A320 (FMGS) idle factor
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A320 (FMGS) idle factor
Does anyone ever bother to alter idle factor (during FMGS initialization from the default factory setting of 0%) ?
I am talking about the Idle factor only, NOT perf factor.
I find that the FM descent point is usually earlier (far too conservative) than intended (something like 3 x + 20 or well more) when the aircraft is capable of descending later/ steeper (3x +10).
Often you will get a message "too steep path ahead" yet the aircraft is fully capable of descending at this so called "steep path".
When you fly a managed descent (even with all the descent winds), it will often fly slower then the managed ECON descent speed as the aircraft has to slow down (raise the nose / reduce VS) to maintain profile. Once again this is because the FMGS calculated a shallower descent profile than what the aircraft will actually fly.
Will modification of the idle factor resolve this error and the FMGS now reflect more what the aircraft is capable of achieveing ? (more accurate descent points and no more steep path messages)
Are there any hazards / risks when modifying the idle factor away from the default factory setting ?
The FCOM says modification of the idle factor is to adjust FM descent predictions. Will it do anything else ?
Unfortunately, no where does it say in the FCOM / Airbus SOP's (or I couldn't find it) that a pilot is to even check or allowed to modify the IDLE factor during FMGS initialisation ?
However the FCOM explains what modification of the idle factor does (as I stated) but doesn't say if / when you can do it.
Are pilots allowed to adjust the idle factor or would this be a deliberate breach of the FCOM / SOP's ?
I am talking about the Idle factor only, NOT perf factor.
I find that the FM descent point is usually earlier (far too conservative) than intended (something like 3 x + 20 or well more) when the aircraft is capable of descending later/ steeper (3x +10).
Often you will get a message "too steep path ahead" yet the aircraft is fully capable of descending at this so called "steep path".
When you fly a managed descent (even with all the descent winds), it will often fly slower then the managed ECON descent speed as the aircraft has to slow down (raise the nose / reduce VS) to maintain profile. Once again this is because the FMGS calculated a shallower descent profile than what the aircraft will actually fly.
Will modification of the idle factor resolve this error and the FMGS now reflect more what the aircraft is capable of achieveing ? (more accurate descent points and no more steep path messages)
Are there any hazards / risks when modifying the idle factor away from the default factory setting ?
The FCOM says modification of the idle factor is to adjust FM descent predictions. Will it do anything else ?
Unfortunately, no where does it say in the FCOM / Airbus SOP's (or I couldn't find it) that a pilot is to even check or allowed to modify the IDLE factor during FMGS initialisation ?
However the FCOM explains what modification of the idle factor does (as I stated) but doesn't say if / when you can do it.
Are pilots allowed to adjust the idle factor or would this be a deliberate breach of the FCOM / SOP's ?
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Greetings,
What are you using, Legacy, Pegasus, or thales New FM?
IDLE and PERF Factors are data enabling the customization of your aircraft, in order to fine tune the predictions of the FMS.
Because the FMS Perf database is a generic one, and engine degradation takes place.
IDLE factor is provided by Flight ops engineering, and is different from one aircraft to another, it has to be entered during DONE, and PREFLIGHT flght phases, inhibited elsewise.
Now vertical profile of A320 and 340 families, is based on optimum i.e., most economical profile, given CI, WIND, TEMPERATURE, TROPOPAUSE height, and aircraft Weight.
The FMS will always try to keep the aircraft on its Optimum profile, using the Optimum Speed.
So if all the data have been entered and are correct (Rubish IN Rubish OUT principal)then the FMS SHOULD NOT behave the way you describe it, unless it is a old Legacy FMS.
TOO STEEP PATH is the result of constraining waypoints altitude wise followed by a speed reduction/restriction, and the FMS has computed that is cannot dissipate the Energy to loose Speed and Altitude (unless you help him with a liitle bit of speed brakes)
IDLE and PERF factors can be modified by crew, the only problem is that the data you are going to use have to be provided by Egineering, because they impact the Economics of your flight i.e. -XX% will delay your descent +XX% will make yo descend early (on IDLE segments) therefore your fuel consumption will increase
The other thing influencing your descent is the CABIN RATE of DESCENT setting
What are you using, Legacy, Pegasus, or thales New FM?
IDLE and PERF Factors are data enabling the customization of your aircraft, in order to fine tune the predictions of the FMS.
Because the FMS Perf database is a generic one, and engine degradation takes place.
IDLE factor is provided by Flight ops engineering, and is different from one aircraft to another, it has to be entered during DONE, and PREFLIGHT flght phases, inhibited elsewise.
