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737 NG FMC waypoint Alt constraints: A, B and both?

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737 NG FMC waypoint Alt constraints: A, B and both?

Old 25th Feb 2023, 22:57
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737 NG FMC waypoint Alt constraints: A, B and both?

Can both a "A"(at or above) and "B" (at or bellow) altitude restrictions be manually entered in the 737 CDU/FMC Route/Legs page?

Example: /FL180AFL200B

Someone told me this was not possible and the only instances where such constraints can be seen are those in which the restriction makes part of the coded procedure, and not by pilot input through the CDU.
Supposedly pilots can only manually enter "A" or "B" alt constraints.

Thanks for any answers,

Last edited by jcomm; 26th Feb 2023 at 07:54.
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Old 26th Feb 2023, 05:02
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Only one altitude constraint can be manually entered. The sort of constraint mentioned, with both an at/above and an at/below altitude at the waypoint, can only come from the database and cannot be manually entered.
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Old 26th Feb 2023, 07:56
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Thank you for the prompt answer @ Snoopy58

Last edited by jcomm; 27th Feb 2023 at 11:33.
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Old 26th Feb 2023, 16:48
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Originally Posted by jcomm
Can both a "A"(at or above) and "B" (at or bellow) altitude restrictions be manually entered in the 737 CDU/FMC Route/Legs page?

Example: /FL180AFL200B

Someone told me this was not possible and the only instances where such constraints can be seen are those in which the restriction makes part of the coded procedure, and not by pilot input through the CDU.
Supposedly pilots can only manually enter "A" or "B" alt constraints. And you might know all these already, just typing out some options.

Thanks for any answers,
As answered above, not possible manually. However there's several way to still comply:

1) Easiest solution: Use an AT LEVEL constraint that is halfway between the above and below (FL190 in your example), and enter that for the DBFIX. Technically compliant, but might not be the most efficient profile, especially with large vertical windows.

2) Other option: Make sure all other constraints are in, no constraint on the DBFIX. Check to see what altitude you will cross the DBFIX. This will tell you if you need to have an ABOVE or a BELOW restriction, and enter that as an ABOVE or BELOW constraint as applicable. Now, you will still have to pay attention if conditions change, so not optimal in busy workload.

3) Quick&Dirty workaround: Create a USERFIX 1NM before the DBFIX you need the ABOVE&BELOW restrictions for. For descend, enter the Below constraint on the pilot USERFIX, enter the Above constraint on the DBFIX. For climb switch that around. This will always ensure you will comply with both the constraints, if you do it the other way around, you could end up missing the constraint by 300".

4) Combine 1 & 2. Check the planned crossing altitude for the DBFIX without any constraint. If it is in the window, make that the AT LEVEL constraint for the DBFIX. If it is below, make it AT LEVEL for the ABOVE restriction, If it is above, make it AT LEVEL for the BELOW restriction.

I am on the bus, so possibly different on the 737, but think this should still work.
I use option 3, but I image some people don't like adding waypoints in the SID/STAR. And you might know all of these already, maybe not everyone does.

Last edited by hans brinker; 26th Feb 2023 at 17:48.
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Old 27th Feb 2023, 11:32
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Thank you so much @ hans brinker for your additional hints / techniques to overcome not being possible to manually enter the "AB" constraints!

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Old 27th Feb 2023, 13:46
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Originally Posted by hans brinker
As answered above, not possible manually. However there's several way to still comply:

1) Easiest solution: Use an AT LEVEL constraint that is halfway between the above and below (FL190 in your example), and enter that for the DBFIX. Technically compliant, but might not be the most efficient profile, especially with large vertical windows.

2) Other option: Make sure all other constraints are in, no constraint on the DBFIX. Check to see what altitude you will cross the DBFIX. This will tell you if you need to have an ABOVE or a BELOW restriction, and enter that as an ABOVE or BELOW constraint as applicable. Now, you will still have to pay attention if conditions change, so not optimal in busy workload.

3) Quick&Dirty workaround: Create a USERFIX 1NM before the DBFIX you need the ABOVE&BELOW restrictions for. For descend, enter the Below constraint on the pilot USERFIX, enter the Above constraint on the DBFIX. For climb switch that around. This will always ensure you will comply with both the constraints, if you do it the other way around, you could end up missing the constraint by 300".

4) Combine 1 & 2. Check the planned crossing altitude for the DBFIX without any constraint. If it is in the window, make that the AT LEVEL constraint for the DBFIX. If it is below, make it AT LEVEL for the ABOVE restriction, If it is above, make it AT LEVEL for the BELOW restriction.

I am on the bus, so possibly different on the 737, but think this should still work.
I use option 3, but I image some people don't like adding waypoints in the SID/STAR. And you might know all of these already, maybe not everyone does.
Option 5: Get a bigger Boeing which CAN handle A/B

Option 6: Disconnect everything and hand fly whilst remembering every constraint. 👍
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Old 28th Feb 2023, 19:05
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Originally Posted by rudestuff
Option 5: Get a bigger Boeing which CAN handle A/B

Option 6: Disconnect everything and hand fly whilst remembering every constraint. 👍
I would love option 5, but sadly can't afford one. Option 6 would definitely freak my FO out 😂.
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Old 28th Feb 2023, 19:06
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Originally Posted by jcomm
Thank you so much @ hans brinker for your additional hints / techniques to overcome not being possible to manually enter the "AB" constraints!


Thank you 😊.
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