View Full Version : approach and idle thrusts

3rd Apr 2010, 11:10
hi everybody,
I have three questions, if I can have answers I would be grateful.

1. I wander the thrust of engines during descent in terms of N1 or thrust.
(during taxi, engines run at idle, right) ?

2.May I have N1% at idle

3. after leaving top of descent what is the flap configuration during descent.

(please add the engine and aircraft types, if also available fuel flow and EGT would be great).

best regards to everyone.

3rd Apr 2010, 17:41
Sounds like a sim game question rather than a technical question...

3rd Apr 2010, 18:24
1) Idle. N1 approx 25%

Taxi- Between Idle and whatever is needed (usually below 30% N1)

2) See above

3) 0. All aircraft ever built. FF aprox 1000 KG/ Hr (B777) EGT approx 350 deg.

3rd Apr 2010, 21:03
during taxi, engines run at idle, right

- it strictly depends on weight: the higher the aircraft weight, the higher is your needed N1 to taxi ...
- the ground idle mainly depends on whether you're using anti-ice or not: the "heavier" is the engine bleed, the higher the idle N1 commanded by FADEC to meet with the demand.

On the whole, the question ain't well formulated to be sincere ... :)


4th Apr 2010, 11:06
thank you Wizofoz.

I also heard about that there is another idle set, called as approach idle which is little higher than regular idle. But I guess assuming idle N1 during descend will be Ok in generally.

4th Apr 2010, 11:09
Dear Intruder, actually I am trying to figure out engine power usage during different phases of flight to use in one of my study. I am not a pilot, just interesting in gas turbines.

4th Apr 2010, 11:20
Dear Fredgrav, thank you.
you stated very well, that I missed the weight and idle thrust relation and the bleed air as well. normaly we just can learn what books say, yet no book says what you commented above. so I see that discussing with users is very useful.

also you are right about the comment to my question, I have lots of thing in my mind to ask actually, this is why I messed like that. please, all of you excuse me.

about the flap, what I really wandered is when do you initiate to extend the flaps (after final approach point ?) and how flap degrees are configurated due to speed, altitude etc (also when do you extend the landing gears).

good days.

4th Apr 2010, 11:53
Hi ex902, don't be warrying about ... :ok:
Can answer with regard to Airbus A319/320/321, even if it should be almost the same for a most commercial aircraft:
1. Flaps/Slats extension is initiated a few miles before reaching the FAF/FAP and green dot (= VMD speed) is normally flown till CONF 1 is selected, at say 2/3 NM from FAF/FAP;
2. Flaps are sequencially extended throughout the decelaration;
3. Normally, landing gear is extended once CONF 2 is selected, but may be extended a bit earlier to help deceleration.

Bear in mind that FLAPS/LANDING GEAR/SPOILERS greatly increase parasite drag, thus increasing Vls (lowest speed that can be flown with A/THR engaged, having an appropriate margin above stall speed).

Hope that helps, ;)

4th Apr 2010, 12:11
Bear in mind that FLAPS/LANDING GEAR/SPOILERS greatly increase parasite drag, thus increasing Vls

Er... could you please explain why extending Flaps / lowering the Gear increase Vls?

4th Apr 2010, 14:27
Don't want to use flaps until down in the thick air and slowed down.

4th Apr 2010, 15:13
Er... could you please explain why extending Flaps / lowering the Gear increase Vls?

Hi rudder,
Vls is 1.23 Vs1g and depends on aircraft weight and Slats/Flaps configuration: furthermore it is directly proportional to the parasite drag experienced. Spoilers, Landing gear and flaps -> (above its optimal lift/drag setting eg. Landing Configuration) all increase the amount of drag produced, thus increasing VLS ... even though in normal landing configuration the increase ain't that significant (it is almost balanced by the relatively high lift produced), it will be by extending speed brakes.


4th Apr 2010, 16:14
Approach idle and Flight idle are 2 forms I know of used to increase idle for flight. Approach idle is more like what it sounds like, approach idle is usually a fadec control based on logic (flap and gear position etc).

Flight idle is usually weight off wheels, the company I work for is operating it's first approach idle aircraft and the writeups can be a pain in the ass to deal with because flight mode/condition is usually not defined well in the PIREP. We are all learning.

Also to mention N1 % changes on baro press and temp. I believe somewhere around a 5%increase from ground to approach is an average norm.

FE Hoppy
4th Apr 2010, 16:38
Here are the definitions of the 4 idle settings on the Ejets.

The IDLE modes are automatically adjusted by the FADEC in order to maintain the minimum thrust necessary to provide the required bleed pressure to the airplane.
Flight Idle N1 varies with altitude and can change as a function of ECS and anti-ice bleed requirements.
The idle mode selections are the following:
Flight Idle.
Approach Idle.
Final Approach Idle.
Ground Idle.

Whenever ICE CONDITION is sensed the N1 for the FLIGHT IDLE and APPROACH IDLE is automatically increased to maintain the minimum bleed pressure required for the ANTI ICE system operation.

With the FINAL APPROACH IDLE there is no N1 automatic increase but a cyan dash is displayed on both EICAS N1 dial to show the minimum thrust required to maintain the bleed pressure required by the ANTI ICE system.

The flight idle mode is active anytime in flight when the approach idle is not selected.

Approach Idle is used in flight to enable rapid acceleration to go-around thrust. Approach idle is activated as follows:
Altitude less than 15000 ft;
In flight;
Flaps 1 or greater; and
Landing gear down.

Final Approach Idle is active as follows:
Altitude lower than 1200 ft AGL;
Landing flaps;
Landing gear down.

Ground Idle is the minimum thrust setting. Ground Idle provides the minimum stable engine thrust level for ground operations.