Now vertical profile of A320 and 340 families, is based on optimum i.e., most economical profile, given CI, WIND, TEMPERATURE, TROPOPAUSE height, and aircraft Weight.
The FMS will always try to keep the aircraft on its Optimum profile, using the Optimum Speed.
So if all the data have been entered and are correct (Rubish IN Rubish OUT principal)then the FMS SHOULD NOT behave the way you describe it, unless it is a old Legacy FMS.
TOO STEEP PATH is the result of constraining waypoints altitude wise followed by a speed reduction/restriction, and the FMS has computed that is cannot dissipate the Energy to loose Speed and Altitude (unless you help him with a liitle bit of speed brakes)
IDLE and PERF factors can be modified by crew, the only problem is that the data you are going to use have to be provided by Egineering, because they impact the Economics of your flight i.e. -XX% will delay your descent +XX% will make yo descend early (on IDLE segments) therefore your fuel consumption will increase
The other thing influencing your descent is the CABIN RATE of DESCENT setting
Last edited by kijangnim; 23rd Aug 2008 at 04:33.
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TOO STEEP PATH is the result of constraining waypoints altitude wise followed by a speed reduction/restriction, and the FMS has computed that is cannot dissipate the Energy to loose Speed and Altitude (unless you help him with a liitle bit of speed brakes)
For example, the profile below (descent gates) are easily achievable (open descent) however the FMS will always say "TOO STEEP PATH".
250 knots, 8000' 30 track miles
250 knots, 5000', 20 track miles
green dot, 3000' 10 track miles
Will modification of the idle factor resolve this problem ?
Last edited by Bagot_Community_Locator; 23rd Aug 2008 at 05:54.
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Greetings,
Remember, FMS will try to keep your flight path within the optimum path,keeping the optimum speed, this is whyit gives you PREDICTIONS to show you how far of you are going to be speed and alitutde wise, and of course with that prospect you can do what the FMS cannot do.
IDLE factor will not solve your problem, check that all data have been entered (wind and temp are very important) check that the cabin press is within the scheduled descent, and if all are correct then he FMS should be able to fly it.
Now regarding TOO STEEP PATH, it just means that the FMS will not be able to fly the path, in the configuration , conditions a that moment in time (may be strong tail wind...)
BTW what box is it?
Remember, FMS will try to keep your flight path within the optimum path,keeping the optimum speed, this is whyit gives you PREDICTIONS to show you how far of you are going to be speed and alitutde wise, and of course with that prospect you can do what the FMS cannot do.
IDLE factor will not solve your problem, check that all data have been entered (wind and temp are very important) check that the cabin press is within the scheduled descent, and if all are correct then he FMS should be able to fly it.
Now regarding TOO STEEP PATH, it just means that the FMS will not be able to fly the path, in the configuration , conditions a that moment in time (may be strong tail wind...)
BTW what box is it?
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@ kijangnim
Are you rated on the A320 family? That the FMS in deed SHOULD be able to corretly calculate a descent profile is indeed true, however reality looks way different, at least on our aircraft. If yours are different, I would like to know why...
We fly CFM-powered A319s (2x23 klbs) and A320s (2x25 klbs) as well as IAE-powered A321s (-100: 2x 30 klbs/-200 2x33 klbs). We have ships with the Legacy FMS as well as many Pegasus and some Thales equipped.
Except for the Thales FMS (only a couple of sectors until now) my personal experience is, that even with all variables entered as good as it gets, the FMS calculates way too conservative.
You can fly well above the FMS profile an get back on it before Speed reduction a 10.000 ft during descent, even when the box calls for 'More Drag' or 'Too Steep Path Ahead' during Descent. The A319 will make up for some 2.000 ft during an Descent from the lower 30s, the A320 for 1.500 and the A321 for more than 1.000.
Only in case you have to fly the entire descent with ENG A/I on, you come close to the predictions and even then you are descending steeper than the box estimated in the mach-segment of the descent (seen by the VDEV-value on the PROG page decreasing during initial descent).
For myself I don't even ignore the descent predictions anymore and calculate for myself usually pretty well. I'm just wondering how other FMSs do it better? What is the experience of other bus-operators?
Regards, MAX
Are you rated on the A320 family? That the FMS in deed SHOULD be able to corretly calculate a descent profile is indeed true, however reality looks way different, at least on our aircraft. If yours are different, I would like to know why...
We fly CFM-powered A319s (2x23 klbs) and A320s (2x25 klbs) as well as IAE-powered A321s (-100: 2x 30 klbs/-200 2x33 klbs). We have ships with the Legacy FMS as well as many Pegasus and some Thales equipped.
Except for the Thales FMS (only a couple of sectors until now) my personal experience is, that even with all variables entered as good as it gets, the FMS calculates way too conservative.
You can fly well above the FMS profile an get back on it before Speed reduction a 10.000 ft during descent, even when the box calls for 'More Drag' or 'Too Steep Path Ahead' during Descent. The A319 will make up for some 2.000 ft during an Descent from the lower 30s, the A320 for 1.500 and the A321 for more than 1.000.
Only in case you have to fly the entire descent with ENG A/I on, you come close to the predictions and even then you are descending steeper than the box estimated in the mach-segment of the descent (seen by the VDEV-value on the PROG page decreasing during initial descent).
For myself I don't even ignore the descent predictions anymore and calculate for myself usually pretty well. I'm just wondering how other FMSs do it better? What is the experience of other bus-operators?
Regards, MAX
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Adjusting the Idle factor will adjust the descent profile, not sure which way though. An average managed descent with the a/c on profile will descend with power on (approx 44.5 N1) just in case you need to use engine anti ice or environmental conditions change. If you get high, the speed increases to the edge of the 20kts bracket and then it reduces power.
Next time you go flying, in the above mentioned conditions, switch on engine anti ice and take a look at the N1, it should say Idle or very close.
I would be very careful and not touch this setting but maybe seek guidance from your tech pilot.
Rgds
MF
Next time you go flying, in the above mentioned conditions, switch on engine anti ice and take a look at the N1, it should say Idle or very close.
I would be very careful and not touch this setting but maybe seek guidance from your tech pilot.
Rgds
MF
Last edited by Man Flex 37.5; 23rd Aug 2008 at 19:53.
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Yes I am rated on type. I fly an IAE A320.
I got no idea what type of FM we are using.
Yes, our A320's will often make up for 1500' during descent (meaning it commenced descent about 5 miles early).
This is from the FCOM :
IDLE FACTOR
IDLE and PERF factors follow the same principle.
The PERF factor is mainly used for prediction during the cruise phase.
The IDLE factor is dedicated to the FM descent segment.
The aim of the IDLE factor is to adjust FM descent predictions. In particular, the Top of Descent (TOD)
position, with the actual engine idle thrust used during descent.
A positive IDLE FACTOR gives an earlier Top of Descent (shallower path).
A negative IDLE FACTOR delays the Top of Descent (steeper path).
Example : An IDLE FACTOR of - 3 decreases the computed descent length by 5 NM.
IDLE Factor at delivery :
The IDLE Factor set at delivery is 0 %.
If a negative factor delays top of descent (as per FCOM), then wouldn't an idle factor of -3 now solve this problem and give a more accurate descent point (we commence descent and not gain the usual 1500 / not go 1500 below VDEV) ?
I got no idea what type of FM we are using.
Yes, our A320's will often make up for 1500' during descent (meaning it commenced descent about 5 miles early).
This is from the FCOM :
IDLE FACTOR
IDLE and PERF factors follow the same principle.
The PERF factor is mainly used for prediction during the cruise phase.
The IDLE factor is dedicated to the FM descent segment.
The aim of the IDLE factor is to adjust FM descent predictions. In particular, the Top of Descent (TOD)
position, with the actual engine idle thrust used during descent.
A positive IDLE FACTOR gives an earlier Top of Descent (shallower path).
A negative IDLE FACTOR delays the Top of Descent (steeper path).
Example : An IDLE FACTOR of - 3 decreases the computed descent length by 5 NM.
IDLE Factor at delivery :
The IDLE Factor set at delivery is 0 %.
If a negative factor delays top of descent (as per FCOM), then wouldn't an idle factor of -3 now solve this problem and give a more accurate descent point (we commence descent and not gain the usual 1500 / not go 1500 below VDEV) ?
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Greetings,
I am on the A330, however, Single Aisle or long range dont matter since they are both common in fact you can plug a thales on any Arbus, except 300 and 380, because they are developped from the same specification 555 from aerospacial.
Having said that, I reinstate what I did before, as per the specification the FMS will keep either as close as possible or be on the optimum profile, this is the conservative side.
Now for MAN FLEX 37.5:
The Idle you have noticed is what is refered to IDLE + Delta, it is there to help manage the descent and stay in the brakets, and not for the Anti-Ice, it allows the FMS to command further thrust reduction when its profile reaches the bottom of the braket during descent, as the last mean of loosing energy.
Optimum profile is flown at optimum speed, so any time you depart from that scenario you do better (or worst than the FMS)
Now I recommand you to have a chat with Enginering department and check is they have a value for your aircraft IDLE FACTOR, if they do Then use it if they dont ask then to survey Engine PERF, I am sure they wll come with something.
Happy Landing
I am on the A330, however, Single Aisle or long range dont matter since they are both common in fact you can plug a thales on any Arbus, except 300 and 380, because they are developped from the same specification 555 from aerospacial.
Having said that, I reinstate what I did before, as per the specification the FMS will keep either as close as possible or be on the optimum profile, this is the conservative side.
Now for MAN FLEX 37.5:
The Idle you have noticed is what is refered to IDLE + Delta, it is there to help manage the descent and stay in the brakets, and not for the Anti-Ice, it allows the FMS to command further thrust reduction when its profile reaches the bottom of the braket during descent, as the last mean of loosing energy.
Optimum profile is flown at optimum speed, so any time you depart from that scenario you do better (or worst than the FMS)
Now I recommand you to have a chat with Enginering department and check is they have a value for your aircraft IDLE FACTOR, if they do Then use it if they dont ask then to survey Engine PERF, I am sure they wll come with something.
Happy Landing
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the FMS will keep either as close as possible or be on the optimum profile, this is the conservative side.
- puts us OCTA (below CTA steps)
- puts us below DME/GPS arrival steps
- flies us at the lower end of the managed speed range (which is slower than the speed we ideally like to fly, esp if ATC want us at a fixed speed)
- might also involve a level segment at low level with a thrust increase or reversion to speed mode with some thrust (how can this be the most efficient way to descend an aircraft compared to idle thrust all the way down ?)
Therefore we usually insert constraints to keep us in CTA and above and DME/GPS arrival steps.
However, when we now insert these constraints (ie 8000' at 30nm, 5000' at 20 miles), the FM now says "TOO STEEP PATH AHEAD". Yet from experience, the aircraft is fully capable of descending this so called steep path without speed brake (in nil wind/ head wind).
That is why most pilots here use the open descent as the managed descent does not seem to work (do what we want it to do), even if you try and modify it as much as you can (create contstraints etc).
Any solution to this problem ?
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Greetings,
I agree with you that the optimum Profile is very seldom matching the ATC environment, but this is Airbus Aerospacial specification, and there is nothing we can do about it, on the other hand it saves money if you stay managed all time.
TOO STEEP PATH under the conditions you have shown is a bit curious, I need to have a snap shot of your MCDU, distance , speed and altitude of the waypoint before 8000 feet and after 5000 feet, spped is important. as well as your cabin altitude rate settings on the MCDU before TOD, if you can add the wind it will be perfect, E-Mail that to me and I will be able to see what is happening. Legacy FMS excluded of course.
I agree with you that the optimum Profile is very seldom matching the ATC environment, but this is Airbus Aerospacial specification, and there is nothing we can do about it, on the other hand it saves money if you stay managed all time.
TOO STEEP PATH under the conditions you have shown is a bit curious, I need to have a snap shot of your MCDU, distance , speed and altitude of the waypoint before 8000 feet and after 5000 feet, spped is important. as well as your cabin altitude rate settings on the MCDU before TOD, if you can add the wind it will be perfect, E-Mail that to me and I will be able to see what is happening. Legacy FMS excluded of course.
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Now for kijangnim:
The Idle plus delta as you mention is used to manage the descent, if you look at a tod point and use open descent you will always go below the profile, if you put engine anti ice on the N1 will increase thus shallowing out the descent path. So to summarise, engine anti ice and other environmental factors all come under the umbrella of managing the descent profile.
The Idle plus delta as you mention is used to manage the descent, if you look at a tod point and use open descent you will always go below the profile, if you put engine anti ice on the N1 will increase thus shallowing out the descent path. So to summarise, engine anti ice and other environmental factors all come under the umbrella of managing the descent profile.
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Greetings,
Airbus always resisted to incorporate ANTI ICE function into the vertical profile of ALL Airbuses
, and the IDLE + DELTA is NOT for ANTI ICE 
it is the extra bit of reduction the FMS will use to keep you within the descent brakets, it is here to accomodate for wind, temperature, differences between forcast entered values and the real world .
We all know that the FCOM is the worst document, and that Airbus is very badly documented, however HWL and THALES have published Pilots Guides, just try to get one and you will see
A lot Airline have complained about it because it is not a all economic, during several conferences ( where I was)
Airbus always resisted to incorporate ANTI ICE function into the vertical profile of ALL Airbuses
, and the IDLE + DELTA is NOT for ANTI ICE it is the extra bit of reduction the FMS will use to keep you within the descent brakets, it is here to accomodate for wind, temperature, differences between forcast entered values and the real world .We all know that the FCOM is the worst document, and that Airbus is very badly documented, however HWL and THALES have published Pilots Guides, just try to get one and you will see
A lot Airline have complained about it because it is not a all economic, during several conferences ( where I was)
Last edited by kijangnim; 24th Aug 2008 at 08:23. Reason: typos
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OK, lets reverse the scenario, imagine that there was no idle factor at all and descent planning used the entered data, if you use engine anti ice, the N1 will increase and you will be above your profile by around 1000 feet on the A319.
