircraft more parameters are used, Flaps at approach setting is used on the Trent.
This 6 sec accel is part of every engine acceptance test. Doing it on the wing on a big fan engine can be tricky!!
 
KELPkid
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RE: Ground Idle Vs Flight Idle

Fri May 02, 2008 10:21 pm



Quoting Tornado82 (Reply 8):
My guess would be a squat sensor in the gear. Similar to the lockouts for thrust reversers on some models.

I'd hope that there's more to the ground sense logic than that. Squat switches are notoriously unreliable in GA planes. I have personally witnessed a gear retraction and prop strike in a Cessna 182RG that should not have happened (the squat switch was somehow thinking "in the air", and the doofus taxiing her around moved the gear handle to "up..."  sarcastic  ).
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fr8mech
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RE: Ground Idle Vs Flight Idle

Fri May 02, 2008 10:35 pm



Quoting TristarSteve (Reply 9):
In fact it is a CAA FAA regulation that the engine should go from idle to take off in 6 secs for a missed approach

I just checked the AMM and you're right, actually you're off by .2 seconds. The PW20XX limit is 5.8 seconds from approach idle (flight) to 95% take-off. I believe the fastest I saw was 2 or so seconds.

That bumps up the ground to 95% time. Next time I'm involved with an engine run (few and tooooo far in between, now days), I'll time ground to flight idle. I'm thinking 3 or 4 seconds.
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pilotpip
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RE: Ground Idle Vs Flight Idle

Fri May 02, 2008 11:56 pm



Quoting KELPkid (Reply 10):

I'd hope that there's more to the ground sense logic than that. Squat switches are notoriously unreliable in GA planes. I have personally witnessed a gear retraction and prop strike in a Cessna 182RG that should not have happened (the squat switch was somehow thinking "in the air", and the doofus taxiing her around moved the gear handle to "up..." ).

There's built in redundancy for that reason. On the 170 the aircraft is able to compare the other switches and throw up to one out if it disagrees. On the 145 you'd get the "LG Gear Disagree" when doing a good crosswind landing. I hated that. You'd be all focused on using good technique and midway through touchdown the master caution starts flashing at you.
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HAWK21M
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RE: Ground Idle Vs Flight Idle

Sat May 03, 2008 4:48 am

Flight Idle is at a slightly higher thrust than Ground Idle to cater to a Go around if needed.The signal comes from the L/G with Time delay,& flap position.
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IFixPlanes
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RE: Ground Idle Vs Flight Idle

Sat May 03, 2008 6:00 am



Quoting HAWK21M (Reply 13):
The signal comes from the L/G with Time delay,& flap position.

What aircraft are you referring to?
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FredT
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RE: Ground Idle Vs Flight Idle

Sat May 03, 2008 6:53 am



Quoting KELPkid (Reply 10):
I'd hope that there's more to the ground sense logic than that.

Many/most aircraft have wheel spin up sensors as well. You already have most of it in place for the autoskid anyway, so why not?

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HAWK21M
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RE: Ground Idle Vs Flight Idle

Sat May 03, 2008 7:57 am



Quoting IFixPlanes (Reply 14):
What aircraft are you referring to?

On the B752,the idle reset solenoid sets high or low idle.

High idle is used only during landings and when engine cowl A/I is on in flight [extra bleed extracted].High idle is used for descent in ice conditions to prevent possible engine problems.High idle is used during landings to reduce the time for engine to spool up to max rpm.

The idle reset solenoid on the FFG when deenergized sets Flight idle position by approx 2 secs.

Lowering the flaps to landing position,turn on Engine A/I,or lower the Gear & idle reset solenoid DeEnergizes to Flight Idle position.On Touchdown & with 5 sec T/D the Engine idle control relay energizes,this gives 5 secs if a Go around needs to be accomplished,the spool up time in flight idle is 4 secs & Ground idle is 6 secs,from Idle to T/O condition.

regds
MEL
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Starglider
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RE: Ground Idle Vs Flight Idle

Sat May 03, 2008 9:49 am

On the B777, two different idle modes are available and automatically selected by the EEC.

"Minimum idle" and "approach idle."

Minimum idle has a lower thrust setting for ground operation and most phases of flight.

Approach idle has a higher thrust setting, and therefore, decreases acceleration time for a go-around.

Approach idle is automatically selected in flight when:
- The flaps are commanded to the landing position
or when:
- Engine ant-ice is on.


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Jetlagged
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RE: Ground Idle Vs Flight Idle

Sat May 03, 2008 3:26 pm

The 747 Classic used landing gear tilt switches and flap position, plus a timer. The tilt switches detect whether the aircraft is in flight. If in flight with TE Flaps set to Landing position (i.e. greater then 23 deg) the engines are set to flight idle. At touchdown, detected by the tilt switches, a timer starts and after 6 seconds the engines switch to ground idle. The 6 second delay is to account for the possibility of a go around after touchdown, and also to help acceleration into full reverse thrust. A GRD IDLE caution light illuminates on the ground any time an engine is at ground idle and the flaps are greater than 23 degrees (landing positions being 25 and 30).

If the logic was purely off flap position the landing roll with no reverse thrust would be increased considerably, due to the higher thrust from flight idle.

The only turbofan engined airliner I know of where ground/flight idle switching is manual is the BAe 146. The thrust levers are moved back from the flight idle detent to the ground idle stop when on ground (inhibited in the air IIRC).
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David L
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RE: Ground Idle Vs Flight Idle

Sat May 03, 2008 6:22 pm

I see why you'd want the engines to be able to spool up from idle to go-around power in less than 6 seconds but, in practice, how often are the engines running at the higher flight idle when the aircraft is in the landing configuration?

As I understand it, the thrust will be relatively high anyway to overcome the high drag of the landing configuration but you'd tend to be aiming for a target airspeed at that point. I'd assume that you'd normally be producing more thrust than the minimum available, in case you need to lose a few knots. I'd also assume that flight idle is designed to be less than the "typical" average thrust used in the landing configuration (or vice versa). Is the higher flight idle simply to ensure that the thrust never drops so low that you lose the ~6 second spool-up time?
 
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RE: Ground Idle Vs Flight Idle

Sat May 03, 2008 8:19 pm



Quoting David L (Reply 19):
Is the higher flight idle simply to ensure that the thrust never drops so low that you lose the ~6 second spool-up time?

In a word Yes.
Ground idle is not a lot lower than flight idle, but on an engine with a small surge margin like the RB211-22B, the accel time from ground idle to flight idle can be 10 secs or so, then only 6 secs to take off.
 
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RE: Ground Idle Vs Flight Idle

Sat May 03, 2008 10:32 pm



Quoting TristarSteve (Reply 20):
In a word Yes.

Thanks. The reason I stressed "never" is just to clarify that the "spoolability" of flight idle is only relevant during those moments when the thrust is reduced to lose speed on approach. I'm just trying to get it clarified here because of previous threads (in non-aviation) that implied that flight idle was the main reason engines are ready spool up quickly to GA power in the landing configuration. It just seems to me that, most of the time on approach, it's the high drag configuration that's responsible for the high thrust and resultant readiness to accelerate to GA power. I gave up looking for clarification in those threads.  Smile
 
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RE: Ground Idle Vs Flight Idle

Sat May 03, 2008 11:45 pm



Quoting Starglider (Reply 17):
Approach idle has a higher thrust setting, and therefore, decreases acceleration time for a

Exactly...Some Fedex 727's have this option..The 2S2F 727's with the Valsan mod have the approach idle function it is basically another solenoid on the fuel control which bumps up the power in the event that a go around is needed.
 
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RE: Ground Idle Vs Flight Idle

Sun May 04, 2008 5:59 am



Quoting David L (Reply 19):
Is the higher flight idle simply to ensure that the thrust never drops so low that you lose the ~6 second spool-up time?

True.On a GA you would want to get Airborne faster.Hence.
regds
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tdscanuck
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RE: Ground Idle Vs Flight Idle

Thu May 08, 2008 5:21 am



Quoting David L (Reply 19):
I see why you'd want the engines to be able to spool up from idle to go-around power in less than 6 seconds but, in practice, how often are the engines running at the higher flight idle when the aircraft is in the landing configuration?

