Question on Boeing FMC Vnav

[DeltaSix]

Howdy heavy drivers, was just wondering if you can help a poor confused :confused: piston driver with some FMC question on VNAV :{

The first question is on the ECON CRZ page, particularly on the To T/D. This gives you the time and position of the top of descent calculated by the FMC.
Do you actually use this ? Doesn't either centre or radar give you the descent clearance, or do you just use this for reference ?
If you do use it, on what circumstances ?

Second - ACT ECON DES page. The DESDIR key. I understand that this causes the FMC to ignore all LEGS page altitude restrictions and descend directly to the AFDS MCP altitude. My dumb question is, when do you use this if you are flying the STAR ? Does the STAR needs to be cancelled before using this or from your experience, are there any other circumstances when this needs to be used ?

Third - ECON CRZ page. "STEP TO and AT ". This is where the Step Climb begins. How do you use this in practical terms ?

Fourth - Is there anything in the FMC that doesn't work well that I need to look out for ?

Thanks guys

D6

[Macrohard]

Why ?

Don't the Boeing manuals explain it well enough?

[blueloo]

Firstly - Honeywell (Pegasus or non-pegasus) or Smiths Industry FMC?


Yes the TOD of descent from FMC is used, - ATC generally give you a clearance like "When ready descend XXX" - the clearance is given generally well before the FMC calc TOD, so set the MCP and at the appropriate time the VNAV will start you down. If there is a big tailwind, then you might use "Descend now", to start into the descent a bit earlier. SOme guys will start down in V/S, some wont. (It depends on what protection you want from the automatics from overspeeding etc).

Cant remember the DESDIRC key - was that a pegasus thingy? If so I dont remember anyone using it - and generally you wouldnt use it on a star, unless specifically told to be at a different altitude to a star constraint, in which case you would probably just re-enter it manually. (I presume it is the same as the ALT intervene on descent on SMiths industry - nobody uses that either)


I remember Step Climb programing an FMC used to be more hassel than its worth (only ever saw about 2 guys succesfully program it to automatically climb). It was easier just to climb when 1800 feet below optimum to 2000 feet above, using either just a normal FMC altitude re-entry, and MCP change. (or some guys would climb at VS/+500.

[Avid Aviator]

1. T of D already well answered.
2. If you have not below and/or at or below restrictions on a STAR, selecting DESDIR (or equivalent) removes these from the LEGS page. If these altitudes were limiting the descent profile, then the profile can be recalculated to the optimum (idle) VNAV profile; if the STAR requirements were not limiting (eg. 9000B at 20nm to run) then obviously it would make no difference. In practice, ATC usually only cancel a single descent limitation, requiring deletion of the appropriate altitude restriction on LEGS. "Direct" is generally used on climb, when a "climb unrestricted" type clearance may be issued.
3. "Step To" is useful for fuel remaining/ETA calculations. Inserting step climb points in the LEGS may give a more accurate ETA/Fuel remaining than relying on FMC logic. The FMC assumes you'll step when it's optimum time, which is great around Australia when there's no traffic. If you're longhauling behind/below someone, and you know you'll be held down to a certain point, you can put this into "the box" to more accurately show the real situation.
4. Yes.
Hope that helps.

[Keg]

SOme guys will start down in V/S, some wont. (It depends on what protection you want from the automatics from overspeeding etc).

Definitely showing your 'light twin' thinking there Blueloo! I raised that point a short time ago and was informed (in no uncertain terms) that use of modes that gave a more direct control of the AFDS to avoid over speeding was not needed in the 744! :eek: :ouch:

Delta six has covered the step to and at pretty well. The 'step to' and 'at' is guidance information from a practical sense. It's telling you where the FMC thinks it's going to climb. If you're going to climb earlier or later then you can tell the FMC that as well by inserting the altitude constraint in the LEGS page. It means that if you've been held lower than desired for a period of time or are taking advantage of favourable winds (or avoiding unfavourable ones) the FMC can provide you with a more accurate fuel over destination and waypoint ETAs if it knows when you intend on stepping up. With no 'hard' constraints in it from the flight crew, it will tell you where the FMC thinks you should step up. This calculation is based on a/c weight only and won't take into account either forecast or actual winds.

[huckleberry58]

1. I use the TOD point as a cue to make my PA, usually 5 minutes before the TOD point and then request descent clearance around 1minute to TOD. If control/centre give me descent clearance at my discretion prior to that point then I wind the MCP ALT to the clearance flight level. If you enter the descent winds it gives quite an accurate descent point.
2. For the 777 there is a DES NOW key which gives approx. 1250ft/min rate of descent until you intercept your VNAV profile. It won't delete your altitude constraints on the leg page unless we press the ALT button on the MCP. Each subsequent push on the button deletes the next constraint, so if there are 3 constraints on the STAR then pressing the button three times will delete all the constraints. In practice it becomes a non issue because the flight profile is higher than the constraints and somewhere around 10,000 to 15,000 feet we go to basic mode (no LNAV or VNAV but HDG SEL and FLCH or V/S or whatever) so constraints wont matter.
3. STEP TO is useful as a guide for when to climb. Depending on the step size you enter into the FMC (RVSM or ICAO which equates to 2000ft or 4000ft) and the accuracy of the enroute winds from the flight plan which have been entered into the FMC and other factors (cost index etc), it will tell you when to go up. This should be roughly the same as your hardcopy flight plan. Normally I will follow the steps but I do check the optimum flight level on the VNAV cruise page and use that as a guide also. For the 777 the FMC will give you 3 levels 1. Optimal 2. Max and 3. Recommended.

