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Difference between Airbus open descent/ climb and Boeing FLCH.

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Difference between Airbus open descent/ climb and Boeing FLCH.

Old 22nd Feb 2020, 17:32
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SRS
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Difference between Airbus open descent/ climb and Boeing FLCH.

Would someone explain the difference ( if there is one ) between Boeing FLCH and Airbus open climb/ descent.
I would like to know how the computer logic works.

as always, Thanks in advance.
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Old 22nd Feb 2020, 18:58
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Don't they work the same way? The AFDS mantains airspeed (managed or selected on the Airbus) with pitch and climb thrust.
On the 737 the FMA announcements are N1 | MCP SPD | roll mode in use.
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Old 23rd Feb 2020, 03:37
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SRS, There is no difference between Boeing FLCH and Airbus Open Climb/Descent.
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Old 23rd Feb 2020, 08:47
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Doesn’t the Boeing FLCH put the autothrottles in HOLD mode during descent? Then the pilot can manually adjust thrust to adjust descent rate. The Airbus open descent doesn’t do that, if you want to adjust thrust manually in OPDES then you need to disconnect the autothrust. I haven’t flown a Boeing but have a few FCOM s and that is my understanding of the system.
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Old 23rd Feb 2020, 08:55
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It's ARM on the 737 and that's correct.
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Old 23rd Feb 2020, 09:05
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Yeah I flew the Avro RJ and was led to believe its auto flight system was very similar to Boeing.
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Old 23rd Feb 2020, 09:23
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The basic mode concepts are similar (between Level change and open climb/descent) however as always with these thing the devil is in the detail. There are even differences between different flavours of the 737 in terms of how the system actually behaves, most noticeably in the gain of the control laws (i.e. how aggressive the AT and AP are).
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Old 23rd Feb 2020, 10:03
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ARM also means keep your arm on the thrust levers also to anticipate the A/T sluggish response from an idle descent.
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Old 23rd Feb 2020, 10:18
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I am unsure if the logic is different on more modern boeing types, but on the 747/757/767 FLCH is very similar to an open clb/des.
There are a few subtleties however.

If engaging FLCH more than about 2000' away from your target alt, the A/C will give you full thrust in climb and idle in descent.
As aerocat mentioned, the A/THR will go into HOLD mode after 5/10 seconds in the descent ONLY. You can then move the trust levers to whatever position you like and the aircraft will maintain speed by adjusting the pitch.

If engaging FLCH less than about 2000' away from your altitude target, the A/C will give you sufficient thrust to reach the commended alt in 1 to 2 minutes.

The 2000' ish alt difference and the time the A/C allows itself to reach the target altitude is from memory as this information as been deleted from my manuals, so please take those figures with a grain of salt.
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But basically, big altitude change, FLCH=OPEN CLB/DES (but you can move the thrust levers in the descent and the a/c will keep them wherever you left them)
Small altitude change, FLCH=V/S, you just don't know what the a/c is actually targeting.

It should be similar logic on the 777/787, not sure about the archaic 737​​​​​​. Please correct if I am wrong

Last edited by Tif; 23rd Feb 2020 at 21:16.
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Old 23rd Feb 2020, 11:48
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On the 777, FLCH has the added functionality of engaging the Autothrottle if it was previously off and the arm switches are on. This particularly handy for rebuilding the auto flight system into a more familiar state after any deliberate or inadvertent manoeuvres.
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Old 23rd Feb 2020, 16:26
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For me the biggest pet peeve is that pushing FLCH opens the speed window. On the Bus the speed stays managed...
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Old 23rd Feb 2020, 17:30
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Originally Posted by AerocatS2A View Post
Doesn’t the Boeing FLCH put the autothrottles in HOLD mode during descent? Then the pilot can manually adjust thrust to adjust descent rate. The Airbus open descent doesn’t do that, if you want to adjust thrust manually in OPDES then you need to disconnect the autothrust. I haven’t flown a Boeing but have a few FCOM s and that is my understanding of the system.
Spot on!

The OPEN DES mode is a selected mode. It maintains a SPD/MACH (selected or managed) with the AP/FD pitch mode while autothrust (if active) maintains IDLE thrust. It is not to be used for final approach.

The OPEN DES mode can be engaged only if the following conditions are met: ‐ The aircraft has been in flight for more than 5 s ‐ LAND mode is not engaged ‐ The FCU selected altitude is lower than present altitude. The OPEN DES mode is engaged by one of the following: ‐ Pulling out the ALT knob ‐ Selecting a manual speed when EXP mode is engaged. Note: When OP DES is engaged: ‐ The FMA displays “OP DES” ‐ The managed LVL/CH dot on the FCU goes out ‐ The system arms the ALT mode.

The OPEN DES mode is disengaged by one of the following conditions: ‐ Manual engagement of another vertical mode ‐ Selection of an altitude higher than present altitude. V/S (FPA) engages on current V/S (FPA). The vertical mode is boxed white. If within 5 s after the reversion to V/S, the flight crew does not confirm the altitude target change by another expected action, a triple click aural warning sounds, and the V/S (FPA) is boxed white and flashes for 10 s.

When OPEN DES is engaged, pitch control maintains the target speed/Mach number, and autothrust maintains idle thrust (or the flight crew maintains it manually). The speed target may be either selected or managed. The OPEN DES mode disregards all altitude constraints.



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Old 23rd Feb 2020, 20:54
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The B version sounds more elaborate.
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Old 23rd Feb 2020, 21:19
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Originally Posted by Banana Joe View Post
ARM also means keep your arm on the thrust levers also to anticipate the A/T sluggish response from an idle descent.
Just like HOLD means to hold on to them.
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Old 24th Feb 2020, 05:41
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The B version sounds more elaborate.
It's not elaborate but is complicated. The throttle going in ARM and HOLD mode at different stages is not very intuitive according to NTSB after SFO and during certification of 787(which is similar) both FAA and EASA were not very comfortable with the design. It's all mentioned in NTSB report. Airbus OP DES or DES modes are straight forward with no fiddling required from the pilot.
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Old 14th Mar 2020, 14:36
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Adding to the above...on the 787, FLCH behaves almost exactly the same as VNAV SPD. While speed is on the elevator, it sets the thrust to achieve the desired altitude change in approximately two minutes. So a 3000 foot FLCH climb or descent will result in about 1500 fpm up or down. Naturally, when there is a very large difference between the present altitude and the MCP altitude, this results in climb thrust or idle thrust. On descents, the autothrottle still goes to hold mode once it is set, as with the older Boeings.
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