Moderators: richierich, ua900, PanAm_DC10, hOMSaR

 
speedracer1407
Topic Author
Posts: 330
Joined: Fri Dec 24, 2004 4:19 pm

Flight Idle And Ground Idle

Wed Aug 16, 2006 5:01 pm

After reading a different thread, I was reminded that I still don't know the what accounts for the difference between flight and ground idle; hopefully, I have the terminology correct. I have a few questions:

Assuming flight Idle is generally higher than ground idle, what causes the difference? Is it part of FADEC software?

In non FADEC engines, is it still some kind of automatic mechanical thing?

Is it simply the result of aerodynamic forces on the engine in flight that makes the difference (windmilling, of some sort)?

is there a detent on the throttle quadrant for it?

Are quicker spool-up times from flight-idle to maximum power a primary reason for the difference, or as before, is it just a natural/practical consequence of high airspeed?

Thanks for any replies,

O
Dassault Mercure: the plane that has Boeing and Airbus shaking in their boots.
 
chksix
Posts: 336
Joined: Mon Sep 26, 2005 6:16 pm

RE: Flight Idle And Ground Idle

Wed Aug 16, 2006 6:01 pm

I'm interested to know the limits for the 737NG. I know ground idle is at 20% but can't find info on Flight Idle.

[Edited 2006-08-16 11:02:49]
The conveyor belt plane will fly
 
Tristarsteve
Posts: 3660
Joined: Tue Nov 22, 2005 11:04 pm

RE: Flight Idle And Ground Idle

Wed Aug 16, 2006 7:50 pm

Flight idle is always higher than ground idle.
Nowadays it is controlled by the FADEC, but on the RB211 there was a FI/GI solenoid connected to the FFR. Ground Idle is set as the slowest practical speed the engine will operate with bleeds on.
Flight idle is set top acheive the idle to take off acceleration speed required by regulations. FAR is 5 secs, and CAA used to be 6 secs. If the engine will not accelerate from ground idle in this time, then in flight the min speed is set to acheive this. It is very necessary in large fan aircraft where flight idle can be much higher than ground idle, but in small fans it is not always necessary. I await MEL to say something, but I can't remember it on the JT8D-15, or the Spey.
 
User avatar
HAWK21M
Posts: 30067
Joined: Fri Jan 05, 2001 10:05 pm

RE: Flight Idle And Ground Idle

Wed Aug 16, 2006 8:42 pm

From What I recollect offhand.On the RB211-535E4.

The Idle Reset Solenoid on the FFG sets High or Low Idle.

High idle is only used during Landings and when Engine Cowl A/I is on in Flt.High Idle is used for Descent in Icing conditions to prevent possible Engine problems and is used during Landings to reduce time for the Engine to spool up to Maximim rpm if needed in case of G/A.

When the Idle Reset Solenoid is De-Energized Idle speed increases.At sea level the Acceleration time decreases by approx 2 secs.[regular 5-6 secs].

In Flt with Flaps up & Gear retracted.Low Idle is used.When Flaps lowered to Landing position & Engine A/I selected or NLG lowered,Idle Reset solenoid is DeEnergised & speed Increases.

On Touchdown The Solenoid Energizes after a Time Delay of 5 seconds.

1.06 EPR Flight Idle on RB211-535C.

regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
chksix
Posts: 336
Joined: Mon Sep 26, 2005 6:16 pm

RE: Flight Idle And Ground Idle

Wed Aug 16, 2006 9:37 pm

Any N1 percentages? Would 40 - 50% N1 be probable?
The conveyor belt plane will fly
 
Tristarsteve
Posts: 3660
Joined: Tue Nov 22, 2005 11:04 pm

RE: Flight Idle And Ground Idle

Thu Aug 17, 2006 3:18 am

Quoting Chksix (Reply 4):
Any N1 percentages? Would 40 - 50% N1 be probable

No lower than that.
On RB211 N1 idle is around 22%, N2 40% and N3 60%.
Flight Idle increases this by about 5% N1.
 
chksix
Posts: 336
Joined: Mon Sep 26, 2005 6:16 pm

RE: Flight Idle And Ground Idle

Thu Aug 17, 2006 4:42 am

Ok thanks! I'm surprised it's that low.
The conveyor belt plane will fly
 
113312
Posts: 667
Joined: Sat Apr 16, 2005 9:09 am

RE: Flight Idle And Ground Idle

Thu Aug 17, 2006 7:10 am

Of course, you're only discussing Flight or Ground Idle for tubofan engines. They really shouldn't be called "Flight" or "Ground" at all but rather High and Low.

Many of the high bypass engines in use basically have a closed "throttle" speed of "Ground Idle" (low) on the ground and in the air. They only switch over to "Flight Idle" (high) in the air with a landing configuration such as gear and flaps extended. The reason for this is to facilitate rapid engine response for go around (aborted landing) and reverse thrust after touchdown.

EMC
 
liedetectors
Posts: 323
Joined: Thu Jul 21, 2005 10:44 am

RE: Flight Idle And Ground Idle

Thu Aug 17, 2006 7:34 am

Quoting Speedracer1407 (Thread starter):
is there a detent on the throttle quadrant for it?

No. Ground Idle is exactly what is sounds like - idle thrust for on the ground. It is the same detent at Flight Idle, but also requires the Weight on Wheels switch to show that it is on the ground. Tripping the WOW switch will tell the FADEC to spool the engines down to Ground Idle.
If it was said by us, then it must be true.
 
troubleshooter
Posts: 420
Joined: Mon Feb 07, 2005 5:22 am

RE: Flight Idle And Ground Idle

Sat Aug 26, 2006 7:24 am

Quoting Chksix (Reply 1):
I'm interested to know the limits for the 737NG. I know ground idle is at 20% but can't find info on Flight Idle.

Note that IDLE is always based on N2 speed.

For the CFM56-7 there are three IDLE modes:
GROUND IDLE
above 58%. Idle speed will increase with increase of OAT, generator load and bleed demand
FLIGHT IDLE
during flight the FADEC maintains a N2 speed of 72%
APPROACH IDLE
the FADEC goes into this mode (72-79% N2) if following conditions occour:
- either cowl anti-ice is swiched on
- either main landing gear down and locked below 15.000 ft
- TE flaps are equal or more than "15" below 15.000 ft
This job sucks!!! I love this job!!!
 
User avatar
IFixPlanes
Posts: 266
Joined: Sun Feb 20, 2005 11:34 pm

RE: Flight Idle And Ground Idle

Sat Aug 26, 2006 2:40 pm

B737NG Idle Control:

never tell an engineer he is wrong ;-)
 
chksix
Posts: 336
Joined: Mon Sep 26, 2005 6:16 pm

RE: Flight Idle And Ground Idle

Sat Aug 26, 2006 5:39 pm

Thank you for that info Troubleshooter and IFixPlanes!
The conveyor belt plane will fly

Who is online

Users browsing this forum: No registered users and 17 guests

Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos