FLCH
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FLCH
With so much technical knowledge on this site, how would one explain FLCH in technical terms....what is the thinking and logic behind the level change?
thanking in advance.
thanking in advance.
Do you have a particular brand in mind?
In my experience FLCH tends to just be flight idle descent or full thrust climb ie speed on pitch. I suppose 'open climb' 'open descent' in some Airbus aircraft is the same thing
I know on some types for a small climb eg 2000' the aircraft will limit the climb rate by using less than full thrust to help avoid TCAS events caused by high closure rates
In my experience FLCH tends to just be flight idle descent or full thrust climb ie speed on pitch. I suppose 'open climb' 'open descent' in some Airbus aircraft is the same thing
I know on some types for a small climb eg 2000' the aircraft will limit the climb rate by using less than full thrust to help avoid TCAS events caused by high closure rates
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I would define it as pitching for speed until preselected altitude in the altitude window on the MCP. This is regardless of thrust available. Because you can disconnect A/T, reduce thrust and the A/P will pitch to maintain speed until reaching the preselected altitude. At that point, it will pitch to maintain that altitude. I reckon if at some point thrust exceeds drag and weight (you advance the thrust levers yourself), it will eventually pitch up and at a later stage stabilize at an altitude that keeps the selected speed. Never tried it in the sim.
We use LVL CHG to driftdown to OEI MAX ALT at MAX CONT thrust.
We use LVL CHG to driftdown to OEI MAX ALT at MAX CONT thrust.
FLCH...
As for the OP...difficult to know what Tech level to pitch the reply at if we don't know the tech level the poster is working at:
Simple answer "FLCH 101 for some Boeings" - using FLCH tries to give you the required change in height in approximately two minutes and at a fixed speed..
..However if the increment is so big the system can't achieve the change in two minutes it will climb you at climb power to get you there as soon as it can, if descending it will descend you at best rate, fixed speed, idle power...
Probably most used for descending/climbing when you aren't trying to follow a planned/published vertical profile, or you just want or need to get the level change done promptly (e.g. an ATC "descend now").
If the Op wants something deeper then I'll leave it to the grown ups.
As for the OP...difficult to know what Tech level to pitch the reply at if we don't know the tech level the poster is working at:
Simple answer "FLCH 101 for some Boeings" - using FLCH tries to give you the required change in height in approximately two minutes and at a fixed speed..
..However if the increment is so big the system can't achieve the change in two minutes it will climb you at climb power to get you there as soon as it can, if descending it will descend you at best rate, fixed speed, idle power...
Probably most used for descending/climbing when you aren't trying to follow a planned/published vertical profile, or you just want or need to get the level change done promptly (e.g. an ATC "descend now").
If the Op wants something deeper then I'll leave it to the grown ups.
How were you taught to climb and descend in your primary trainer (Cessna/Piper)?
To climb: Apply full power and select the pitch attitude to achieve 70kts. Then maintain the speed with small elevator inputs.
To descend: Close the throttle and select the pitch attitude to achieve 70 kts. Then maintain the speed with small elevator inputs.
In both cases, the power is ‘fixed’, inasmuch that the throttle is either wide open or fully closed. The elevators are controlling the speed (which of course doesn’t have to be 70kts; that was just an example).
FLCH is the same in that the thrust levers go to “full power” for a climb, and “idle” for a descent*. The elevators control the speed.
The FMAs show that FLCH, VNAV SPD and TO/GA are the only ones where the auto-throttle is in a “THRUST” mode and the elevators are in “SPD” mode.
All other modes have a “defined vertical trajectory” which is controlled by the elevators; the auto-throttle must therefore control the speed.
This applies to ALT HOLD, V/S, G/S, GP and VNAV PTH.
*I agree with Wiggy that later models have an intelligent FLCH and TO/GA, which don’t use just full power and idle, but to try and explain the principle to someone unfamiliar with AP and AT modes, the “binary model” may help understanding. I would also leave out VNAV to start with!
To climb: Apply full power and select the pitch attitude to achieve 70kts. Then maintain the speed with small elevator inputs.
To descend: Close the throttle and select the pitch attitude to achieve 70 kts. Then maintain the speed with small elevator inputs.
In both cases, the power is ‘fixed’, inasmuch that the throttle is either wide open or fully closed. The elevators are controlling the speed (which of course doesn’t have to be 70kts; that was just an example).
FLCH is the same in that the thrust levers go to “full power” for a climb, and “idle” for a descent*. The elevators control the speed.
The FMAs show that FLCH, VNAV SPD and TO/GA are the only ones where the auto-throttle is in a “THRUST” mode and the elevators are in “SPD” mode.
All other modes have a “defined vertical trajectory” which is controlled by the elevators; the auto-throttle must therefore control the speed.
This applies to ALT HOLD, V/S, G/S, GP and VNAV PTH.
*I agree with Wiggy that later models have an intelligent FLCH and TO/GA, which don’t use just full power and idle, but to try and explain the principle to someone unfamiliar with AP and AT modes, the “binary model” may help understanding. I would also leave out VNAV to start with!
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There are also a couple of traps when using FLCH on a Boeing.
1) If you are descending in FLCH on an approach with say 2000’ in the alt sel window and are approaching that altitude and you then put 4000 feet in the window for the missed approach it will immediately start a climb to that altitude and make you high on profile.
2) If you are descending in FLCH and disconnect the autopilot and raise the nose (ignoring the flight director) the speed will reduce below target speed and the autothrottle will not help you out. Nor will the low speed function of autothrottle wake up
chip in.
