Couple of Flight Sim Questions

[Tomliner]

If it is not out of order to ask,for traffic descending toward Edinburgh (EGPH) on,say, a Tweed 1A SID,what would be a most likely FL at Margo?
Secondly,a question for the Boeing FMC experts.When an instruction is received from ATC of the type 'descend to FL XXX to be level 20 before Trent'for example,how would this be entered in the FMC? Would it be done using the fix pages?The sim which I am refering to is the LevelD 767.
I hope these questions don't infringe on the rules of this Forum,but I thought who better to ask than the professionals.Best Wishes

[bucket_and_spade]

Dunno about the first question but to make the altitude constraint given by ATC in your second question, and assuming a Boeing FMC, I'd insert the waypoint "TNT/-20" in the LEGS page (the -20 means 20 miles before) before the waypoint "TNT" (assuming TNT is on your route and in the LEGS page). Next I'd put "/XXX" opposite this new waypoint on the right and voila, the VNAV mode will try to make that constraint.

[Tomliner]

Thanks for that Bucket and Spade.I should have spotted that in the FMC instructions.I have just looked,it is in there,I must have missed it.Probably an age thing!

[Doug E Style]

Tomliner,

There is normally a restriction of FL260 at MARGO on a TWEED 1A arrival into Edinburgh (or the LANAK arrival into Glasgow for that matter). As often as not though, at that stage the aircraft will be on radar vectors but ATC will still clear you to FL260 to be level abeam MARGO.

Regards.

[Tomliner]

Thanks for that info Doug E Style.I was aware of that restriction but the reason for asking is that when using the LevelD 767,it seems difficult to descend rapidly enough to achieve the next restrictions as shown on the chart,even with speedbrakes.More practice needed!I know it's not real,but it is interesting.

[G SXTY]

It's probably more realistic than you think. Many arrivals into EDI involve some sort of dirty dive - The TWEED 1D tells you to expect 7,000' at TARTN, which is always entertaining . . .

[Rainboe]

it seems difficult to descend rapidly enough to achieve the next restrictions as shown on the chart,even with speedbrakes.

You have another trick. As long as you are not ATCspeed restricted, youu can get an even better descent by bumping up speed towards Mach/IAS limits. You really drop out of the sky with incredible descent rates, but there is a limit of 3000fpm within 3000' of your sector safety altitude. If even that won't work, you just have to tell ATC.

[JEM60]

I'm an ex-PPl and a Flight Simmer. I would be grateful if a professional pilot would take the trouble to give me a suitable rate of descent, with no restrictions, say into FuertaVentura in the Canaries for instance, in a 737/A320. type. I am aware of the trebling of the height loss=distance to go formula, which gives, say, 99miles to loose 33,000 feet. Are rates of descent of 3,000 feet or more per min current and acceptable.Sorry if this is a childish question, but cannot fly the real thing anymore. Cheers, and love reading Pprune. Thankyou.

[Tomliner]

Thanks Guys or Gals for those interesting replies.Re the FL at TARTN,I may have read some discussion about it somewhere.Possibly the ATC section in Pprune.Away from flight sim and in the real world,would a descent of 3000fpm be uncomfortable for the pax?

[pre3mhjt]

Not really, the pressurisation system should cope with most rates of descent that you can throw at it in normal ops, the main constraint would be the 3000fpm within 3000ft of the applicable MSA/SSA, other than that you can really use any descent rates that you feel are appropriate, obviously taking into account IAS/Mach limits and cabin pitch angle (hopefully the service would be finished by this point and the seatbelt signs on though!).

[JEM60]

Pre3Mhjt.Many thanks for that

[Rainboe]

The VNAV on the 737 is not very good or stable. From a high altitude descent, you can often get descent rates initially of 4,500fpm, especially when it thinks it is too high or having trouble maintaining the computed profile, even if you have entered various flight level winds to help it calculate more accurately. It common to have to manually take over in LChng or VS when the speed is rapidly approaching barbers pole, and it will breach speed limits unless you do take over. It seems very unstable sometimes, not at all like the 747 system which is identical, but works well. ROD varies a lot, but generally reduces lower down to 2000fpm+. One has to carefully check what speed/altitude parameters are input on STARs or set required ones yourself as this affects the computed profile, which is displayed as a sort of GS needle on the PFD. The descent calculation is unreliable sometimes, and it can suddenly recompute and come up with a different profile. Descending into Leeds is especially bad for some reason, the descent can suddenly change by several thousand feet. Then if you get any directs, it recomputes the whole thing again. Compared to the 747, it is not very good.

[JEM60]

Thankyou Rainboe. I have never found one of your posts boring. Cheers.

[EGCC4284]

3 x each 1000ft to lose plus a mile for every 10kts of speed to reduce.

So at 36,000ft at 280kts, it would be about 120 miles to descend and slow to 160kts etc etc etc

Dont forget that a stong head or tail wind will alter these figures

[Rainboe]

Jem

Thankyou Rainboe. I have never found one of your posts boring. Cheers.

Which post was that?!

[wobble2plank]

The airbus has a variety of modes with which to trap you in the descent. Whilst the 100 miles gives you about 30,000' works the good old Frenchies had to complicate the descent modes!

If you have a way point with a vertical limit on the final approach to your field you can commence descent by 'pushing' the altitude selector and giving the auto pilot a 'managed' descent. This will allow the AP to speed up and slow down within a defined bracket (displayed in magenta on the speed tape) whilst adhering to the altitude limitations as best it can. If they go amber it can't make them and calls for 'MORE DRAG' in the PFD and MCDU. Very ambitious if you are high and heavy! Pulling the altitude selector with the next altitude dialled up gives you 'open descent'. This will descend you, at the best rate, ignoring waypoint altitude constraints, for a given speed. In other words, you dial up 3000' in the height window and pull and it will barrel down as fast as it can whilst holding your selected speed. Want to go down quicker? Use the speed brake. Want to go down really quick? Use the gear! Also you have V/S mode, this will ignore speed to give you the rate of descent, as dialled up in the FCU. That is, if the speed goes below selected then auto thrust will keep it there. However, if you dial in -5000fpm and pull V/S, watch the speed go off the tape! Generally V/S is used for 'finesse' and open descent for 'get me in the ball park'.

The upshot is that if you have a well planned MCDU route with well defined vertical, lateral and speed waypoints then give the 'bus managed nav and descent and it will get you there. If not, there are lots of options to play with.

Enjoy.

[Tomliner]

As a complete amateur,it seems to me that the descent procedures are not just a case of 'let VNAV get on with it'.Does the procedure vary with different airlines.For example,do some airlines prefer the use of FL change against VNAV or vice versa?Hope I'm not getting out of my depth here!

[Rainboe]

No, they both have their place, along with VS, for flying any managed descent and approach.

[Atreyu]

Using VNAV depends on the kind of FMS you have, and company approval for using some/all of it's features. On the humble ARJ, we can't use VNAV as we have no approval for its use; even though 5 of the fleet have the GNLU FMS kit (GNLU can accept ALT/SPD constraints), which is roughly similiar to a 737NG.

FL260 by Margo
FL180 10d before ESKDO?(I think this is a new one? Our plates have changed)
FL160 at SLP (defined by TLA 10d)
7000' at TWEED (the old plate used to say " 7000' or FL Equivalent" again this may have changed)

Generally if you check with ATC, the Speed Limit Point isn't enforced and you can maintain high speed until FL100. Also in reality it tends to be vectors from before MARGO, eventually vectoring to the east of the city of Edinburgh for a RW24 arrival, or south west of the city near the towns of Bathgate and Livingston for a RW06 arrival.

Regarding the TNT restriction, you can use the methods described or for a rough guide use your Mach no. as miles per minute, i.e. at Mach .70 you're doing roughly 7 miles per minute

Example.

FL350 cleared FL280 20nm before TNT

If you were 50nm from TNT, and needed to be level at FL280 20nm before TNT, you have 30nm to lose 7000', at 7 miles per minute (M.70) you have 3 minutes roughly (7x3=28) so the descent rate required would be in the region of 2300fpm. This is ballpark stuff and might need adjusting regarding strong head or tailwind but should see you there or there abouts. If you haven't got fancy Nav kit this method works well.

If you do have a sophisticated FMS, with a lower level selected on the MCP, and being in a descent mode (LVL change or VS) it should paint a green semi-circular line on the ND, which tells you the aircraft will be level at that point at the current vertical speed, this line takes into account current headwind/tailwind and airspeed and is very accurate, if you program in this distance constraint as bucket_and_spade suggests i.e. TNT/-20, then alter your vertical speed to match the green line onto this new waypoint, you will level exactly at the level selected on the MCP at the point your 'aiming' for.

Be careful regarding this though, not sure how MSFS works regarding its ATC system, but in reality you can be cleared to a higher level and told to 'expect' a lower level by XYZ

Example

"Callsign 123, Descend FL300, expect FL260 20 before Trent"

Now having selected 30000 on the MCP, the descent profile green line will only work out a descent to FL300, so my ballpark method shown above can give a rough guide on what kind of descent rate will be required to maintain the profile until you are cleared further descent when you can then fine tune if you need to.

Sorry for the long post!!! I haven't really discussed VNAV since we don't use it and I'm not overly familiar with its operation.

Atreyu

[Tomliner]

Thanks to you all.Atreyu,that info is very interesting.Re ATC in Flight Sim,the default ATC is pretty unrealistic,although I believe that more accurate add on software is available.A number of add-on payware aircraft are available with very detailed FMCs and systems.Aircraft such as the 747-400,767,737NG and various Airbus types.Best Wishes