View Full Version : Enroute ATC requirements

25th Apr 2002, 09:40
G'day All,

Just wanting some background information regarding the use of an FMS/FMC when an aircraft is given an ATC "requirement".

As operators how do you manage an ATC requirement given as a "distance" to run eg Reach FL370 by 75 ABC given that ABC is in the legs page of the FMS. Is it easier for ATC's like myself to just give you a requirement by a waypoint as opposed to a distance from the waypoint (obviously outside the area of potential conflict).

Alternatively, how does a pilot calculate when he needs to change level when given a "time" based requirement. eg reach FL350 by time 0324. Time now 0315. Is the V/S functionality involved or is it mentally cacluated??.

Thanks in advance.


25th Apr 2002, 11:17
Altitude constraints with respect to a waypoint are very easy to enter in the FMC. I just enter ABC/75 (or -75 if before ABC) and that creates a new waypoint where i can enter the altitude resctriction. The rest will be done by the FMC (if within aircraft performance). If you say "reach within xxx minutes" or "at 0324" i have to do the calculations and fly it in the V/S-mode without any FMC-guidance. Everything for the B733 only, don't know about other aircraft types.


25th Apr 2002, 12:19
For en-route climb requirements with a time constraint then a little mental maths is needed. Firstly, to use your example, determine that you can make FL350! We then need to use slightly pessimistic assumptions to make sure we acheive the target set by ATC - assume that the a/c will climb at 500' per min ( it should if it can make the altitude, and will probably do better). If you have to climb by 2000' then begin the climb before 0319.

25th Apr 2002, 14:43
Up here, these days, I find that ATC usually asks if I can make a specific level by a specific DME distance. I take a rough, if somewhat educated stab, based on the performance I know that I can expect from the aeroplane. But, then, I've flown the same aircraft for the past 6 years, so I'm getting the hang of it now! :D

Mostly, I find that I can make the climb within the constraint. This is just as well because, for damned sure, if I say "no", I'll get held down at a fuel inefficient level until the other traffic is sighted and passed. Such is life.

Carnage Matey!
25th Apr 2002, 15:04
Well my Airbus draws a nice 'level off' symbol on the map at top of climb or descent so you can get a quick idea from that and your knowledge of aircraft performance. Our FMC also has a Progress page which will give you time and distance predictions for vertical changes, comparing both normal and expedite modes. If you want to be really fancy you can enter a PBD (Place-Bearing-Distance) waypoint in the flight plan, attach a level constraint and let the FMC do it for you exactly. General practice on being level by a certain time is to calculate roughly what V/S you need, work in a fudge factor for the time spent level-ish at each end then climb or descend at that rate.

26th Apr 2002, 02:59
On the B744 and, I suppose, the other glass Boeings, distance is a piece of the proverbial.
Provided the CRZ page alt is at or above the constraint altitude, enter the constraint on the FIX page and it will provide a distance, ETA as well as a point on the ND map. An alternative, as previously noted, is to make the required distance a waypoint - ABC/-nn or place/bearing/distance - and look on the LEGS page to see predicted altitude.
WRT time based requirements, the above will also work for initial climb or during a climb. If, however we are refering to a step climb, eg "be level FL350 by time tttt" whilst cruising FL310, as I understand it, mental arithmetic is in order. You can, of course, set a time on the FIX page to remind you that something needs to be done.