Every single flight...they go back to flight idle as they enter the flair.

Tom.
 
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RE: Ground Idle Vs Flight Idle

Thu May 08, 2008 5:57 am



Quoting SEPilot (Reply 1):
Could it be that ground-idle is so low that there is more of a risk of flame-out, and thus is inadvisable to use in flight?

Ground idle in flight was way too long expression to be used during fast cockpit verbal comunication, so another nickname is been invented by FAA. it's "STALL".
 
miamiair
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RE: Ground Idle Vs Flight Idle

Thu May 08, 2008 11:10 am

Toss in a screwball here:

Low-speed Ground Idle

On the C-130 you can select this setting, but you won't get juice out of the generators, so usually 1 and 4 will go to LSGI.
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David L
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RE: Ground Idle Vs Flight Idle

Thu May 08, 2008 11:23 am



Quoting Tdscanuck (Reply 24):
Every single flight...they go back to flight idle as they enter the flair.

Ah, I meant what proportion of the descent is in flight idle? Sorry.  Smile
 
DeltaGuy
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RE: Ground Idle Vs Flight Idle

Thu May 08, 2008 12:36 pm

Flight idle settings are better than what alot of aircraft have had to use for approaches, at least in the military sense. Older aircraft powered by turbojets, like the A-3 Skywarrior, A-6 Intruder, etc etc, used to have to keep speedbrakes out on approach, and keep the engines spooled up, in case of a G/A...the aircraft would simply not have enough response time for a GA.

In the Gulfstream, ground idle is around 46.6, and flight idle is around 67%.

DeltaGuy
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tdscanuck
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RE: Ground Idle Vs Flight Idle

Thu May 08, 2008 2:35 pm



Quoting Tdscanuck (Reply 24):
Quoting David L (Reply 19):
I see why you'd want the engines to be able to spool up from idle to go-around power in less than 6 seconds but, in practice, how often are the engines running at the higher flight idle when the aircraft is in the landing configuration?

Every single flight...they go back to flight idle as they enter the flair.



Quoting David L (Reply 27):

Ah, I meant what proportion of the descent is in flight idle? Sorry.

Probably not a tremendously high proportion now, since you usually spool up some as you enter landing configuration to counter the increased drag. However, continuous descent approaches are starting to show up which, at least in theory, would allow you to hold flight idle all the way to touchdown.

Tom.
 
David L
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RE: Ground Idle Vs Flight Idle

Thu May 08, 2008 4:21 pm



Quoting Tdscanuck (Reply 29):
Probably not a tremendously high proportion now, since you usually spool up some as you enter landing configuration to counter the increased drag.

That's what I thought. Sorry to hijack the thread but I wanted to clear up some "misinformation" I'd seen elsewhere.  Smile

Quoting Tdscanuck (Reply 29):
However, continuous descent approaches are starting to show up which, at least in theory, would allow you to hold flight idle all the way to touchdown.

I've heard a bit about those but I wasn't sure whether they are designed to use flight-idle or just a constant "lowish" thrust. Pihero gave us a detailed link about it but it was a while ago and I've forgotten most of it.  duck 
 
UAL747
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RE: Ground Idle Vs Flight Idle

Thu May 08, 2008 5:21 pm

So, how many pounds of thrust would a GE-90-115B be producing during cruise at normal conditions and altitude? I assume it's quite a bit less than the max.

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Jetlagged
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RE: Ground Idle Vs Flight Idle

Fri May 09, 2008 4:50 pm



Quoting UAL747 (Reply 31):
So, how many pounds of thrust would a GE-90-115B be producing during cruise at normal conditions and altitude? I assume it's quite a bit less than the max.

Not only will the engines be running at less than takeoff power in percentage terms during cruise, but at cruise altitude the air is much less dense. At 35,000 feet air density is about 30% of the sea level value, so thrust and drag will be about 30% of their sea level values, all other things being equal.
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UAL747
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RE: Ground Idle Vs Flight Idle

Fri May 09, 2008 11:24 pm



Quoting Jetlagged (Reply 32):
At 35,000 feet air density is about 30% of the sea level value, so thrust and drag will be about 30% of their sea level values, all other things being equal.

True, and I know engines are supposed to be more effecient at cruising altitude, but don't they perform better at sea level in colder temperatures? Hence the reason for the 16,000ft Runway at DEN? If the air is less dense, it would seem that the engine would somehow have to compensate to get enough air jammed into the compressors to produce the same amount of thrust as it would at sea level.

I know I'm wrong, but can you see my logic?

UAL
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roseflyer
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RE: Ground Idle Vs Flight Idle

Fri May 09, 2008 11:54 pm



Quoting Blackbird (Reply 6):
How do the logic-circuits determine that the plane is in the air and not in the approach and landing configuration, and in the air in the approach and landing configuration? (I would guess flap position, altitude, and AoA/Airspeed?)

On a 737 in air mode requires the gear struts to not be compressed. That is all that is needed for the plane to be in air.

There is a lot of logic that goes on such as flap position, wheel speed, gear position, and engine thrust. The PSEU monitors various parameters to decide whether the plane is in air or on the ground and what configuration it is in. It's fun to design tests to trick the plane to do things that it is not intended to do.
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KELPkid
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RE: Ground Idle Vs Flight Idle

Sat May 10, 2008 12:56 am



Quoting RoseFlyer (Reply 34):
It's fun to design tests to trick the plane to do things that it is not intended to do.

Wasn't there a 727 incident around 1979/1980 where the crew got the plane to roll over and enter a high speed dive because the captain pulled enough circuit breakers to trick the plane into allowing the spoilers to be deployed in flight or something similar?
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jetmech
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RE: Ground Idle Vs Flight Idle

Sat May 10, 2008 2:11 am

Quoting UAL747 (Reply 31):

According to the GE website ( http://www.geae.com/education/engines101/ ), the 115B has a cruise thrust of 11,000 lbs, an idle thrust upon landing (flight idle?) of 8,350 lb's, and a ground idle thrust of 3,080 lb's

Quoting Jetlagged (Reply 32):

You also lose thrust as the aircraft ( and engine ) gain forward velocity. The forward velocity of the engine gradually reduces the amount by which the engine can increase the velocity (accelerate) of the air that passes through it, thus, thrust drops.

Thrust (Newtons) = mass flow rate (kg/s) * (exhaust velocity - inlet velocity) (m/s)

Regards, JetMech

[Edited 2008-05-09 19:27:32]
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Tristarsteve
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RE: Ground Idle Vs Flight Idle

Sat May 10, 2008 11:31 am



Quoting KELPkid (Reply 35):
Wasn't there a 727 incident around 1979/1980 where the crew got the plane to roll over and enter a high speed dive because the captain pulled enough circuit breakers to trick the plane into allowing the spoilers to be deployed in flight or something similar?

The flight engineer had gone to the toilet. The Capt pulled the slat cbs and then extended the flaps a little. He thought that it would save fuel. The flight engineer got back in his seat, and reset the tripped cb without asking. The slats extended and all hell broke loose.
 
DeltaGuy
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RE: Ground Idle Vs Flight Idle

Sat May 10, 2008 1:35 pm



Quoting Blackbird (Reply 6):
How do the logic-circuits determine that the plane is in the air and not in the approach and landing configuration, and in the air in the approach and landing configuration? (I would guess flap position, altitude, and AoA/Airspeed?)

Many aircraft have a WOW switch (and no, that's not a switch that expresses amazement  Wink), or Weight on Wheels...they'll detect strut compression, or wheel spin up. That's a hard indication that the aircraft is indeed on the ground, ground idle can be applied, and that the ground spoilers can be extended. Most Gulfstreams use a system called the 'Nutcracker', because the switch on each main strut almost resemble nutcrackers. The system requires both to sense being on the ground, but only one needs to sense being in the air for the air mode to engage.

There was an incent at PBI some years ago with a GV, on short final, where the ground spoilers deployed and the aircraft dropped 100' out of the sky, pretty much driving the struts through the wings. Why? The nutcrackers sensed ground mode, because a mechanic had put popsicle sticks through them to trick them into thinking they were in the air mode while the a/c was on jacks. When the crew went to takeoff, the gear wouldn't come up (nutcrackers still think theyre in ground mode), so when they went to land, and power was pulled to idle, the g/s's came up. Messy.

