Handling changing transition altitudes in FMCs

[AnthonyGA]

How do FMCs handle variable transition altitudes, in parts of the world where the transition altitude changes a lot (from airport to airport or region to region)?

In the simulated FMCs I've seen, there's only one spot to enter a transition altitude, and there doesn't seem to be any provision for specifying other transition altitudes elsewhere on the route. And the FMC seems to "forget" the transition altitude and snap back to 18000 during the flight. Do real FMCs have more flexibility? How is it done?

[BOAC]

If the database has full airport info, the TA and TL are stored. If not, being an American product, it defaults to 18,000' and needs to be manually adjusted.

There are not normally other TA's on a route, but there will be other TLs if descent is required away form destination TOD.

[AnthonyGA]

I suspected that might be the problem. I'll check the database used by the sim and see if it provides transition altitudes by location (I suspect that it doesn't, alas!).

[BOAC]

I may well be wrong but I'm pretty sure that you will need SIDs and STARS in the DB to get this option.

[AnthonyGA]

The sim does have SIDs and STARs in its database, but the internal language used to define them is somewhat simpler than the situations one encounters in real life, and I don't know if any provision has been made to specify transition altitudes. I'll have to look into it. As it is I still find the whole arrangement of transition altitudes in the UK and Europe to be very confusing compared to the US; I've figured out that Heathrow is 6000 and Charles de Gaulle is apparently 4000, but that's it.

[BOAC]

PM if you need more TA's, although any set of plates, Jepp or Thales/Aerad will show them. In my experience it is common in the 'lower level' airlines to require crews to insert (if required) TAs and TLs.

We need an FMC genius to give us the database answer!

[AnthonyGA]

I'm a bit puzzled here ... do you have to give the FMC both a TA and a TL? Can't it derive the TL from the TA and the current altimeter setting? (This assumes that the crew keeps the local QNH up to date somehow while in the flight levels; I don't know if that actually happens.)

FMCs are very complex and while the sim add-on I use is extraordinarily faithful to the real thing, it cannot simulate every detail of FMC operation, or at least the current version probably cannot. For example, my impression is that a real FMC can fly a smooth DME arc, but in simulation the arc is simply defined as a large number of closely spaced internal waypoints (but maybe a real FMC does that too).

[8che]

Anthony,

The FMC Database will make sure the correct TA is displayed. Thats if the airport is in the database. If it is not then you can simply put it in manually yourself.

In the real world the TA is actually not particulary important. To avoid level busts you would set 1013 whenever you are cleared to a FL and QNH whenever you are cleared to an altitude, the TA doesnt really come in to it. ( See current tech log thread)

[BOAC]

To correct 8che - it depends on the equipment. I speak only of 737 Classic and NG, and certainly it is not necessarily true in the Classic. The database NEEDS the TA as separate data for an airfield, even if that airfield is 'in the database'.

Anthony - no, the 737's I have flown cannot 'deduce' TL, even with QNH entry. DME arcs cannot be flown on all 737s - it again depends on the equipment, and on the older ones the 'arc' cannot even be put into the route. On the later ones, the arc is often in the database for an arrival and it will be flown fine.