737 NG TOD calculation
 

Hi all!
I'm a flight simulation enthusiast and I'm currently "flying" a good model of 737 NG (600-700-800-900).
Recently I had a discussion with the developers, cause they told me that after the flight planning in the FMC, the system needs the insertion of at least a destination runway (or the supposed arrival procedure) in order to be able to calculate the Top Of Descent point.
Now, reading the 737NG FCOM I did not understand exactly that, and so I'm here to ask a real 737NG pilot if that assumption is true or if the FMC can calculate the (estimated) TOD anyway knowing only the destination airport.
Thank you for your patience!

The FMC works the TOD backwards, i.e from destination to Origin.
A STAR and Approach will have height/or speed restrictions plus forecast winds and the use of anti-ice and ISA Dev/ QNH will all contribute to where the TOD begins.

In reality we put in the expected arrival and Approach so it will give us a planned TOD.

Thank you!
So they told me the real procedure.
Glad to hear that the model is correct!

Even without a STAR or a runway, the box knows the elevation of the landing airport so will have a rough TOD point displayed - It just needs a start point (your cruise level) and an end point (landing elevation). When you enter the approach, it will be more exact. 3 times and a bit is a rough guide though. ie. 100nm at 30,000'. 50nm at 15,000'. Then just build in your slow down. 1 extra mile for a 10 knot slow down whilst conducting a continuous descent. All the best!

Well, this is exactly what I intended, and it took to me some time trying to convince development guy about this capability, but, withou success.
At the moment in the model I'm using, no TOD is calculated without at least a destination runway insertion.
Not a big deal, but it is a matter of principle :ok:

Thank you!

I can´t ever recall seeing a TOD in the real aircraft prior to loading an arrival.
Thats with 10.6 through 10.8 I think.
That may just be because I have always loaded an arrival prior to reaching that point.
ps kudos to you zetato for stating that this is about flight sim in your first sentence. thanks.

You're welcome, and yes, I'm an avionic technician in real life, but not on 737, and I like to fly advanced online simulation at home, so I'm really interested to know real procedures about the models I'm using :)

If the final fix is the arrival airport nav aid (ie JFK VOR @ JFK), 150/0100 (150 kts @ 100') and it will figure a TOD.

Prior to that, without a STAR or approach, it shows the airplane arriving at JFK at the final cruise altitude which means there is no TOD.

That's one way to generate a TOD before a STAR/APP is selected.

Will an altitude/speed entry have to be made at the arrival airport nav aid? (Or does the FMC assume a speed/height of 150/100?

It's been suggested to me (personally unconfirmed) that an airport ICAO (e.g. KJFK) as a final waypoint in some FMCs is sufficient to generate a T/D. This might explain the varied reports we're getting here. Not all company routes contain the destination airport as a final waypoint.

Ok, I'm starting to be a little confused....:O
Usually I prepare my FP in the FMC inserting Departure, Arrival, the route part, the Perf Init data, flaps, cg etc. until Preflight Complete is obtained.
Then, a soon as ATC assigns it to me, I insert the SID.
At this point, all work like a charm, once in flight on the CDU Progress page I can read the distance to TOC, but, once in Cruise phase, I have no distance from TOD shown, unless I insert a runway or a procedure (STAR or APP) for the Arrival Airport.
I was thinking, as "piston broke again" said, that the FMC was able to compute a raw TOD knowing the arrival airport elevation, and then had refine the calculation once others data have been inserted (procedures, winds, runway etc.), but it seems not so confirmed so far. :confused:
Anyway, thanks to all the guys partecipating at this discussion, for me is really interesting :ok:

In reality you would put a STAR and runway and expected SID.

There would be a route discontinuity, which maybe the reason why as the route will just come to a stop without a runway selected. If you don't want a STAR just use a 10 mile fix on the runway centre line at 3000 210kts.

Ok, understood.
No, I always use STAR and Approach procedures if available, if they're not, then I use to enter one or more reference point as you said.
My question was only regarding the case I'm flying in cruise waiting to know the arrival STAR from ATC without have entered anything more (for the destination) than the arrival ICAO.

I can definately see the attraction of doing it how line pilots do it day to day, so hopefully this helps. In real life, nobody that I have ever flown with has ever put a height and speed at a destination VOR or Aerodrome Reference Point in order to get an approximation of their TOD. In general, (at my airline, others will be different) the pilots have an idea of what arrival they will get and put that in the FMC in anticipation of the clearance, then, when they have recieved the clearance from ATC, they conduct a thorough briefing with about 80nm until TOD. Ideally the briefing would be complete with 20 or 30 nm left until TOD and then a PA can be made telling the pax what the destination wx is etc.
as far as setting up on the ground goes, most will do all the things you have stated but in addition will put their departure and destination airfields in the fix page with a 25nm ring around them. This is to provide a terrain clearance awareness should there be an emergency on departure or arrival. They will brief the minimum terrain clearance height within 25nm. They also make sure the enroute and descent winds are loaded and are similar to the flight planned winds. This can have a big effect on fuel used etc.

NSEU - you'd have to enter the speed/altitude on the R side of the FMC.

Farmer - Rarely done but I have used it. The best recollection I can think of right now is when the FMC arrival fuel is much higher than planned. "What the heck is it looking at??" Check and it shows over the last fix(co-located with destination ie JFK) at cruise altitiude. Enter a speed and low altitude and it computes a more realistic ETA and fuel vs. having to spend more time selecting DEP/ARR, STAR, ILS XX, etc, etc. It's just a quick snapshot to check flight plan data vs. FMC data with minimal effort and it can be deleted or will be removed when the box is 'cleaned up' after inserting the correct STAR/runway combination later in the flight.

Honestly with 20+ yrs, and numerous a/c types and FMC versions, I can't remember if my current FMC 'unwinds' to destination altitude automatically. Something to refresh my brain cells with on my next trip. :ok:

Good stuff, misd-agin, I have never thought of doing that. I will keep it in my back pocket for a rainy day :)

zetato - I am out of touch with the latest FMC updates, but on the 737 I understood it always used to calculate a TOD, as NSEU says, with a coded ICAO destination on Routes to arrive at 1500' above the airfield. As you are aware, however, it is not normal to have just the ICAO as DEST - normally at least a runway would be entered which would give you a ball park TOD (or just take 25 mins off ETA:))

Thanks, BOAC. Perhaps the NG is similar to the 747-400

Looking at the final LEGS page of a company route for a 747-400, you might see something like this (if a STAR or descent constraint has not been programmed).

_____ACT RTE 1 LEGS___ 9/9
ANOBO (38NM) .857/FL380 (small font)
RYN.....(108NM) - - -/- - - - -
VTBS....(77NM) - - -/- - - - -
_ _ _ _ _

The FMC doesn't put .875/FL380 at the last two waypoints. It seems to be sensing that these are close to the destination (and perhaps has generated a simple descent profile)

I thought about this thread when I set up my FMC this morning (the real thing, not the simulator). Software was 10.8.. I think?

From the last point in the flight plan I put in direct to the destination. On the FMC DESC page it did tell me distance to T/D (Top of Descent). However, it did not depict the point on the Navigation Display.

Ehi guys, thank you very very much for the answers.
I'll try all the "tricks" you suggested me and check which are the best for my intentions.
I was never hoping to have such a follow to my question, and in case will post other questions in the future! :ok:

See you!

172 Driver, just out of curiosity, did you enter the destination waypoint prior to reaching T/C? On the 747-400, I don't think I've seen both the T/C and T/D displayed on the ND at the same time, even with a short route and STAR programmed.

Thanks
NSEU