I agree that the idle factor is uneconomic, we always descend with power on during managed descent, the descent also takes longer now as we have reduced our cost index.
I agree that the idle factor is uneconomic, we always descend with power on during managed descent, the descent also takes longer now as we have reduced our cost index.
Any solution to this problem ?
We had a descent card. It was fantastic. Most blokes within the community would hit 1000' and the power would come up- with an idle descent from 35000'.
Weight, wind and speed were the obvious factors and major variables.
The airplane was flown hard and fast. Meeting ATC gate requirements- 20 track miles 300 to 340kts and 5000-6000'.
VNAV was used as a back-up and only ever proved its worth on complex STARS. Or VNAV was tricked by crew to thinking it had more miles to run- final waypoint to airfield, then the ILS approach strung out separately worked a treat. 250 below 10 didn't exist either.
Great days. Domestic Oz flying was corrupted when QF long haul started flying short haul routes.
Last edited by Gnadenburg; 24th Aug 2008 at 08:54.
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Greetings Man Flex,
Indeed
"if you use engine anti ice, the N1 will increase and you will be above your profile by around 1000 feet on the A319."
Remember AIRBUS doesnot want Anti ice function to cater profile change (Boeing Does have a function) and AIRBUS did not embed ANTI ICE or Speed Barkesin the Profile.
Descent:
The aircraft will descent at IDLE + delta (i.e., Speed on Elevator) until it reaches the first constraining constraint then continue geometric descent (i.e Speed on N1)
Now the only way to fly IDLE all the way is to clear all constraints present in the fms on the descent segments and up a constraining contraint at XXXX feet. Is it safe
well no some constraint are linked to terrain profile airspace and traffic 
Indeed
"if you use engine anti ice, the N1 will increase and you will be above your profile by around 1000 feet on the A319."
Remember AIRBUS doesnot want Anti ice function to cater profile change (Boeing Does have a function) and AIRBUS did not embed ANTI ICE or Speed Barkesin the Profile.
Descent:
The aircraft will descent at IDLE + delta (i.e., Speed on Elevator) until it reaches the first constraining constraint then continue geometric descent (i.e Speed on N1)
Now the only way to fly IDLE all the way is to clear all constraints present in the fms on the descent segments and up a constraining contraint at XXXX feet. Is it safe
well no some constraint are linked to terrain profile airspace and traffic
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Anti Ice
Just to highlight the information that is crucial to answer the original question:
The Airbus descent profile is calculated for the situation "Engine Anti Ice ON during the whole descent."
If actually, you have Engine Anti Ice OFF during a (large) portion of your descent, you will always fall short on energy. Cause - with EAI ON, your idle N1 is slightly increased, giving you slightly more thrust.
Reason for calculating with EAI ON: Airbus wants to protect you from arriving at the bottom of your descent with too much energy, which could happen if calculation was made with EAI OFF while actually during descent EAI was used (ON).
The Airbus descent profile is calculated for the situation "Engine Anti Ice ON during the whole descent."
If actually, you have Engine Anti Ice OFF during a (large) portion of your descent, you will always fall short on energy. Cause - with EAI ON, your idle N1 is slightly increased, giving you slightly more thrust.
Reason for calculating with EAI ON: Airbus wants to protect you from arriving at the bottom of your descent with too much energy, which could happen if calculation was made with EAI OFF while actually during descent EAI was used (ON).
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IDLE + Delta explanation
Greetings,
I have met people saying that they believe in Santa
Image of Descent Airbus FMS2 - Photobucket - Video and Image Hosting
check this out
Greetings,
I have met people saying that they believe in Santa
Image of Descent Airbus FMS2 - Photobucket - Video and Image Hosting
check this out
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Idle factor parameter
Hello dear ones,
I'm working on a project to optimize descent profiles and on one part I need to calculate the IDLE factor parameter.
So I'd like your help, especially if any of you engineers have already had to calculate the idle factor for their aircraft.
I'm working on a project to optimize descent profiles and on one part I need to calculate the IDLE factor parameter.
So I'd like your help, especially if any of you engineers have already had to calculate the idle factor for their aircraft.
