747-468 FMS Calculation of Opt/Max Altitude

[Semaphore Sam]

I'm curious about how the 400 FMS calculates step-climbs & opt/max altitudes. I have a sneaking suspician that ISA Dev is not a factor, but can't prove it. When forcast winds & temps are entered, 1) do these entered temps affect time/distance-to-go to opt step time, or 2) is it just present temp, or 3) is temp (both entered & measured) totally ignored? I have the Honeywell Pilot's Guide, and temp factors aren't mentioned; just weight & wind. (I assume the 777 DOES use temp devs; the 400 FMS, being earlier & cruder, may not).

[G.Khan]

The FMS in the -400 certainly does take temp. deviation into account, if you want to see it happen, next time plug in a + instead of a - in front of the temperature, associated with the winds, at altitude!
The FMS also takes into account the buffet limitations, before showing optimum and maximum FLs, that are pre programmed depending on what aviation authority they operate under, 1.3 for UK CAA and 1.2, (I think), for FAA.

[magnum]

Maybe we can extend that question to the B777. I have noticed several times that max/opt alt indication increased significantly after the step climb was completed. That would have meant that the ISA Dev must have decreased significantly over the temp that the FMC had used for its calculation.
That would mean that the FMC displayed opt/ max alt can be off significantly if the temp at higher altitude is lower than actual alt temperature minus 2° per 1000'.. right?

[Semaphore Sam]

So, Mr. Khan & Mr. Magnum, a hypothethical:

You're cruising at FL310; actual SAT is -36°C (ISA +10°C), but the forecast SAT for FL350 (entered during pre-flight, with winds) is -64° (ISA -10°C). Are the Opt/Max altitudes & time/distance to Step-Climb figured on actual (-36° - 8° = -44°C extrapolated up to FL350), or entered/forecast (-66°C at FL350, extrapolated down, or in this case, extrapolated level)?

As Mr. Khan has implied, I'm rather dense, so please patronize me a little.

[G.Khan]

Sorry, certainly didn't mean to imply anything, just telling you what I believe happens. Re your question I THINK the aircraft will use actual temps when it receives them but we used to cross check and update anyway.

The winds and temps are initially for planning purposes and only the actual conditions are relevant to actual performance capability. Been retired a bit now and rusty!

<img src="redface.gif" border="0">

[mutt]

Semaphore Sam, Have you forgotten that we exist? I know that we don’t make the best coffee in the world, but its free!!!!! Anyway the next time that you are down in Kandara drop in and I will give you the long answer to your question.

The following is taken from the “FMC supplementary performance data for the Boeing B747-400 with CF6-80C2 Engines, ref DO44U702-2”.

Altitude Data

The FMC generates optimum and maximum altitude data to assist the pilot in choosing the best cruise altitude for a given trip. The FMC also uses the optimum altitude data to calculate an optimum step point. The calculated maximum altitude is used to ensure that the predicted path of the airplane remains within its performance capabilities. The Maximum altitude is the lower of the thrust limited altitude and the limits of the VNAV operational envelope.

Short Trip Optimum Altitude

The equivalent trip distance is computed by correcting the actual trip distances for the effects of climb derate selection, origin and destination altitudes, forecast winds and temperatures, takeoff gross weight, the use of thermal anti-ice protection during descent and the desired minimum cruise time.

Economy Cruise Mode Long Trip Optimum Altitude

The optimum altitude data shown in figs xx (Long Trip Optimum Altitude and Maximum Altitude ISA-ISA+10 and ISA+20) are identical as the economy cruise long trip optimum altitude is unaffected by temperature deviation.

Maximum Altitude

Maximum Altitude data applicable to the ECON mode are provided versus gross weight in figs xx for temperature deviations of ISA-ISA+10 and ISA+20.

Let me know when you are going to Cairo again and I will come along to test this out.

Cheers.

Mutt.

[Semaphore Sam]

Mr G.Khan: Thanks for the help; I tested out the FMS during cruise the other day; regardless of actual temp, the FMS changes time/distance to step and opt/max altitudes when gross changes are made on the wind/temp inputs for future waypoints (in other words, not just actual, but planned temps are used).

Mutt: Anytime you'd like to come to Cairo (or anywhere else), it'd be my pleasure; I really enjoyed that last ride. The Honeywell guide (for which I paid $50 in Seattle) is not very forthcoming on how the FMS uses data, except for an excellent discussion of how actual present winds & forecast (pilot input) winds are mixed for calculations.

See ya!

[TAT Probe]

If the FMS uses the usual Honeywell algorithms, then temperature isn't a factor. The FMS plans step climbs and optimum altitudes based on Cost Indices and entered winds only. The reason for ignoring temperature is that it is in effect irrelevent in calculating the altitude for best Specific fuel Consumption (which is really all it is doing). As OAT increases, so does Fuel Flow, but also so does TAS, so the calculation of NAM/Kg Fuel remains constant for any given weight, regardless of OAT.

All clever stuff, but these FMS programs are not really that clever, it's just that they save us having to look up reams of tables in a book.

[Cornish Jack]

SS
Rather late to the party, but my Boeing 744 training manual indicates re. max altitude - 'the calculations include the approx. fuel burn-off in climb from current to max operating alt; max altitude is limited by max certified alt and the max alt at which a 1.3g manoeuvre can be accomplished (1.2g for the FAA) at the planned cruise and climb speed; the max altitude is constrained by the more limiting of,.. a rate of climb of 100fpm at planned climb speed and max climb thrust,... or same rate of climb at planned cruise speed and max climb thrust.
(The max climb thrust may be max cruise thrust if so specified on the CDU PERF FACTORS page.)
The max alt calculation is (obviously) used for Optimum altitude and Step Climb point calculations. It is used internally to generate a message indicating that a cruise alt too high for corresponding cost-index, weight, speed limit and R o C. If too high a cruise alt is entered, the fuel predictions are suppressed.
Ther is no specific mention in this particular section of the book of wind and temp inputs to the equation.