Correct use of Altitude Constraints B737CL
Thread Starter
Joined: Oct 2000
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From: Wild Blue Yonder
Correct use of Altitude Constraints B737CL
At my airline, everyone is new to the 737 Classic. Still a bit of growing going on with the FMS and CDU entry. The use of the altitude data entry for waypoint crossing is not clearly spelled out in the FCOM with only one page speaking to it. I personally believe that is because the A or B suffix should be self explanatory when applied to waypoint altitude constraints, however, I am seeing a lot of crews wrongly applying the suffix to EVERY waypoint they are cleared to descend and level off at.
The logic employed, which I think is incorrect application of the FMC feature, is that it will recalculate all the altitudes behind the waypoint, aiding in descent. This may be correct IF there is continued descent, but not when one is leveling off at the fix specified.
Example:
"Boeing 123 cross LUCKY at FL130 at 250knots" I am thinking the entry should simply be LUCKY 250/130 and not 250/130B because there is no constant descent to increase the descent gradient since there is a level flight segment.
Looking at the computed numbers for the waypoints after doesn't show any change to backup the B suffix entry logic.
Trying to substantiate my logic against the inappropriate use of the at or below entry.
The logic employed, which I think is incorrect application of the FMC feature, is that it will recalculate all the altitudes behind the waypoint, aiding in descent. This may be correct IF there is continued descent, but not when one is leveling off at the fix specified.
Example:
"Boeing 123 cross LUCKY at FL130 at 250knots" I am thinking the entry should simply be LUCKY 250/130 and not 250/130B because there is no constant descent to increase the descent gradient since there is a level flight segment.
Looking at the computed numbers for the waypoints after doesn't show any change to backup the B suffix entry logic.
Trying to substantiate my logic against the inappropriate use of the at or below entry.
Joined: Oct 2006
Posts: 658
Likes: 1
From: Ankh Morpork, DW
The FMC entry should be the same as the clearance. To put FL130B is not the same clearance as at FL130. There's little written on this because there should be no need for further explanation.
Is English not the primary language at your airline? When I worked in Asia, I noticed some little issues caused by the nuances of prepositions.
Stamp out this "tribalism" as it is not correct and could be unsafe.
Is English not the primary language at your airline? When I worked in Asia, I noticed some little issues caused by the nuances of prepositions.
Stamp out this "tribalism" as it is not correct and could be unsafe.
Joined: Apr 2012
Posts: 683
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From: In Space
FL130A = At or above
Fl130B = At or below
If your cleared to a waypoint with an altitude/FL , as you said LUCKY FL130 then putting /130 in the box will be suffice as the FMC will calculate a path to reach it if not a CDU message will say unable next altitude.
Fl130B = At or below
If your cleared to a waypoint with an altitude/FL , as you said LUCKY FL130 then putting /130 in the box will be suffice as the FMC will calculate a path to reach it if not a CDU message will say unable next altitude.
