I have just completed a quick circuit of FlyTampa Amsterdam Schiphol in the PMDG 737-600 NGX (the original version), and I noticed that the flightplan that I had constructed in the FMS did not correctly calculate the Top of Descent location. It was displayed at the landing runway threshold! In short, I had to use LVL CHG to start the descent. No major problem, but I am curious why this error occurred. I have completed literally hundreds (probably thousands) of these short flights, and the FMS always correctly calculates the ToD position on the ND. Can anyone shed some light on why this may have happened? I tried it several times with slightly different routes, but it failed to calculate ToD correctly with all of them.

It is worth noting that I have done circuits at this airport before without problem, except that on those occasions I used fictional waypoints to build the flightplan. This time, I used a real world SID, and then a return to the airport via the SPY VOR.

Edited August 14, 20223 yr by Christopher Low

No replies? Does nobody have any idea why this happens? I have just encountered it again during a short trip from EGPD Aberdeen to EGPN Dundee.

Would your cruising altitude be too high? 

I am always informed if the cruise altitude is not viable....and at 5000 feet, it is comfortably below acceptable limits

It may be related to how it computes the TOD. From the PMDG forum, I see someone saying

"It computes a path, where the aircraft will descent as cost effective as possible (as laid out by the given CI) under the given external circumstances such as weather, etc. while respective any speed and/or altitude restrictions there may be for the arrival."

So, it's not just using rate of descent. Could this be the issue?

The problem here is that there are never any speed/altitude restrictions or weather related effects in my flights. I always use direct arrivals (as opposed to STARs), and the weather in my flights is always set to clear and calm (in other words, no clouds and no wind). It works perfectly for the vast majority of my short flights (even if there are no waypoints between the departure and arrival), so I am puzzled why it is not working for these two. I do not get any warning messages in the FMS regarding speeds/cruise altitude etc. In fact, I do get "unable to achieve xx speed" in several flightplans, but that does not affect the ToD calculation (and is irrelevant anyway, as I manage the speeds as per my own requirements during the approach and landing phase of any flight).

@Christopher Low, I have no answer for your ToD issue but I've never had this problem with my flights.

If you want to try this short flight it might shed some light on the problem. Manchester to London takes 40mins. Here's the plan for a 23R departure and 27R approach.

LIST2R LISTO UL612 HON HON1H

Cruise is FL190 and you should see a ToD around HON.

I just let RC4 do all the work. I then just get vectors to the ILS. No FMC involved.

59 minutes ago, IanHarrison said:

I just let RC4 do all the work. I then just get vectors to the ILS. No FMC involved.

But for those who don’t use RC4 they rely on the calculated ToD.

10 minutes ago, Ray Proudfoot said:

But for those who don’t use RC4 they rely on the calculated ToD.

Yes, of course.

Are there that many who don't use ATC at all?

Just now, IanHarrison said:

Are there that many who don't use ATC at all?

Plenty use VATSIM but as I’ve never tried it I don’t know if their controllers initiate your descent or whether you request it close to the ToD.

The plan I posted earlier is about as simple as they come. If there’s no ToD point shown on the ND or in the CDU a reinstall might be in order.

The ToD is calculated correctly in 99.9% of my flights, so I do not think that it is an install issue. In fact, if I create fictional "direct to" waypoints that closely approximate the flightplans of the two flights in question, the ToD is displayed. It makes me wonder if it is a small bug that only rears its head under unusual circumstances. After all, most of my own flights are hardly "by the book"

For the record, the EGPD-EGPN flightplan is as simple as departure from runway 34 at Aberdeen on the ADN SID, and then turn south to intercept the IAN27 approach into Dundee.

Edited August 21, 20223 yr by Christopher Low

18 minutes ago, Christopher Low said:

For the record, the EGPD-EGPN flightplan is as simple as departure from runway 34 at Aberdeen on the ADN SID, and then turn south to intercept the IAN27 approach into Dundee.

I’ll try this this week. What cruise altitude did you set?

“After all, most of my own flights are hardly "by the book"”

Well arriving back at your departure airport is unusual but Concorde managed it on the “Round the Bay” Mach 2 hop so I’m sure a humble 737 can do it. 😁

This flight was Aberdeen to Dundee, so I did not arrive back at my starting destination (although a fair number of my flights do exactly that). My cruise altitude was only 5000 feet, because I like to enjoy the scenery during my flights

It is worth noting that I am using the original 737 NGX, not the NGXu.

Edited August 21, 20223 yr by Christopher Low

Chris, with a cruise altitude of 5000ft you don’t really need a ToD point. You only have to descend 1500ft to pick up the glideslope at most UK airports. 😉

I respectfully suggest you choose another aircraft for your low-level sightseeing. 😉 The Carenado PC12 is excellent for that type of flight.

For 737 flights I’d suggest a minimum of 150nm. You’d always get a ToD then. I think the flight may need a ToC before a ToD is calculated.