So, point is, the logic systems behind the flight idle and configuration changes can be tricky, paying attention to it and the horror stories during ground school is mucho important.

DeltaGuy
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Jetlagged
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RE: Ground Idle Vs Flight Idle

Sat May 10, 2008 3:23 pm



Quoting TristarSteve (Reply 37):
Quoting KELPkid (Reply 35):
Wasn't there a 727 incident around 1979/1980 where the crew got the plane to roll over and enter a high speed dive because the captain pulled enough circuit breakers to trick the plane into allowing the spoilers to be deployed in flight or something similar?

The flight engineer had gone to the toilet. The Capt pulled the slat cbs and then extended the flaps a little. He thought that it would save fuel. The flight engineer got back in his seat, and reset the tripped cb without asking. The slats extended and all hell broke loose.

And nothing to do with ground/flight sensing logic.
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UAL747
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RE: Ground Idle Vs Flight Idle

Sat May 10, 2008 8:51 pm



Quoting JetMech (Reply 36):
According to the GE website ( http://www.geae.com/education/engines101/ ), the 115B has a cruise thrust of 11,000 lbs, an idle thrust upon landing (flight idle?) of 8,350 lb's, and a ground idle thrust of 3,080 lb's

Wow, that is amazing that it only requires 22,000lbs of thrust out of the 230,000lbs available to keep that big of an aircraft going about 550MPH! No wonder the thing is a rocket on take offs!

UAL
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tdscanuck
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RE: Ground Idle Vs Flight Idle

Sat May 10, 2008 11:18 pm



Quoting UAL747 (Reply 33):
True, and I know engines are supposed to be more effecient at cruising altitude, but don't they perform better at sea level in colder temperatures?

They produce most thrust at dense air (sea level/cold) conditions...that's not necessarily the same thing as most efficient (max SFC).

Tom.
 
EssentialPowr
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RE: Ground Idle Vs Flight Idle

Mon May 12, 2008 2:56 am



Quoting Blackbird (Thread starter):
Why do aircraft have two idle settings? Why is there a ground idle and a flight-idle?

I didn't read any of these posts, so forgive me...

"Idle" for a gas turbine really has no meaning; they are steady state, continuous cycle engines that do not have discrete points in their PV diagrams, unlike recip engines....

With me??


Power levels are scheduled higher for in flight regimes as opposed to ground, as the engine typically has higher loads in flight as opposed to on the ground. Selection of anti ice for ex, requires quite a bit of power. The engine must be scheduled to a higher "idle" power level in flight to account for this instant demand and still provide adequate stall and acceleration margins.
 
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fr8mech
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RE: Ground Idle Vs Flight Idle

Mon May 12, 2008 3:06 am



Quoting EssentialPowr (Reply 42):
"Idle" for a gas turbine really has no meaning

The maintenance mauals I'm familiar with all refer to ground idle and flight (or approach) idle.

As I recall, the introduction of anti-ice (for example) does not change the idle speed, but does require a power adjustment by the flight crew. This is indicated by the magenta caret on the B757/B767 and a low N1 light on the classic jumbos. Maybe some of the later generation aircraft make the adjustment to the idle setting.
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VC-10
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RE: Ground Idle Vs Flight Idle

Mon May 12, 2008 4:07 am

From the A346 FCOM

Modulated Idle -

Selected - in Flt, when the flaps are retracted and the gear is up.
On ground, provided reverse is not selected

Approach Idle -

Selected - In flight, when the falps are extended to Flap 2, Flap 3 0r full, or when the landing gear is down
Varies only in relation to altitude
Allows the engine to rapidly accelerate from idle to go-around.

Anti-ice

When anti-ice is selected, wing and/or engine, the idle N1 is automatically increased
 
tdscanuck
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RE: Ground Idle Vs Flight Idle

Mon May 12, 2008 5:01 am



Quoting EssentialPowr (Reply 42):

"Idle" for a gas turbine really has no meaning; they are steady state, continuous cycle engines that do not have discrete points in their PV diagrams, unlike recip engines....

With me??