The Asiana 777 that crashed at SFO fell into both of the above traps in quick succession. Some people think its a defect in the system that it can happen. I don’t. I think you just have to understand how it works.
1) If you are descending in FLCH on an approach with say 2000’ in the alt sel window and are approaching that altitude and you then put 4000 feet in the window for the missed approach it will immediately start a climb to that altitude and make you high on profile.
2) If you are descending in FLCH and disconnect the autopilot and raise the nose (ignoring the flight director) the speed will reduce below target speed and the autothrottle will not help you out. Nor will the low speed function of autothrottle wake up
chip in.
The Asiana 777 that crashed at SFO fell into both of the above traps in quick succession. Some people think its a defect in the system that it can happen. I don’t. I think you just have to understand how it works.
Hardly ever, if by “approach” you mean within approx 3000ft of terrain.
It would be much better to use V/S or FPA to capture the correct profile from above, or maybe ALT to capture it from below. Of course, intelligent use of VNAV could also work.
These modes offer more finesse than FLCH.
It would be much better to use V/S or FPA to capture the correct profile from above, or maybe ALT to capture it from below. Of course, intelligent use of VNAV could also work.
These modes offer more finesse than FLCH.
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Asiana Flight 214 comes to mind.
One crew at my previous company nearly stalled a TCAS. Got an RA during descent, so TCS'd the aircraft to follow the RA. Didn't realise FLCH is SPDe not SPDt. Pithced up with autothrust commanding Idle.
After that, we were trained to fly FLCH with all automatics. If you want the AP out, change vertical mode first. I will be honest, it has caught me out once.
One crew at my previous company nearly stalled a TCAS. Got an RA during descent, so TCS'd the aircraft to follow the RA. Didn't realise FLCH is SPDe not SPDt. Pithced up with autothrust commanding Idle.
After that, we were trained to fly FLCH with all automatics. If you want the AP out, change vertical mode first. I will be honest, it has caught me out once.
I think Air France got caught out at TLV a couple of times with the similar Airbus “Open Descent” mode as well.
A good rule is to follow the flight director. If you choose not to follow it, turn both of them off.
Another good rule is to read and understand the FMAs, left to right
Boeing: Autothrottle///Ailerons///Elevators
—————————-——AP status———————
Airbus: Autothrust///Elevators///Ailerons///AP status
And another one is to ask yourself, “What is actually controlling my speed?” Auto-Thrust/Throttle or Elevators? If it is the elevators, what is the Autothrottle/thrust actually doing? Will it “wake up” or is it dormant? Should I use a different vertical mode? Should I manually control the Thrust Levers?
A good rule is to follow the flight director. If you choose not to follow it, turn both of them off.
Another good rule is to read and understand the FMAs, left to right
Boeing: Autothrottle///Ailerons///Elevators
—————————-——AP status———————
Airbus: Autothrust///Elevators///Ailerons///AP status
And another one is to ask yourself, “What is actually controlling my speed?” Auto-Thrust/Throttle or Elevators? If it is the elevators, what is the Autothrottle/thrust actually doing? Will it “wake up” or is it dormant? Should I use a different vertical mode? Should I manually control the Thrust Levers?
Last edited by eckhard; 13th Mar 2019 at 20:32. Reason: Formatting of FMAs
Anybody yet pointed out that FLCH uses/accomplishes asymptotic capture of selected altitude?? ... at least that's the theory! 
S'alright Bloggs, it just means adjusting closure rate to avoid overshoot 
S'alright Bloggs, it just means adjusting closure rate to avoid overshoot
Over the years there has been much speculation and confusion with appropriate use of FLCH. When I have come across such confusion (many times in 12,000 x 777) I have explained that it is an IAS lock, much the same as the IAS button on the MD 80 series. There are a few other things that attach to it but essentially it will try to maintain selected speed.
Others have explained that there are traps for the unwary, however if you know what is going on it is a safe and expedient way to manage your flight profile. Frankly it was my preferred (non STAR) mode for descent prior to G/S capture (from above), particularly when operating in a speed controlled environment.
Maui.
Others have explained that there are traps for the unwary, however if you know what is going on it is a safe and expedient way to manage your flight profile. Frankly it was my preferred (non STAR) mode for descent prior to G/S capture (from above), particularly when operating in a speed controlled environment.
Maui.
Over the years there has been much speculation and confusion with appropriate use of FLCH. When I have come across such confusion (many times in 12,000 x 777) I have explained that it is an IAS lock, much the same as the IAS button on the MD 80 series. There are a few other things that attach to it but essentially it will try to maintain selected speed.
As I said earlier, FLCH mode is one of the few where the Autothrottle does not control speed.
In fully auto-coupled flight, everything should work out but if you decide to fly manually and then deviate from the flight-director.......watch out!
Last edited by eckhard; 15th Mar 2019 at 16:49.
Wee monkey. Why would you do either of those things?
Why would you reset to missed approach altitude before you have captured G/S?
Why would you ignore the flight director without making some compensation for what is inevitable?
Why would you not invoke airmanship and awareness?
Is it not a reasonable assumption that, before disengaging the autopilot, the brain should be engaged?
Maui
Why would you reset to missed approach altitude before you have captured G/S?
Why would you ignore the flight director without making some compensation for what is inevitable?
Why would you not invoke airmanship and awareness?
Is it not a reasonable assumption that, before disengaging the autopilot, the brain should be engaged?
Maui
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