[DeltaSix]

Thanks to all who replied, appreciate it very much.

huckleberry58, on point 1, what if you don't get a descent clearance before the FMC's calculated TOD ? do you just press the ALT HOLD button to keep the aircraft from descending ?
After you pass the TOD, and if you did use the ALT HOLD button, do you just switch on the VNAV again to re-acquire the VNAV descent profile ?

On point 2, DES NOW - which altitude will this try and capture ?... LEGS page?assuming that MCP ALT is lower.

Also, can someone please tell me where I went wrong with this exercise, Take off from point A, levelled off at 3000, I've programmed the FMC before take off that waypoint B's altitude constraint is 9000B ( the SID transition point) and point C at 12000B, I've set the MCP ALT to FL340 which is the same as what I've entered as the CRZ ALT and engaged the VNAV after 3000. The aircraft ignored the altitude constraints and kept on climbing to FL340. At point B it was at 11,000 and point C was at FL150. what went wrong ?
The programmed SID was definitely ACTIVATED and EXECUTED. Is there something that I've missed doing ?

Thanks a million guys.

D6

[Chimbu chuckles]

Excuse me if I am wrong but by your profile I am guessing this was not in a real aircraft or real sim? If this is in FS2004 or similar all bets are off.

In the 767-300ER

If you don't reset the MCP ALT to a lower altitude approaching the calculated TOD you get a 'reset alt' message and it maintains level flight until you do. If you press 'des now' in the descent page of the FMC the aircraft will eventually capture the alt in the MCP window having descended gradually to capture the 'optimum' descent profile it has calculated and done it's best to meet any descent restraints in the VNAV profile.

In a climb such as you describe there is no reason for the altitude constraint busts provided you are in VNAV and not FLCH or VS. On descents they sometimes struggle to meet such constraints because the overspeed protections won't allow a high enough ROD to meet them....strong tailwinds for example might overpower the FMCs ability to drive the aeroplane down in time. What I often do is put in a false or stronger than forecast tailwind in the descent page....other people put in fake icing levels...both cause the FMC to move the descent point a little further away from desto thus giving the FMC VNAV a fighting chance. If descent winds are forecast as negligible I would put in a 25kt tailwind for instance.

Another thing we commonly do is delete all the hard altitude constraints from the LEGS page and just use the MCP window. An example is the DVR 4G at LHR...it has a 6000' alt constraint until well past Epsom NDB and a bunch of others I can't remember off the top of my head. If you leave them in you have to fiddle with the FMC when cleared above them (pressing 'CLIMB DIR')...if you delete them you simply reset the MCP window to assigned alt and hit VNAV or FLCH. Because we always get cleared initially to the 6000' SID alt limit and more often than not cleared higher well before the SID constraint is met we just delete them all and use the MCP....you don't want to be head down fecking with the FMC in the middle of a busy noise abate departure.

You asked if there were any other traps...don't **** with VS...it has no overspeed protections and can cause the aeroplane to climb or descend away from the MCP alt. We do use it for various things...like CDA approaches at LHR...but it is a mode with more exposure to drama than FLCH, which will always drive you towards the alt in the MCP window, or VNAV, which does the same thing but via the profile you have prgrammed in the FMC.

Low level ALT CAP is another real pain in the arse. It can happen on climb or descent. On climb it may happen if you are light and have a low level restriction...say 3000'...as you get airborne and call departures they clear you higher immediately but because of the initial high ROC you have 'ALT CAP'ed approaching 1000' and despite selecting a higher altitude in the MCP window the aircraft continues to level off at 3000'...until one of you notice ALT CAP in the FMA and hit VNAV or FLCH to get out of it.

On descent it can happen when being progressively cleared to lower levels while tracking for an ILS...highish RODs with an assigned altitude of say 3000' and 'APP' armed...LOC Captured and ALMOST captured glideslope and the aeroplane again ALT CAPs and starts to level off causing the glideslope to head south rapidly:( A flurry of hands winding the MCP ALT down to some very low value and engaging FLCH or VS in an attempt to change the momentum of a 136T aeroplane in time to avoid an unstable approach and ensueing GA is not a great look.:E

Chuck.

[huckleberry58]

For point 1 as Chimbu chuckles said, 2 minutes prior to TOD of descent we will get an EICAS msg 'RESET MCP ALT'. If we don't do anything the aircraft will maintain its flight level, no need to press ALT hold. The 777 will always maintain the more conservative altitude which has been set on the MCP or FMC, in the case of a descent it will be the higher of the two and for a climb it will be the lower. There is alot of integration between the MCP and FMC in the 777 so pressing ALT hold is not required. If we are past TOD, once we get descent clearance we would set the MCP altitude and press the ALT button on the MCP to start a descent and the aircraft would try to recapture the VNAV path. If we do use ALT hold then yes, I would re-engage VNAV (or possibly FLCH) to re-acquire the profile.
Point 2. Yes, if the MCP alt is lower then when using DES NOW it will descend at approx. 1250ft/min until intercepting the original VNAV descent profile and descend to the next FMC altitude restriction on the LEGS page.
Your exercise seemed ok.