Setting Cruise Altitude in FMC

[Pete491 0]

Guys,

 

I just bought the 747 and was setting up my first flight in the FMC. I have been flying the PMDG 737 for quite a while and thought that the FMC's would be similar. They are very much the same except in entering the cruise altitude. I must be doing something wrong.

 

I start by completing the RTE page. Orig, Dest and CO ROUTE. All seems good. I go to PERF INIT. Enter cruise ALT, FL330. set ZFW, Reserves, Cost Index. Go to Takeoff Ref set flaps and speed ref. Back to RTE, Activate, check altitude in LEGS pages and see cruise altitude is set at FL390 not FL330. Back to PERF INIT and double check that CRZ ALT is at FL330. It is.

 

I must be missing a step somewhere.

 

Ideas?

 

Thanks!

[scandinavian13 5,192]

What's your step size and OPT ALT?

[GodAtum 69]

I have the same issue. My step size is ICAO and OPT is FL 354.

[Pete491 0]

Changed step size to 0 and that fixed the problem. All legs are at the cruise altitude.

 

[scandinavian13 5,192]

It's not a problem, guys. It's a user issue.

 

When you set your cruise altitude and leave the STEP SIZE as ICAO or some interval, the FMC follows the OPT ALT as closely as possible given your initial cruise altitude (that you supplied), using the interval in the STEP SIZE. With a STEP SIZE other than zero, you're telling the aircraft to assume that it will climb based on its weight schedule to fly a more optimal profile: as you get lighter, you want to take advantage of higher altitudes for fuel efficiency.

 

As Pete noted, changing the STEP SIZE to 0 forces the system to assume you're going to stay at whatever initial altitude that you provided. Check out the type rating lessons, here, which explain how to properly set up and fly the aircraft (along with the FMC section of the manuals).

[hopskip 253]

Aircraft like the 747-400 can carry lots of fuel. Indeed a 747-400 can carry something like 160 tonnes of fuel (that's more weight in fuel than a full 737 loaded to capacity).

 

Aircraft wings can only produce so much lift, and the higher you go the thinner the air is, meaning the less lift a wing can generate. However thinner air creates less drag, so the higher an aircraft flies the less fuel it uses for the same amount of time. (unless it's so high that it starts to need to hold it's nose high/angle of attack increases so much that the drag from maintaining altitude increases more than the reduction of drag from being higher up).

 

This means that as the aircraft flies along, it's optimum altitude (for minimum fuel burn) increases slowly over the duration of the flight. If you start a 14 hour long flight, the Optimum Altitude may be as low as 29,000ft or so. 5 hours later it will be up near 33,000ft. By the time 12 hours has passed the optimum altitude may even be above 39,000ft.

 

When sending a Flight Notification to Air Traffic Control, Flight Planners/dispatch may need to indicate when they will be climbing or decending as they cross Flight region boundaries (or country borders, or changing from Easterly headings to westerly headings) on the flightplan. An example of such would be the following:

 

N495F320 RPN UL419 TAPAR N115 ROJON/N498F340 N115 ALS

 

In this segment the flightplan departs RPN (VOR) at 32,000ft to fly along the UL419 Airway to TAPAR, turns onto the N115 airway to ALS, but while on the N115 airway the aircraft will climb to 34000ft at/before ROJON.

 

How do we know the aircraft will want to be at 34,000ft at ROJON?

 

use a STEP SIZE of something larger than 0 (in this case it's been set to 2000) and look at the legs page

Not a bug, but an actual useful feature.

 

So what happens when we get to ROJON and the Legs page says to go to 34,000ft?

 

nothing. Unless you use the Mode Control Panel MCP to select 34000ft on the autopilot, and then go to the VNAV/CRZ page and insert 34000ft into the cruise altitude and excecute. And you wouldn't do that without an ATC clearance even with it annotated in the flightplan.

 

What is showing up in the LEGS page is the "predicted best place" to initiate your step climb - It's saying you can climb, not saying you must (and deftinatley not saying you will).

[scandinavian13 5,192]

What is showing up in the LEGS page is the "predicted best place" to initiate your step climb - It's saying you can climb, not saying you must (and deftinatley not saying you will).

 

Well-written, Trent!

[megs8888 20]

Thanks