777 VNAV and Altitude Constraints
 

Hi,

I have a question regarding the calculation of VNAV profiles and altitude constraints on the 777. In calculating the climb profile, altitude constraints along the route will be observed by the calculated VNAV profile. However, at some point, an altitude constraint on the route will become a constraint relating to the descent path, rather than the climb path.

In calculating the climb path, is there a specific rule which determines whether an altitude constraint in the route is treated as a constraint during the climb phase?

To illustrate by means of a simple example, suppose a route was entered from EGLL to KJFK consisting of great circle waypoints every 20nm. If an altitude constraint of 6000ft was entered on the first waypoint (eg 20nm out of EGLL) this would be treated as a climb constraint. Similarly a constraint at the 40nm point. How far out along the route would an altitude constraint stop being considered in the calculation of the climb profile? Is it related to the distance from origin, distance from destination or some function of the trip length.

Thanks in advance for any info.

Don't think it's that complex. If your refering to the /6000B constrain on an LHR SID the aircraft climbs to whatever is set on the mcp or the constraint whichever is lower, until you press the alt sel knob to erase the constraint.

If you then turned round and flew a descent into LHR or continued to JFK the constraint would apply the other way round. It would adjust the top of descent point to acheive /6000B by that waypoint.

I'm not sure it has anything to do with acheiving cruise alt/lvl.

Hope that helps.

If I understand you correctly (I don't fly the 777); After you have entered route + ZFW + CRZ alt, the box will have the means to calculate your T/C and T/D. If you have a constraint before your T/C, then naturally this will be a constraint in the VNAV climb path. If for some reason you have a constraint in the CRZ path lower than entered CRZ alt, you'll get the message unable CRZ alt. And finally, any constraints after T/D will be for your VNAV descent path.

On another type the FMS looks at total track miles and makes the first half CLB constraints and the second half DES constraints. Its not perfect for short sectors!!

I don't know the exact answer to this question, but I think you may be missing a key point here that may explain things (or maybe not). The T7 FMC (assuming it operates similarly to other Boeing models) will calculate T/C and T/D points as well as crossing altitudes for each waypoint, plus S/C points when warranted. It would seem logical that the FMC would switch over after it calculates arrival at the final climb altitude programmed, or immediately in the event of a early descent. An "UNABLE CRZ ALT" indication would mean that the T/C and T/D points (as calculated) are coincident, in which case the switchover point would be exactly halfway through the flight.

edit: bollocks, RYR738 beat me to it!

During the climb the 777 will follow any altitude constraints set in the vertical profile while in VNAV. Between constraints it will climb according to the climb derates and speeds set in the FMC. If there is a cruise constraint set which is below the FMC selected cruise altitude, the FMC will level off at the constraint altitude in VNAV PATH at selected VNAV CLB speed. Fuel and eta predictions will still assume step climbs as weight reduces. If you keep a cruise altitude restraint for the whole cruise segment ( you would not do that, as in practise you would nominate the constraint as a new CRZ ALT) the aircraft levels off in VNAV ALT at ECON CLB SPEED. The FMC calculates the T/D from the constraint altitude. See 777 tech manual 11-30-20(-23)