How to enter an FMC speed constraint?

[dnc62]

Can anyone tell me how to enter a speed constraint into the FMC against a waypoint without also inputting a level constraint as well? For example, let's say the FMC is saying you will cross a waypoint PARCH with a speed and altitude of 240/8860. If ATC issue an instruction to "cross PARCH at 210kts at 9000ft" you would input 210/9000, these would be bold to indicate a requirement. If the instruction was "cross PARCH at 210kts" without a level constraint, how is that inserted into the FMC? If you put it in as 210 it takes it as a level. If you make up a level to go with the speed it will assume it is a constraint. So basically I want to force it to cross at a speed but allow the FMC to compute the correct level for the profile. And before anyone says type in 8860, that is obviously not the answer. 8860 is the computed level for a profile at 240kts not 210kts. At 210kts it would be lower. I'm sure there is an easy answer as it is quite a common instruction in the real world.
Thanks

[Avi]

You can enter 210/9000B or even a higher altitude (the B is the important point here).

[andrej]

If you want only speed restrain, you enter 210/ (XXX/). For further reading and options/ways to enter constraints into the FMS, check page 387 of the manual.

Cheers,

[Hardy Heinlin]

To be even more on the safe side, you may enter an A constraint instead of a B. (The B may cause problems with your CRZ ALT entries.) Enter the lowest possible altitude, e.g. 400 above runway level, or even zero, or even below zero. E.g.

210/0400A

210/000A

210/-1000A


|-|ardy

(The NG FMC will allow speed-only constraints.)

[dnc62]

Thank you all for the replies. I'll experiment with those solutions.
Cheers

[dhob]

For the Legacy 744 FMC, a speed cannot be entered without an altitude constraint with it, i.e. 210/XXXX or 210/XXXXA or 210/XXXXB etc. It will not accept 210/. For your specific example, it depends whether your restriction is on a SID (climb) or on a STAR (descent). If climbing, then enter 210/250B, which allows the FMC to cross where it wants (at 8860) and you won't reach the altitude prior to the waypoint. There exists an anomaly in the Legacy FMC with combined speed and altitude constraints, when reaching first of the two constraints, they both are then dropped. Again, for your example, if climbing, and 210/8000A was entered, then when the aircraft reaches 8000', both the 210 knot restriction and 8000A restriction are deleted from the legs page. Reverse it for a descent, i.e. 210/2000A will allow the FMC to calculate its most efficient decent path and ensure the speed restriction remains until sequencing the waypoint.
For the NG FMC, speeds can be entered without an altitude, in all phases of flight, including cruise. For NAT HLA Oceanic operations, this is helpful to be able to enter your Oceanic CLX assigned Mach at the Oceanic entry point as .85/. At the exit point, E/, can be entered which resumes ECON speed.
The only issue NG FMC to be aware of is Altitude Intervene is just that, it only deletes altitude constraints, not speed constraints. If your are cleared to climb unrestricted on a SID with a XXX/XXXXX constraint, selecting altitude intervene will leave you with XXX/. The speed must be deleted from the legs page manually.

[Hardy Heinlin]

The only issue NG FMC to be aware of is Altitude Intervene is just that, it only deletes altitude constraints, not speed constraints.

Thanks for the hint. I'm aware of it, but I don't know how the FMC CLB and DES pages display speed-only constraints in 1R. For example, when you have a speed-only constraint of 270 kt, I can imagine 4 options:

(A) 1R displays dashes in the altitude field: 270/-----

(B) 1R displays the predicted altitude (small font): 270/4260

(C) 1R displays the speed only: 270

(D) 1R is blank


To me, (A) looks like the best solution.
(B) is nice, but the small font might be misinterpreted as an altitude constraint because the crew is used to the display logic of the legacy FMC.
(C) could be confused with an altitude constraint, e.g. 270 feet
(D) is ... nothing :-)


Regards,

|-|ardy