How does "idle" have any more meaning for a recip engine? Most recips can run slower than their set idle points...it's just a semi-arbitrary throttle setting (and usually adjustable).

Quoting EssentialPowr (Reply 42):

Power levels are scheduled higher for in flight regimes as opposed to ground, as the engine typically has higher loads in flight as opposed to on the ground. Selection of anti ice for ex, requires quite a bit of power. The engine must be scheduled to a higher "idle" power level in flight to account for this instant demand and still provide adequate stall and acceleration margins.

You can turn anti-ice on on the ground...if you had to change the idle schedule for anti-ice (on a FADEC engine, you don't have to), you'd have to have the same change on the ground as you do in flight.

Tom.
 
EssentialPowr
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Joined: Wed Sep 06, 2000 10:30 pm

RE: Ground Idle Vs Flight Idle

Mon May 12, 2008 10:26 pm



Quoting Fr8mech (Reply 43):
The maintenance mauals I'm familiar with all refer to ground idle and flight (or approach) idle.

Right, as a practical term.

Quoting Fr8mech (Reply 43):
As I recall, the introduction of anti-ice (for example) does not change the idle speed, but does require a power adjustment by the flight crew.

On the 737, in flight idle is higher than on the ground by design.

Quoting Tdscanuck (Reply 45):
How does "idle" have any more meaning for a recip engine? Most recips can run slower than their set idle points

I didn't say that. I said:

Quoting EssentialPowr (Reply 42):
"Idle" for a gas turbine really has no meaning; they are steady state, continuous cycle engines that do not have discrete points in their PV diagrams, unlike recip engines....



Quoting Tdscanuck (Reply 45):
You can turn anti-ice on on the ground...if you had to change the idle schedule for anti-ice (on a FADEC engine, you don't have to), you'd have to have the same change on the ground as you do in flight.

Of course! But doesn't it make sense to schedule the engine to a higher idle power setting in flight to provide for increased engine stall margin and better acceleration when engine power level is low and bleed demand is high? Of course. Even so, the erj has an warning that will pop up in flight when bleed demand is greater than supply, even though those engines are scheduled for a higher flight idle than ground.
 
tdscanuck
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RE: Ground Idle Vs Flight Idle

Tue May 13, 2008 4:10 am



Quoting EssentialPowr (Reply 46):
Quoting Tdscanuck (Reply 45):
How does "idle" have any more meaning for a recip engine? Most recips can run slower than their set idle points

I didn't say that. I said:

Quoting EssentialPowr (Reply 42):
"Idle" for a gas turbine really has no meaning; they are steady state, continuous cycle engines that do not have discrete points in their PV diagrams, unlike recip engines....

I don't really see the difference. You said idle has no real meaning for a gas turbine, and contrasted it to a recip engine. That seems to imply that idle does have meaning for a recip engine.

Tom.
 
EssentialPowr
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Joined: Wed Sep 06, 2000 10:30 pm

RE: Ground Idle Vs Flight Idle

Tue May 13, 2008 4:25 am



Quoting Tdscanuck (Reply 47):
You said idle has no real meaning for a gas turbine

I would call it a power setting.

Quoting Tdscanuck (Reply 47):
That seems to imply that idle does have meaning for a recip engine.



Quoting Tdscanuck (Reply 47):
I don't really see the difference

You don't see the difference between a Brayton (gas turbine) cycle and an Otto cycle? Why would a recip in a lot of applications require a flywheel? In other words, a fly wheel is just added mass. Why is there such a need (yes, not an a/c application, but neither a recip or gas turbine are a/c specific)??
 
tdscanuck
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Joined: Wed Jan 11, 2006 7:25 am

RE: Ground Idle Vs Flight Idle

Tue May 13, 2008 5:41 am



Quoting EssentialPowr (Reply 48):
Quoting Tdscanuck (Reply 47):
That seems to imply that idle does have meaning for a recip engine.

Quoting Tdscanuck (Reply 47):
I don't really see the difference

You don't see the difference between a Brayton (gas turbine) cycle and an Otto cycle?

Obviously, there are differences between the Brayton and Otto cycles. However, I have no idea how that relates to the concept you put forward that "idle" doesn't have meaning for gas turbines.

Tom.

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